# Newton’s Third Law of Motion

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Newton’s Third Law of Motion
Action And Reaction

Introduction If you lean over – you fall
If you lean over with your hands against the wall – you don’t fall Why? Because the wall is pushing on you and holds you in place

Forces and Interactions
Force – a push or pull Newton saw an interaction Mutual action between forces Hammer and nail Hammer moves nail Nail halts the hammer

Newton’s Third Law Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object

Third Law Forces Action force – Reaction force –
It doesn’t matter which is which Partners in the interaction Equal and opposite Neither can exist without the other

Examples You push on floor, the floor pushes on you
Car pushes on road, the road pushes on the car Swimming – you push water backward, the water pushes you forwards More friction – more force

Questions Does a stick of dynamite contain force?
No, force is an interaction between two objects. An object may be capable of creating a force, but it can’t possess it. Dynamite possesses ENERGY

Questions A car accelerates along the road. Strictly speaking, what is the force that moves the car? The road pushes the car. It provides a horizontal force.

Identifying Action and Reaction
Sometimes it is hard to find the action/reaction pairs What are the forces on a falling boulder? Gravity? Weight? Ground? No – none of these

Recipe for finding forces
Identify the interaction Action: Object A exerts force on object B Reaction: Object B exerts force on object A Falling boulder – Earth exerts force on boulder, Boulder exerts force on Earth

Action and Reaction on Different Masses
Boulder-Earth interaction – the forces are EQUAL Does the earth fall into the boulder? Yes, but not as far The reactions are equal and opposite Because Earth is so large, we can’t sense its very small acceleration

Question We know the Earth pulls on the moon. Does the moon also pull on the earth? If so, which pull is stronger? Yes, the Earth and moon both pull on each other. Both pulls are equal and opposite.

Cannon/Cannon ball When fired, the cannon ball exerts a force on the cannon The cannon recoils or “kicks” Why doesn’t the cannon move as fast as the cannon ball? A given force exerted on a small mass will result in a greater acceleration than on a larger mass

Rocket propulsion Air escaping from a balloon
The balloon accelerates the opposite direction as the escaping air. Rocket – each molecule of escaping gas acts like the cannon ball

Rocket in air and space Once though the rocket needed air to “push” against. Actually, rockets work better without air drag (in space)

Momentum How can a karate expert break a stack of bricks?
Why does it hurt more to fall on concrete than a wooden floor? Why do you follow through in golf, baseball or bowling?

Momentum Momentum = mass x velocity p = m x v
How much momentum does a 400 kg bike moving 40 m/s have? p= mv = (400 kg) ( 40 m/s) = 16,000 kgm/s

Helicopters Helicopters – blades push air down. Air pushes chopper up.
Lift – upward reaction force When lift equals weight of chopper, it can hover When lift is greater, the chopper rises

Birds and Airplanes Fly due to action/reaction forces
Wings of bird reflect air downward. The air then pushes upward Airplanes must have a continuous flow of air in order to have lift. The engines push air back. The air pushes the jet forward

Do Action and Reaction Forces Cancel ?
Why do the equal forces not cancel to zero How can there be acceleration? There are MANY equal and opposite forces in a system. If two people kick the ball at the same time – together they cancel out

Question Suppose a friend who hears about Newton’s law say that you can’t move a football by kicking it because the reaction force by the kicked ball would be equal and opposite to your kicking force. The net force would be zero, so no matter how hard you kick, the ball won’t move! What do you say to your friend?

Answer Obviously kicking the ball will accelerate it. Your kick acts on the ball. It accelerates the ball. The reaction force acts on your foot – it decelerates your foot.

The horse-cart problem
If a horse pulls on a cart, doesn’t the cart pull back equally? Three views Farmer – get the cart to market Horse Horse-cart system

Farmer Only concerned about force exerted on the cart
Force on cart/mass of cart = acceleration Doesn’t care about the reaction on the horse

Horse system Opposite reaction force by the cart restrains the horse
So how can the horse move forward? The horse must push on the GROUND – which pushes back.

Horse-Cart system The pull of the horse on the cart and the reaction of the cart on the horse are internal forces. They contribute nothing to the acceleration The acceleration comes from the horse-cart system and the ground

Stalled car You can’t move the car by sitting in your seat and pushing against the dashboard You must get outside and push against the ground. Don’t worry if you get confused, Newton struggled with his law too.

Action Equal Reaction What if you hit a wall? What if you hit paper?
It hits back You can’t hit it harder than it can hit you back What if you hit paper? It can not hit back as hard

Conclusion Push the world hard – it pushes back hard
Push the world gentle – it pushes back gentle You can not touch without being touched!!!

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