The Law of InteractionThe Law of Interaction

Newton’s Third Law of MotionNewton’s Third Law of Motion

33rdrd Law Law

For every action force, there is For every action force, there is an equal and opposite reaction an equal and opposite reaction force.force.

In equation: FIn equation: FA on B A on B = -F= -FB on A B on A

Forces occur in pairs.Forces occur in pairs. Example: for object A and B:Example: for object A and B:

Action force: “A pushes B”Action force: “A pushes B”Reaction force: “B pushes A”Reaction force: “B pushes A”

The action and reaction forces occur at exactly The action and reaction forces occur at exactly the same time!the same time!

If A pushes B, then B at the same time pushes If A pushes B, then B at the same time pushes A A EVEN IF B HAS NO LIFE!EVEN IF B HAS NO LIFE!

““Equal”Equal”In Newton’s Third Law, “equal” means:In Newton’s Third Law, “equal” means:Equal in magnitude (strength)Equal in magnitude (strength)

The action and reaction forces have exactly the same The action and reaction forces have exactly the same MAGNITUDE.MAGNITUDE.

Example: If Bob pushes the wall with 400 N of force, Example: If Bob pushes the wall with 400 N of force, then the wall pushes back Bob with 400 N of force too.then the wall pushes back Bob with 400 N of force too.

““Opposite”Opposite”In Newton’s Third Law, “opposite” means:In Newton’s Third Law, “opposite” means:Opposite in direction.Opposite in direction. Example: Example: If Bob’s foot kicks the soccer ball forward, then If Bob’s foot kicks the soccer ball forward, then

the soccer ball kicks Bob’s foot backwards.the soccer ball kicks Bob’s foot backwards.

33rdrd Law LawAccording to Newton, According to Newton, whenever objects A and whenever objects A and B interact with each B interact with each other, they exert forces other, they exert forces upon each other. When upon each other. When you sit in your chair, you sit in your chair, your body exerts a your body exerts a downward force on the downward force on the chair and the chair chair and the chair exerts an upward force exerts an upward force on your body. on your body.

33rdrd Law Law

There are two forces There are two forces resulting from this resulting from this interaction - a force interaction - a force on the chair and a on the chair and a force on your body. force on your body. These two forces are These two forces are called called actionaction and and reactionreaction forces. forces.

Action: earth pulls on you

Reaction: you pull on earth

Action and Reaction on Different Masses

Consider you and the earth

Newton’s 3rd Law in NatureNewton’s 3rd Law in Nature Consider the propulsion of a Consider the propulsion of a

fish through the water. A fish fish through the water. A fish uses its fins to push water uses its fins to push water backwards. In turn, the water backwards. In turn, the water reactsreacts by pushing the fish by pushing the fish forwards, propelling the fish forwards, propelling the fish through the water.through the water.

The size of the force on the The size of the force on the water equals the size of the water equals the size of the force on the fish; the direction force on the fish; the direction of the force on the water of the force on the water (backwards) is opposite the (backwards) is opposite the direction of the force on the direction of the force on the fish (forwards).fish (forwards).

33rdrd Law Law

Flying gracefully Flying gracefully through the air, through the air, birds depend on birds depend on Newton’s third Newton’s third law of motion. As law of motion. As the birds push the birds push down on the air down on the air with their wings, with their wings, the air pushes the air pushes their wings up their wings up and gives them and gives them lift.lift.

Consider the flying motion of birds. A bird flies by Consider the flying motion of birds. A bird flies by use of its wings. The wings of a bird push air use of its wings. The wings of a bird push air downwards. In turn, the air reacts by pushing the bird downwards. In turn, the air reacts by pushing the bird upwards. upwards.

The size of the force on the air equals the size of the The size of the force on the air equals the size of the force on the bird; the direction of the force on the air force on the bird; the direction of the force on the air (downwards) is opposite the direction of the force on (downwards) is opposite the direction of the force on the bird (upwards).the bird (upwards).

Action-reaction force pairs make it possible for birds Action-reaction force pairs make it possible for birds to fly.to fly.

Other examples of Newton’s Other examples of Newton’s Third LawThird Law

The baseball forces The baseball forces the bat to the left (an the bat to the left (an action); the bat action); the bat forces the ball to the forces the ball to the right (the reaction). right (the reaction).

33rdrd Law Law Consider the motion of Consider the motion of

a car on the way to a car on the way to school. A car is school. A car is equipped with wheels equipped with wheels which spin backwards. which spin backwards. As the wheels spin As the wheels spin backwards, they grip the backwards, they grip the road and push the road road and push the road backwards.backwards.

Action: tire pushes on roadReaction: road pushes on tire

33rdrd Law LawThe reaction of a rocket The reaction of a rocket is an application of the is an application of the third law of motion. third law of motion. Various fuels are burned Various fuels are burned in the engine, producing in the engine, producing hot gases. hot gases. The hot gases push The hot gases push against the inside tube of against the inside tube of the rocket and escape out the rocket and escape out the bottom of the tube. the bottom of the tube. As the gases move As the gases move downward, the rocket downward, the rocket moves in the opposite moves in the opposite direction.direction.

Action: rocket pushes on gases

Reaction: gases push on rocket

Newton’s 3Newton’s 3rdrd Law Law

Suppose you are taking a space walk Suppose you are taking a space walk near the space shuttle, and your near the space shuttle, and your safety line breaks. How would you safety line breaks. How would you get back to the shuttle?get back to the shuttle?

Newton’s 3Newton’s 3rdrd Law Law The thing to do would be to take one of the tools The thing to do would be to take one of the tools

from your tool belt and throw it as hard as you can from your tool belt and throw it as hard as you can directly away from the shuttle. Then, with the help of directly away from the shuttle. Then, with the help of Newton's second and third laws, you will accelerate Newton's second and third laws, you will accelerate back towards the shuttle. As you throw the tool, you back towards the shuttle. As you throw the tool, you push against it, causing it to accelerate. At the same push against it, causing it to accelerate. At the same time, by Newton's third law, the tool is pushing back time, by Newton's third law, the tool is pushing back against you in the opposite direction, which causes against you in the opposite direction, which causes you to accelerate back towards the shuttle, as desired. you to accelerate back towards the shuttle, as desired.

ReviewReviewNewton’s First Law:

Objects in motion tend to stay in motion and objects at rest tend to stay at rest unless acted upon by an unbalanced force.

Newton’s Second Law: Force equals mass times

acceleration (F = ma).

Newton’s Third Law:

For every action there is an equal and opposite reaction.

1stlaw: Homer is large and has much mass, therefore he has much inertia. Friction and gravity oppose his motion.2nd law: Homer’s mass x 9.8 m/s/s equals his weight, which is a force.

3rd law: Homer pushes against the ground and it pushes back.