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Mechanical EquilibriumA force is needed to change an object's state of motion.
- A force is a push or pull. A force is always required to change an obejct's state of motion. - The combination of all forces acting on an object is called the net force. The net force on an object changes its motion. - When you hold a rock at rest in your hand, you are pushing upward on it with as much force as Earth's gravity pulld down on it. The net force on the rock is zero. - The scientific unit of force is the newton, abbreviated N. -Tension is a "stretching force." - Weight is the force of gravity acting downward on an object. - A vector is an arrow that represents the magnitude and direction of a quantity. A vector quantity is a quantity that needs both magnitude and direction for a complete description. Force is an example of a vector quantity. - A scalar quantity is a quantity that can be described by magnitude only and has no direction. Time, area, and volume are scalar quantities. You can express the equilibrium rule mathematically as Ef = 0. - Mechanical equilibrium is a state wherein no physical changes occur; it is a state of steadiness. - Whenever the net force on an object is zero (Ef = 0), the objec is said to be in mechanical equilibrium -- this is known as the equilibrium rule. The symbol E stands for "the sum of" and F stands for "forces." - For a suspended object at rest, the forces acting upward on the object must be balanced by other acting downward to make the vector sum equal zero. For an object at rest on a horizontal surface, the support force must equal the object's weight. - A support force is the upward force that balances the qeight of an object on a surface. A support force is often called the normal force. - An upward support force is positive and a downward weight is negative. - The weight of a book sitting on a table is a negative force that squeezes downward on the atoms of the table. The atoms squeeze upward on the book. The compressed atoms produce the positive support force. |
Essential Questions:How is force related to motion?
How do unbalanced forces afect motion? Why do objects in motion stay in motion? How does a skateboarder use Newton's three Laws of Motion? How would Newton explain the often heard phrase, "The force be with you"? Will a specific force produce the same motion on different objects? Why does it take longer to cook grits in Denver than in Savannah? What floats your boat? How are mass and volume related to density? How do the arrangement and energy of particles determine the phases of matter? How do changes in pressure, volume, or temperature of a gas relate to each other? |
Objects at rest are said to be in static equilibrium; objects moving at constant speed in a straight-line path are said to be in dynamic equilibrium.
- Equilibrium is a state of no change. An object under the influence of only one force cannot be in equilibrium. Only when there is no force at all, or when two or more forces combine to zero, can an object be in equilibrium.
- Both static equilibrium and dynamic equilibrium are examples of mechanical equilibrium.
- Equilibrium is a state of no change. An object under the influence of only one force cannot be in equilibrium. Only when there is no force at all, or when two or more forces combine to zero, can an object be in equilibrium.
- Both static equilibrium and dynamic equilibrium are examples of mechanical equilibrium.
The Parallelogram Rule: To find the resultant of two nonparallel vectors, construct a parallelogram wherein the two vectors are adjacent sides. The diagonal of the parallelogram shows the resultant.
- The sum of two or more vecotrs is called their resultant.
- Combining vectors is simple when they are parallel. If they are in the same direction, they add. If they are opposite directions, they subtract. To find the resultant of nonparallel vectors, use the parallelogram rule.
- When applying the parallelogram rule to two perpendicular vectors that are equal in magnitude, the parallelogram is a square. The resultant is square root of two, or 1.414, times one of the vectors.
- When an object is suspended at rest from two non-vertical ropes, there are three forces acting on it: a tension in the left rope, a tension in the right rope, and the object's weight. The resultant of rope tensions must have the same magnitude as the object's weight.
- The sum of two or more vecotrs is called their resultant.
- Combining vectors is simple when they are parallel. If they are in the same direction, they add. If they are opposite directions, they subtract. To find the resultant of nonparallel vectors, use the parallelogram rule.
- When applying the parallelogram rule to two perpendicular vectors that are equal in magnitude, the parallelogram is a square. The resultant is square root of two, or 1.414, times one of the vectors.
- When an object is suspended at rest from two non-vertical ropes, there are three forces acting on it: a tension in the left rope, a tension in the right rope, and the object's weight. The resultant of rope tensions must have the same magnitude as the object's weight.
Implications of Mechanics on Objects (video)
Forces act on objects causing them to move. Mechanics is the field of science designated to the study of moving objects. This lesson describes how
forces act on objects resulting in motion. Examples are used to describe how forces interact resulting in both simple and complex movement.
forces act on objects resulting in motion. Examples are used to describe how forces interact resulting in both simple and complex movement.
Newton's First Law of Motion -- Inertia
Aristotle, the foremost Greek scientist, studied motion and divided it into two types: natural motion and violent motion.
- During Aristotle's time, natural motionon Earth was thought to be either straight up or straight down: It was "natural" for heavy things to fall and for very light things to rise.
- Violent motion was imposed motion and it was the result of forces that pushed or pulled.
- The proper state of objects was thought to be one of rest, unless they were being pushed or pulled or were moving toward their natural resting place.
Copernicus reasoned that the simplest way to interpret astronomical observations was to assume that Earth and the other planets move around the sun.
- Copernicus' idea of motion in space was extremely controversial at the time, because most people believed that earth was at the center of the universe.
- Copernicus worked on his ideas in secret to escape persecution. At the urging of his close friends, he published his ideas.
- During Aristotle's time, natural motionon Earth was thought to be either straight up or straight down: It was "natural" for heavy things to fall and for very light things to rise.
- Violent motion was imposed motion and it was the result of forces that pushed or pulled.
- The proper state of objects was thought to be one of rest, unless they were being pushed or pulled or were moving toward their natural resting place.
Copernicus reasoned that the simplest way to interpret astronomical observations was to assume that Earth and the other planets move around the sun.
- Copernicus' idea of motion in space was extremely controversial at the time, because most people believed that earth was at the center of the universe.
- Copernicus worked on his ideas in secret to escape persecution. At the urging of his close friends, he published his ideas.
Galileo
Galileo argued that only when friction is present -- as it usually is -- is a force needed to keep an object moving.
- One of Galileo's greatest contributions to physics was demolishing the notion that a force is necessary to keep an object moving. A force is any push or pull.
- Friction is the force that acts between materials that touch as they move past each other.
- Galileo found that a ball rolling on a smooth horizontal plane has almost constant velocity, and if friction were entirely absent, the ball would move forever. Galileo also stated that the tendency of a moving body to keep moving is natural and that every object resists change to its state of motion.
- The property of a body to resist changes to its state of motion is called inertia.
- One of Galileo's greatest contributions to physics was demolishing the notion that a force is necessary to keep an object moving. A force is any push or pull.
- Friction is the force that acts between materials that touch as they move past each other.
- Galileo found that a ball rolling on a smooth horizontal plane has almost constant velocity, and if friction were entirely absent, the ball would move forever. Galileo also stated that the tendency of a moving body to keep moving is natural and that every object resists change to its state of motion.
- The property of a body to resist changes to its state of motion is called inertia.
Newton's Law of Inertia
Newton's first law states that every object continues in a state of rest, or of uniform speed in a straight line, unless acted on by a nonzero net force.
- Isaac Newton's laws of motion replaced the Aristotelian ideas that had dominated thinking for about 2000 years.
- Newton's first law is usually called the law of inertia.
- Force are needed to overcome any friction that may be present. Forces are also needed to set objects in motion initially.
- Once an object is moving in a force-free environment, it will move in a straight line indefinitely.
The more mass an object has, the greater its inertia and the more force it takes to change its state of motion.
- Mass is the quantity of matter in an object. Mass is a measure of the inertia of an object. Mass is measured in the fundamental unit of kilograms.
- Weight is the force of gravity on an object. Weight depends on an object's location. The mass of an object is the same whether the object is located on Earth, on the moon, or in outer space.
- Mass and weight are proportional ti eacg itger ub a given place. Objects with great mass have great weight; objects with little mass have little weight.
- In most parts of the world, the measure of matter is commonly expressed in units of mass. The SI unit of mass is the kilogram and its symbol is kg.
- The SI unit of force is the newton. The SI symbol of the newton is N and is written with a capital letter because it is named after a person.
The law of inertia states that objects in motion remain in motion if no unbalanced forces act on them.
- Copernicus announced the idea of a moving Earth in the sixteenth centruy. This controversal idea stimulated much argument and debate.
- Newton's work showed that objects on earth move with Earth as Earth moves around the sun. The law of inertia also shows that obects within moving vehicles move with the vehicles.
- Notions of motion today are very different from those of our distant ancestors.
- Isaac Newton's laws of motion replaced the Aristotelian ideas that had dominated thinking for about 2000 years.
- Newton's first law is usually called the law of inertia.
- Force are needed to overcome any friction that may be present. Forces are also needed to set objects in motion initially.
- Once an object is moving in a force-free environment, it will move in a straight line indefinitely.
The more mass an object has, the greater its inertia and the more force it takes to change its state of motion.
- Mass is the quantity of matter in an object. Mass is a measure of the inertia of an object. Mass is measured in the fundamental unit of kilograms.
- Weight is the force of gravity on an object. Weight depends on an object's location. The mass of an object is the same whether the object is located on Earth, on the moon, or in outer space.
- Mass and weight are proportional ti eacg itger ub a given place. Objects with great mass have great weight; objects with little mass have little weight.
- In most parts of the world, the measure of matter is commonly expressed in units of mass. The SI unit of mass is the kilogram and its symbol is kg.
- The SI unit of force is the newton. The SI symbol of the newton is N and is written with a capital letter because it is named after a person.
The law of inertia states that objects in motion remain in motion if no unbalanced forces act on them.
- Copernicus announced the idea of a moving Earth in the sixteenth centruy. This controversal idea stimulated much argument and debate.
- Newton's work showed that objects on earth move with Earth as Earth moves around the sun. The law of inertia also shows that obects within moving vehicles move with the vehicles.
- Notions of motion today are very different from those of our distant ancestors.
Newton's First Law of Motion: Examples of the Effect of Force on Motion (video)
This lesson describes Newton's first law of motion, also known as the law of inertia. The interaction between force and motion is explained. Several examples are used to discuss the implications of this law on earth and in space.
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Linear MotionAn object is moving if its position relative to a fixed point is changing.
- When we describe the motion of one object with respect to another, we say that the object is moving relative to the other object. - Unless stated othersise, when we discuss the speeds of things in our environment, we mean speed with respect to the surface of Earth. You can calculate the speed of an object by dividng the distance covered by time. - Galileo is credited as being the first to measure speed by considering the distance covered and the time it takes. - Speed is how fast an object is moving. - Any combination of units for distance nd time that are useful and convenient are legitimate for describing speed. - Some units that describe speed are miles per hour (mi/h) and kilometers per hour. The slash symbol (/) is read as "per." - The speed of an object at any instant is called instantaneous speed. - The average speed of an object is the total distance covered divided by the time. - Average speed does not indicate variations in the speed that may take place during the trip. - A simple arrangement of the definition of average speed gives the total distance covered: total distance covered = average speed X travel time Speed is a description of how fast an object moves; velocity is how fast and in what direction it moves. - Velocity is speed in a given direction. - A quantity such as velocity, which specifies direction as well as magnitude, is called a vector quantity. - Quantities that require only magnitude for a description are scalar quantities. - Constant speed means steady speed. - Constant velocity means both constant speed and constant direction, which is in a straight line. - If either an object's speed or its direction ( or both) is changing, then the object's velocity is changing. You can calculate the acceleration of an object by dividing the change in its velocity by time. - Acceleration is the rate at which the velocity is changing. -In physics, the term acceleration applies to decreases as well as increases in speed. - Acceleration also applies to changes in direction. - Acceleration is defined as the rate of change in velocity, rather than speed. - Acceleration, like velocity, is a vector quantity because it is directional. - If an object's speed, direction, or both changes, the object changes velocity and accelerates. - When the direction is not changing, acceleration may be expressed as the rate at which speed changes. - Since acceleration is the change in velocity or speed per time interval, its units are those of speed per time. |
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Speed and Velocity: Concepts and Formulas (video)Did you know that an object's speed and velocity may not be the same? This lesson describes the concepts of speed and velocity relating to objects in motion. We'll looks at a specific example to help learn how to calculate both speed and velocity.
What is Acceleration? - Definition and Formula (video)Did you know a jet can be traveling the speed of sound and not be accelerating? This lesson describes the difference between speed, velocity and acceleration. Examples are used to help understand the concept of acceleration as well as calculate acceleration with a mathematical formula.
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The acceleration of an object in free fall is about 10 meters per second squared (10m/s^2).
- Gravity causes objects to accelerate downward once they begin to fall.
- In real life, air resistance affects the acceleration of a falling object.
- An object moving under the influence of the gravitational force onlyu is said to be in free fall. Freely falling objects are affected only by gravity.
- The elapsed time is the time that has elapsed, or passed, since the beginning of any motion.
- For free fall, it is customary to use the letter g to represent the accleratin becuase the acceleration is due to gravity.
- Although g varies slightly in different parts of the world, its average value is nearly 10 m/s^2.
- The instantaneous speed of an object falling from rest is equal to the acceleration multiplied by the amount of time it falls ( the elapsed time).
- The instantaneous speed v of an objecy, when the object from rest after an elapsed time t can be expressed in equation form as v = gt. Note that the letter v symbolizes both speed and velocity.
- At the highest point of a rising object, when the object is changing its direction of motion from upward to downward, its instantaneous speed is zero.
- As an object rises, its speed decreases at the same rate it increases when moving downward -- at 10 meters per second each second.
- The instantaneous speed at points of equal elevation in a moving object's path is the same whether the object is moving upward or downward.
For each second of free fall, an object falls a greater distance than it did in the previous second.
- The initial speed of fall is zero and takes a full second to get to 10 m/s.
- Whenever an object's initial speed is zero and the acceleration a is constant, that is, steady and "non-jerky," the equations for the velocity and distance traveled are:
v = at and d = 1/2 at ^2
- Gravity causes objects to accelerate downward once they begin to fall.
- In real life, air resistance affects the acceleration of a falling object.
- An object moving under the influence of the gravitational force onlyu is said to be in free fall. Freely falling objects are affected only by gravity.
- The elapsed time is the time that has elapsed, or passed, since the beginning of any motion.
- For free fall, it is customary to use the letter g to represent the accleratin becuase the acceleration is due to gravity.
- Although g varies slightly in different parts of the world, its average value is nearly 10 m/s^2.
- The instantaneous speed of an object falling from rest is equal to the acceleration multiplied by the amount of time it falls ( the elapsed time).
- The instantaneous speed v of an objecy, when the object from rest after an elapsed time t can be expressed in equation form as v = gt. Note that the letter v symbolizes both speed and velocity.
- At the highest point of a rising object, when the object is changing its direction of motion from upward to downward, its instantaneous speed is zero.
- As an object rises, its speed decreases at the same rate it increases when moving downward -- at 10 meters per second each second.
- The instantaneous speed at points of equal elevation in a moving object's path is the same whether the object is moving upward or downward.
For each second of free fall, an object falls a greater distance than it did in the previous second.
- The initial speed of fall is zero and takes a full second to get to 10 m/s.
- Whenever an object's initial speed is zero and the acceleration a is constant, that is, steady and "non-jerky," the equations for the velocity and distance traveled are:
v = at and d = 1/2 at ^2
On a speed-versus-time graph the slope represents speed per time, or acceleration.
- On a speed-versus-time graph, if the line forms a straight line, time and speed are directly proportional to each other. - The slope of the line is the vertical change divided by the horizontal change for any part of the line. - On a distance-versus- time graph for a falling object, the relationship is quadratic and the curve is parabolic. Air resitance noticeably slows the motion of things with large surface areas like falling feahters or pieces of paper. But air resistance less noticeably affects the motion of more compact objects like stones and baseballs. - Air resistance can affect the acceleration of objects outside a vacuum. - In many cases, however, the effect of air resistance is small enough to be neglected. - With negligible air resistance, falling objects can be considered to be falling freely. Acceleration is the rate at which velocity itself changes. - When we wish to specify how fast something freely falls from rest after a certain elapsed time, we are talking about speed or velocity. The appropriate equation in these cases is v = gt. - When we wish to specify how far an object has fallen, we are talking about distance. The appropriate equation in these cases is d = 1/2 gt^2. |
Newton's Laws and Weight, Mass & Gravity (video)Did you know that mass and weight are not the same? This lesson describes the difference between the two as well as the effect of gravity on weight. Examples are used to teach you how to calculate weight based on mass and acceleration of gravity.
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Projectile Motion
A vector quantity includes both magnitude and direction, but a scalar quanity includes only magnitude.
- Sketches in physics often include arrows, where each arrow reperesents the magnitude and the direction of a certain quantity.
- Velocity is a vector quantity, as is acceleration.
- Scalars can be added, subtracted, multiplied, and divided like ordinary numbers.
The resultant of two perpendicular vectors is the diagonl of a rectangle constructed with the two vectors as sides.
- An airplane's velocity is a combination of the velocity of the airplane relative to the air and the velocity of the air relative to the ground ( the wind velocity).
- For two velocity vectors that are perpendicular, the result of adding the two vectors, called the resultant, is the diagonal of the rectangle described by the two vectors.
- To add equal-magnitude vectors, a square is constructed, and the resultant is the diagonal of the square. For any square, the length of the diagonal is square root of two, or 1.414 times either of its sides.
The perpendicular components of a vector are independent of each other.
- Two vectors at right angles that add up to a given vector are known as the components of the vector they replace.
- The process of determining the components of a vector is called resolution.
- Any vector drawn on a piece of paper can be resolved into vertical and horizontal components that are perpendicular.
- Sketches in physics often include arrows, where each arrow reperesents the magnitude and the direction of a certain quantity.
- Velocity is a vector quantity, as is acceleration.
- Scalars can be added, subtracted, multiplied, and divided like ordinary numbers.
The resultant of two perpendicular vectors is the diagonl of a rectangle constructed with the two vectors as sides.
- An airplane's velocity is a combination of the velocity of the airplane relative to the air and the velocity of the air relative to the ground ( the wind velocity).
- For two velocity vectors that are perpendicular, the result of adding the two vectors, called the resultant, is the diagonal of the rectangle described by the two vectors.
- To add equal-magnitude vectors, a square is constructed, and the resultant is the diagonal of the square. For any square, the length of the diagonal is square root of two, or 1.414 times either of its sides.
The perpendicular components of a vector are independent of each other.
- Two vectors at right angles that add up to a given vector are known as the components of the vector they replace.
- The process of determining the components of a vector is called resolution.
- Any vector drawn on a piece of paper can be resolved into vertical and horizontal components that are perpendicular.
The horizontal component of motion for a projectile is just like the horizontal motion of a ball rolling freely along a level surface without friction. The vertical component of a projectile's velocity is like the motion for a freely falling object.
- A cannonball shot from a cannon, a stone thrown into the air, a ball rolling off the edge of a table, a spacecraft circling Earth -- all of these are examples of projectiles. - A projectile is any object that moves through the air or space, acted on only by gravity ( and air resistance, if any). - When no horizontal force acts on a projectile, the horizontal velocity remains constant. - The horizontal component of motion for a projectile is completely independent of the vertical component of motion. The downward motion of a horizontally launched projectile is the same as that of free fall. - When projectiles are launched horizontally, gravity acts only downward, so the only acceleration is downward. - The vertical distance fallen has nothing to do with the horizontal component of motion. - The path traced by a projectile accelerating only in the vertical direction while moving at constant horizontal velocity is a parabola. The vertical distance a projectile falls below an imaginary straight-line path increases continually with time and is equal to 5t^2 meters. - The maximum horizontal range for projectiles is attained at a projection angle of 45 degrees. - When the effect of air resistance on a projectile's motion is significant, the range is diminished and the path is not a true parabola. - If air resistance is negligible, a projectile hits the ground with the same speed it had originally when it was projected upward from the ground. |
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Newton's Second Law of Motion -- Force and Acceleration
Unbalanced forces acting on an object cause the object to accelerate.
- The combination of forces acting on an object is the net force; acceleration depends on net force.
- Doubling the force on an object doubles its acceleration.
- An object's acceleration is directly proportional to the net force acting on it.
For a constant force, an increase in the mass will result in a decrease in the acceleration.
- The same force applied to twice as much mass results in only half the acceleration.
- For a given force, the acceleration produced is inversely proportional to the mass. Inversely means that the two values change in opposite directions.
Newton's second law states that the acceleration produced by a net force on an object is directly proportional to the magnitude of the net force, is in the same direction as the net force, and is inversely proportional to the mass of the object.
- Newton's second law describes the relationship among an object's mass, an object's acceleration, and the net force on an object.
- In equation form, Newton's second law is written as follows:
net force F
acceleration = mass or a = m
- Acceleration is equal to the net force divided by the mass.
The force of friction between the surfaces depends on the kinds of material in contact and how much the surfaces are pressed together.
- Friction acts on materials that are in contact with each other, and it always acts in a direction to oppose relative motion.
- Liquids and gases are called fluids because they flow. Fluid friction occurs when an object moves thorugh a fluid.
-Air resitance is the friction acting on something moving through air.
- A diagram showing all of the forces acting on an object is called a free-body diagram.
For a constant force, an increase in the area of contact will result in a decrease in the pressure.
- Pressure is the amount of force per unit of area.
In equation form, pressure is defined as follows:
force F
pressure = area of application or P = A
- Pressure is measure in newtons per square meter, or pascals (Pa). One newton per square meter is equal to one pascal.
- The smaller the area supporting a given force, the greater the pressure on that surface.
All freely falling objects fall with the same acceleration because the net force on an object is only its weight, and the ratio of weight to mass is the same for all objects.
- A 10-kg cannonball and a 1-kg stone dropped from an elevated position at the same time will fall together and strike the ground at practically the same time.
- Since mass and weight are proportional, a 10-kg cannonball experiences 10 times as much gravtational force as a 1-kg stone.
The air resistance force an object experiences depends on the object's speed and area.
- The force due to air resistance diminishes the net force acting on falling objects.
- Terminal speed is the speed at which the acceleration of a fallin object is zero because friction balances the weight.
- Terminal velocity is terminal speed together with the direction of motion.
- The combination of forces acting on an object is the net force; acceleration depends on net force.
- Doubling the force on an object doubles its acceleration.
- An object's acceleration is directly proportional to the net force acting on it.
For a constant force, an increase in the mass will result in a decrease in the acceleration.
- The same force applied to twice as much mass results in only half the acceleration.
- For a given force, the acceleration produced is inversely proportional to the mass. Inversely means that the two values change in opposite directions.
Newton's second law states that the acceleration produced by a net force on an object is directly proportional to the magnitude of the net force, is in the same direction as the net force, and is inversely proportional to the mass of the object.
- Newton's second law describes the relationship among an object's mass, an object's acceleration, and the net force on an object.
- In equation form, Newton's second law is written as follows:
net force F
acceleration = mass or a = m
- Acceleration is equal to the net force divided by the mass.
The force of friction between the surfaces depends on the kinds of material in contact and how much the surfaces are pressed together.
- Friction acts on materials that are in contact with each other, and it always acts in a direction to oppose relative motion.
- Liquids and gases are called fluids because they flow. Fluid friction occurs when an object moves thorugh a fluid.
-Air resitance is the friction acting on something moving through air.
- A diagram showing all of the forces acting on an object is called a free-body diagram.
For a constant force, an increase in the area of contact will result in a decrease in the pressure.
- Pressure is the amount of force per unit of area.
In equation form, pressure is defined as follows:
force F
pressure = area of application or P = A
- Pressure is measure in newtons per square meter, or pascals (Pa). One newton per square meter is equal to one pascal.
- The smaller the area supporting a given force, the greater the pressure on that surface.
All freely falling objects fall with the same acceleration because the net force on an object is only its weight, and the ratio of weight to mass is the same for all objects.
- A 10-kg cannonball and a 1-kg stone dropped from an elevated position at the same time will fall together and strike the ground at practically the same time.
- Since mass and weight are proportional, a 10-kg cannonball experiences 10 times as much gravtational force as a 1-kg stone.
The air resistance force an object experiences depends on the object's speed and area.
- The force due to air resistance diminishes the net force acting on falling objects.
- Terminal speed is the speed at which the acceleration of a fallin object is zero because friction balances the weight.
- Terminal velocity is terminal speed together with the direction of motion.
Newton's Second Law of Motion: The Relationship Between Force and Acceleration (video)
This lesson defines Newton's second law of motion. Examples are used to illustrate how unbalanced forces cause objects to accelerate. The examples are used to practice calculating acceleration and force for objects in motion.
Newton's Third Law of Motion -- Action and ReactionA force is always part of a mutual action that involves another force.
- A mutual action is an interaction between one thing and another. - An example of interaction occurs when a hammer exerts a force on a nail, an the nail exerts a force on the hammer. Newton's third law states that whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object. - Newton's third law describes the realtionship between two forces in an interaction. Newton's third law is often stated: "To every action there is always an equal opposing reaction." - In an interaction, one force is called the action force. The other force is called the reaction force. The action and reaction forces are equal in strength and opposite in direction. - When you walk on a floor, you push against the floor, and the floor simultaneously pushes against you. To identify a pair of action-reaction forces, first identify the interacting objects A and B, and if the action is A on B, the reaction is B on A. - When a boulder falls to Earth, the action is Earth exerting a force on the boulder, and the reaction is the boulder simultaneously exerting a force on Earth. - A rocket accelerates becuase the rocket pushes exhaust gas and the exhaust gas pushes on the rocket. |
Newton's Third Law of Motion: Examples of the Relationship Between Two Forces (video)This lesson describes Netwon's third law of motion. Examples are provided to illustrate how interacting objects experience forces. The lesson explains how objects accelerate as a result of force. Applications of Newton's third law are illustrated in nature, machines, and space.
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A given force exerted on a small mass produces a greater acceleration than the same force exerted on a large mass.
- Recall that newton's second law states that acceleration is proportional to the net force and inversely proportional to the mass.
- When a boulder falls toward Earth, Earth also moves toward the boulder. Because Earth has a huge mass, its acceleration toward the boulder is infinitesimally small. a rocket accelerates becuase it continually recoils from the exhaust gases ejected from its engine.
- Recall that newton's second law states that acceleration is proportional to the net force and inversely proportional to the mass.
- When a boulder falls toward Earth, Earth also moves toward the boulder. Because Earth has a huge mass, its acceleration toward the boulder is infinitesimally small. a rocket accelerates becuase it continually recoils from the exhaust gases ejected from its engine.
Defining Systems
Action and reaction forces do not cancel each other when either of the forces is external to the system being considered.
- Consider an imaginary system consisting of an orange siting on a cart. An external force, provided by an apple pulling the cart, causes the system to accelerate in accord with Newton's second law.
- The fact that the orange simultaneously exerts a force on the apple, which is external to the system, may affect the apple (another system), but not the orange. You can't cancel a force on the orange with a force on the apple.
- Now consider a larger system, enclosing both the orange and the apple. The force pair is internal to the orange-apple system. therefore these forces do not cancel each other.
- A force external to the system, in this case friction, is needed for acceleration. When the apple pushes against the floor, the floor simultaneously pushes on the apple -- an external force on the system -- and the system accelerates.
- Consider an imaginary system consisting of an orange siting on a cart. An external force, provided by an apple pulling the cart, causes the system to accelerate in accord with Newton's second law.
- The fact that the orange simultaneously exerts a force on the apple, which is external to the system, may affect the apple (another system), but not the orange. You can't cancel a force on the orange with a force on the apple.
- Now consider a larger system, enclosing both the orange and the apple. The force pair is internal to the orange-apple system. therefore these forces do not cancel each other.
- A force external to the system, in this case friction, is needed for acceleration. When the apple pushes against the floor, the floor simultaneously pushes on the apple -- an external force on the system -- and the system accelerates.
The Horse-Cart Problem
If the horse in the horse-cart system pushes the ground with a greater force than it pulls on the cart, there is a net force on the horse, and the horse-cart system accelerates.
- Imagine a horse that believes its pull on a cart carrying a farmer will be canceled by the opposite and equal pull by the cart on the horse, thus making acceleration impossible.
- From the farmer's point of view, the net force on the cart, divided by the mass of the cart, will produce an acceleration.
- In the system of the horse, the opposite reaction force by the cart on the horse restrains the horse. The horse moves forward by interacting with the ground. When the horse pushes backward on the ground, the ground simultaneously pushes forward on the horse.
- In the horse-cart system as a whole, the pull of the horse on the cart and the reaction of the cart on the horse are forces that act and react within the system. They cancel and can be neglected. It is the outside reaction by the ground that pushes the system.
For every interaction between things, there is always a pair of oppositely directed forces that are equal in strength.
- Suppose that, for some reason, you punch a wall. You cannot hit the wall any harder than the wall can hit you back.
- Hold a sheet of paper in midair and tell your friends that the heavyweight champion of the world could not strike the paper with a force of 200 N (45 pounds). The paper is not capable of exerting a reaction force of 200 N, and you cannot have an action force without a reaction force.
- Imagine a horse that believes its pull on a cart carrying a farmer will be canceled by the opposite and equal pull by the cart on the horse, thus making acceleration impossible.
- From the farmer's point of view, the net force on the cart, divided by the mass of the cart, will produce an acceleration.
- In the system of the horse, the opposite reaction force by the cart on the horse restrains the horse. The horse moves forward by interacting with the ground. When the horse pushes backward on the ground, the ground simultaneously pushes forward on the horse.
- In the horse-cart system as a whole, the pull of the horse on the cart and the reaction of the cart on the horse are forces that act and react within the system. They cancel and can be neglected. It is the outside reaction by the ground that pushes the system.
For every interaction between things, there is always a pair of oppositely directed forces that are equal in strength.
- Suppose that, for some reason, you punch a wall. You cannot hit the wall any harder than the wall can hit you back.
- Hold a sheet of paper in midair and tell your friends that the heavyweight champion of the world could not strike the paper with a force of 200 N (45 pounds). The paper is not capable of exerting a reaction force of 200 N, and you cannot have an action force without a reaction force.