Newton’s Third Law of Newton’s Third Law of Motion – Action and Motion – Action and

ReactionReaction

Chapter 7Chapter 7

ObjectivesObjectives

Define force as part of an interaction.Define force as part of an interaction. State Newton’s third law of motion.State Newton’s third law of motion. Describe how to identify a pair of action-Describe how to identify a pair of action-

reaction forces.reaction forces. Explain why the accelerations caused by Explain why the accelerations caused by

an action force and a reaction force do not an action force and a reaction force do not have to be equal.have to be equal.

Explain why an action force is not Explain why an action force is not cancelled by a reaction force.cancelled by a reaction force.

Explain how a horse-cart system Explain how a horse-cart system accelerates.accelerates.

Explain what must occur in every Explain what must occur in every interaction between things.interaction between things.

7.1 Forces and Interactions7.1 Forces and Interactions Recall (Ch. 2) that a force is a Recall (Ch. 2) that a force is a

push or pull.push or pull. According to Newton, a force According to Newton, a force

is always part of a mutual is always part of a mutual action that involves another action that involves another force.force.

This mutual action is called This mutual action is called an an interactioninteraction..

For example, a bat exerts a For example, a bat exerts a force on a ball but the ball force on a ball but the ball also exerts a force on the bat.also exerts a force on the bat.

So for every interaction, there So for every interaction, there are a pair of forces involved. are a pair of forces involved.

7.2 Newton’s Third Law7.2 Newton’s Third Law According to Newton, According to Newton,

whenever one object whenever one object exerts a force on a second exerts a force on a second object, the second object object, the second object exerts an exerts an equal and equal and oppositeopposite force on the first force on the first object.object.• One force is called the One force is called the

action force.action force.• The other force is called the The other force is called the

reaction force.reaction force. In other words,In other words, for every for every

action, there is an equal action, there is an equal and opposite reaction.and opposite reaction.

7.2 Newton’s Third Law7.2 Newton’s Third Law

Important points to remember:Important points to remember: In every interaction, the forces In every interaction, the forces

always occur in pairs.always occur in pairs. The action/reaction forces are always The action/reaction forces are always

equal in magnitude.equal in magnitude. The action/reaction forces are always The action/reaction forces are always

opposite in direction.opposite in direction.

ExamplesExamples

The athlete pushes the bar upwards.

The bar pushes the athlete downwards.

The girl pushes on the wall and the wall pushes on her.

The swimmer pushes backward on the water and the water pushes forward on her

7.2 Newton’s Third Law7.2 Newton’s Third Law Notice that interactions Notice that interactions

generally depend on generally depend on friction.friction.

That is, if an action That is, if an action force cannot be force cannot be exerted (because the exerted (because the surface is ice, for surface is ice, for example), there cannot example), there cannot be a reaction force.be a reaction force.

There is no resulting There is no resulting forward motion.forward motion.

7.3 Identifying Action and 7.3 Identifying Action and ReactionReaction

In the examples given previously, a pattern for In the examples given previously, a pattern for identifying the action/reaction pair is identifying the action/reaction pair is apparent.apparent.

First, identify the objects “A” and “B”, then First, identify the objects “A” and “B”, then follow this simple “recipe”: follow this simple “recipe”:

ACTION: A exerts a force on BACTION: A exerts a force on B

REACTION: B exerts on force on AREACTION: B exerts on force on A Example:Example:

ACTION: foot pushes ACTION: foot pushes

against sandagainst sand

REACTION: sand pushesREACTION: sand pushes

against footagainst foot

PracticePracticeA car is cruisingdown the street.

A rocket is liftingoff toward space.

A ball is dropped offa very tall building.

Determine the action-reaction forces of Determine the action-reaction forces of the above interactions.the above interactions.

               Bowling ball pushes pin leftwards.

             Baseball is caught in glove..

A sledgehammer drives a stake.

Check Your UnderstandingCheck Your Understanding While driving, Anna observed a bug While driving, Anna observed a bug

striking the windshield of her car. The bug striking the windshield of her car. The bug hit the windshield and the windshield hit hit the windshield and the windshield hit the bug. Which of the two forces is the bug. Which of the two forces is greater: the force on the bug or the force greater: the force on the bug or the force on the windshield?on the windshield?

We know that the Earth pulls on the moon. We know that the Earth pulls on the moon. Does the moon also pull on the Earth? If Does the moon also pull on the Earth? If so, which pull is stronger?so, which pull is stronger?

In tug-of-war, if you pulling on the rope is In tug-of-war, if you pulling on the rope is the action force, is the reaction force the the action force, is the reaction force the ground pushing back on you or your ground pushing back on you or your opponent pulling back on the rope?opponent pulling back on the rope?

7.4 Action and Reaction on 7.4 Action and Reaction on Different MassesDifferent Masses

In the interaction between the In the interaction between the boulder and Earth, the boulder and Earth, the boulder will push on the Earth boulder will push on the Earth with as much force as the with as much force as the Earth will push on the Earth will push on the boulder. The FORCES will be boulder. The FORCES will be the same.the same.

The masses of the 2 are quite The masses of the 2 are quite unequal, however.unequal, however.

Since a = F/m, the boulder Since a = F/m, the boulder (low m) will experience a (low m) will experience a much greater acceleration much greater acceleration than the Earth (high m).than the Earth (high m).

The Earth DOES accelerate The Earth DOES accelerate also but we cannot “sense” it also but we cannot “sense” it because the acceleration is because the acceleration is infinitesimally small. infinitesimally small.

Force and MassForce and Mass Think about the firing of Think about the firing of a cannon. a cannon.

The force the cannon The force the cannon exerts on the cannonball exerts on the cannonball is exactly equal and is exactly equal and opposite to the force opposite to the force exerted by the exerted by the cannonball on the cannonball on the cannon.cannon.

The cannonball moves The cannonball moves forward and the cannon forward and the cannon “kicks” or “kicks” or recoilsrecoils. They . They both accelerate but not both accelerate but not by the same amount due by the same amount due to their differing masses.to their differing masses.

Force and MassForce and Mass Let F represent both the action & reaction Let F represent both the action & reaction

forces.forces.M M represents the mass of the cannon and represents the mass of the cannon and mm

represents the mass of the ball.represents the mass of the ball. For the cannonball: For the cannonball: FF = = aa (large acceleration) (large acceleration) mm

For the cannon: For the cannon: FF = = aa (small acceleration) (small acceleration) MM The acceleration of the cannonball is great The acceleration of the cannonball is great

compared to the acceleration of the cannon.compared to the acceleration of the cannon. A force exerted on a small mass produces a A force exerted on a small mass produces a

greater acceleration than the same force greater acceleration than the same force exerted on a large mass.exerted on a large mass.

Force and MassForce and Mass Extend this idea to a rocket launch.Extend this idea to a rocket launch. Combustion of rocket fuel produces Combustion of rocket fuel produces

“expanding gases”. Each molecule of “expanding gases”. Each molecule of exhaust gas acts like a tiny molecular exhaust gas acts like a tiny molecular cannonball shot downward.cannonball shot downward.

In reaction. the rocket recoils from the In reaction. the rocket recoils from the molecular cannonballs shot downward molecular cannonballs shot downward and climbs upward.and climbs upward.

No atmosphere is necessary for this No atmosphere is necessary for this action & reaction.action & reaction.

Understanding LiftUnderstanding Lift Using Newton’s 3Using Newton’s 3rdrd law, we law, we

can explain how a helicopter, can explain how a helicopter, bird, or plane gets its lifting bird, or plane gets its lifting force.force.

The whirling blades of the The whirling blades of the copter (or the wings of a bird copter (or the wings of a bird or the wings of an airplane) or the wings of an airplane) force air downward; the air force air downward; the air forces the blades (or wings) forces the blades (or wings) upward.upward.

The upward reaction force is The upward reaction force is called called lift.lift.

When lift equals the weight When lift equals the weight of an object, the object can of an object, the object can hover; when lift is greater, it hover; when lift is greater, it can climb upward.can climb upward.

Understanding LiftUnderstanding Lift

A plane, though, must continuously A plane, though, must continuously push air downward to remain in the air.push air downward to remain in the air.

A continuous supply of air is produced A continuous supply of air is produced by the forward motion of the plane.by the forward motion of the plane.

The forward motion is supplied by jets The forward motion is supplied by jets or propellers. They push air backwards or propellers. They push air backwards and, in turn, the air pushes the plane and, in turn, the air pushes the plane forward.forward.

Check your UnderstandingCheck your Understanding

Explain why the acceleration of a Explain why the acceleration of a recoiling gun is much less than the recoiling gun is much less than the acceleration of the bullet.acceleration of the bullet.

Are rockets able to accelerate in Are rockets able to accelerate in space? Why or why not?space? Why or why not?

A tug-of-war takes place between Ms. A tug-of-war takes place between Ms. P’s physics class and Mrs. D’s. If the P’s physics class and Mrs. D’s. If the floors are polished and slippery and if floors are polished and slippery and if Mrs. D’s entire class is wearing socks Mrs. D’s entire class is wearing socks only, who will win and why?only, who will win and why?

7.5 Defining Systems7.5 Defining Systems

•Since action-reaction forces are equal and opposite, why don’t they cancel to zero?•Simply put, the forces do not cancel because they are forces on different systems (different objects).

7.5 Defining Systems7.5 Defining Systems

•To explain this answer, we must consider the systems involved.

•There are 2 systems in the diagram above.

1 2

7.5 Defining Systems7.5 Defining Systems In system 1, we see a vector (arrow) In system 1, we see a vector (arrow)

extending outside the system. This is a extending outside the system. This is a force force onon the system, so the system the system, so the system accelerates (Newton’s 2accelerates (Newton’s 2nd nd Law).Law).

The force was provided by the apple, The force was provided by the apple, which is outside the system. Any reaction which is outside the system. Any reaction force (orange on the apple) is outside the force (orange on the apple) is outside the system, so it does not affect the orange. system, so it does not affect the orange.

Action and reaction forces do NOT cancel Action and reaction forces do NOT cancel when either force is outside the system when either force is outside the system being considered.being considered.

What if we put a dotted line around the What if we put a dotted line around the apple instead of the orange? Our system apple instead of the orange? Our system will be the apple so . . . will be the apple so . . .

7.5 Defining Systems7.5 Defining Systems

In system 2, since the action/reaction In system 2, since the action/reaction forces are forces are withinwithin the system, they the system, they DO cancel each other. They do not DO cancel each other. They do not cause the system to accelerate.cause the system to accelerate.

A force outside the system – A force outside the system – FRICTION – is needed.FRICTION – is needed.

The apple pushes on the floor; the The apple pushes on the floor; the floor pushes on the apple. The floor pushes on the apple. The system accelerates. system accelerates.

7.5 Defining Systems7.5 Defining Systems

Another example: a Another example: a baseball has an baseball has an immeasurable number immeasurable number of forces between the of forces between the atoms WITHIN the ball.atoms WITHIN the ball.

These forces hold the These forces hold the ball together; they DO ball together; they DO NOT cause the ball to NOT cause the ball to accelerate.accelerate.

An external force, like An external force, like a bat, is needed to a bat, is needed to accelerate the ball.accelerate the ball.

7.6 The Horse-Cart Problem7.6 The Horse-Cart Problem

Take a look at the Take a look at the cartoon on pg. cartoon on pg. 115 in your test.115 in your test.

This “problem” This “problem” can be thought of can be thought of as containing 3 as containing 3 different systems.different systems.– The cartThe cart– The horseThe horse– The horse and The horse and

cartcart

7.6 The Horse-Cart Problem7.6 The Horse-Cart Problem The cart system: the cart will accelerate The cart system: the cart will accelerate

because there is an external force being because there is an external force being place on it by the horse. There is also an place on it by the horse. There is also an external force of friction but this can be external force of friction but this can be discounted because the wheels are smooth discounted because the wheels are smooth and shiny.and shiny.

The horse system: there is a reaction force The horse system: there is a reaction force from the cart that is external to this system. from the cart that is external to this system. It will therefore act to restrain the horse. It will therefore act to restrain the horse. However, the horse also interacts with the However, the horse also interacts with the ground – the horse pushes backward and ground – the horse pushes backward and the ground pushes forward. the ground pushes forward.

7.6 The Horse-Cart Problem7.6 The Horse-Cart Problem If the horse in the horse-cart system pushes the If the horse in the horse-cart system pushes the

ground with a greater force than it pulls on the ground with a greater force than it pulls on the cart, there is a net force on the horse and the cart, there is a net force on the horse and the horse-cart system will accelerate.horse-cart system will accelerate.

The horse-cart system: the pull of the horse on The horse-cart system: the pull of the horse on the cart and the cart on the horse are internal the cart and the cart on the horse are internal forces and do not accelerate the system. The forces and do not accelerate the system. The interaction between the horse-cart system and interaction between the horse-cart system and the ground is responsible for motion.the ground is responsible for motion.

Acceleration = Acceleration = force of the push on the groundforce of the push on the ground mass of the horse and cartmass of the horse and cart

7.7 Action Equals Reaction7.7 Action Equals Reaction

For every For every interaction interaction between things, between things, there is always there is always a pair of a pair of oppositely oppositely directed forces directed forces that are equal in that are equal in strength.strength.

PracticePractice

List the forces acting on List the forces acting on thethe– Tractor systemTractor system– Elephant systemElephant system– Man systemMan system

PracticePractice

Consider the interaction depicted above between Consider the interaction depicted above between foot A, ball B, and foot C. The three objects interact foot A, ball B, and foot C. The three objects interact simultaneously (at the same time). Identify the simultaneously (at the same time). Identify the two two pairspairs of action-reaction forces. Use the notation of action-reaction forces. Use the notation "foot A", "foot C", and "ball B" in your statements."foot A", "foot C", and "ball B" in your statements.

Under what conditions would the ball accelerate? Under what conditions would the ball accelerate?

PracticePractice

If A and B have If A and B have equal mass, what equal mass, what will happen when will happen when they push off from they push off from each other?each other?

Suppose A has a Suppose A has a greater mass than greater mass than B. B.

AB