Newton’s Third Law of Motion

Summary of Newton’s Laws So Far

• Newton’s 1st Law of Motion explains the Law of Inertia

– This law predicts the behavior of objects when all forces acting on them are balanced

• Newton’s 2nd Law of Motion describes quantitatively (with math) how unbalanced forces affect motion

– It also addresses the nature of forces experienced by two interacting objects

• So, Newton’s first two laws of motion explain how the

motion of a single object changes

Forces and Objects

• Our daily experiences might lead us to think that forces are always applied by one object on another

• A horse pulls a buggy

• A person pushes a grocery cart

• A magnet attracts a nail

• However, things are not so one-sided!

• Come to find out, whenever 2 objects interact, they exert forces on each other

Newton’s Third Law • Newton’s third law describes what happens when

one object exerts a force on another object

• Let’s look at an example:

• A hammer strikes a nail

• The hammer exerts a force on the nail

• Yet, the nail must also exert a force on the hammer because the hammer’s state of motion is changed

• And according to Newton’s 1st law, this requires an outside force!

• This is the essence of Newton’s Third Law of Motion!

The 3rd Law and Action – Reaction Pairs

• The forces exerted by two objects on each other are often called an action-reaction force pair

• Either force can be considered the action force or the reaction force

• According to Newton’s third law of motion:

For every action, there is an equal but opposite reaction

• In other words, forces always act in equal but opposite pairs

Something to Keep in Mind…

Action and reaction force pairs act on different objects!

Newton’s Third Law • Again, Newton’s Third Law of Motion

states that for every action, there is an equal (in size) but opposite (in direction) reaction

• This means that when you push on a wall, the wall pushes back on you with a force equal in strength to the force you exerted.

Action - Reaction • You constantly use action-reaction

force pairs as you move about.

• For example when you jump, you push down on the ground.

• The ground then pushes up on you.

• The size of the force on the ground equals the size of the force on you

• The direction of the force on the ground (down) is opposite the direction of the force on you (up)

• It is this upward force that pushes you into the air!

Think About It . . .

What happens if you are standing on a

skateboard or a slippery floor and push against

a wall? You slide in the opposite direction

(away from the wall), because you pushed on

the wall but the wall pushed back on you with

equal and opposite force.

Why does it hurt so much when you stub

your toe? When your toe exerts a force on a

rock, the rock exerts an equal force back on

your toe. The harder you hit your toe against

it, the more force the rock exerts back on your

toe (and the more your toe hurts).

Rocket Launches and The 3rd Law • When the rocket fuel is ignited, a hot gas

is produced.

• As the gas molecules collide with the inside engine walls, the walls exert a force that pushes them out of the bottom of the engine

• This is the thrust force and what propels the rocket forward in the opposite direction!

Action: rocket pushes on gases

Reaction: gases push on rocket

The Rocket Example Summary

• The downward push on the gas by the engine walls is the action force

• The reaction force is the upward push on the rocket engine by the gas molecules

• This is the thrust that propels the rocket upward!

Newton’s Third Law

• A bug with a mass of 5 grams flies into the windshield of a moving 1000kg bus.

• Which will have the most force?

– The bug on the bus

– The bus on the bug

Newton’s Third Law

• The force would be the same.

• Force (bug)= m x A

• Force (bus)= M x a

Think I look bad? You should see the other guy!

FINISH YOUR NEWTON’S LAWS FOLDABLE!

TIME FOR SOME MORE PRACTICE!

Turn to Pages

POST-ACTIVITY DISCUSSION OF BALLOON RACES

Analysis Conclusion to Be Handed In

• Remember, Newton’s Third Law of Motion says that whenever one object exerts a force on another object, the second object exerts an equal and opposite force on the first object. However, note that the two forces (action and reaction) do not act on the same object

• Were the forces acting on your balloon rocket equal and opposite? Why or why not? Use your knowledge of ALL Newton’s Laws to support your answer.