Newton’s Third Law

Did you know that when you push against a wall, it pushes back?

No, the wall isn’t “getting pushy.”

But why don’t you move the wall? Why don’t you fall?

It all has to do with Newton’s Third law…

Newton’s Third LawEvery action force has an equal and opposite

reaction force.

What is a force?An action or reaction force is a push or a pull.

Every time a force is applied, a reaction force occurs at the same time.

For example, when a ball is thrown against a wall, the wall “pushes” it back. That is what we call bouncing.

If force are “equal and opposite”, why does anything ever move?

Here is a famous problem: A horse is pulling on a cart, and the cart pulls back with the same amount of force. If all forces are equal, how can the horse and cart move?

Answer: The horse moves because the force he exerts with his hooves is greater than the force of the wagon pulling him back.

What pushes the horse forward? It's the ground! The horse pushes backward on the ground, so the ground pushes forward with an equal force. If the horse can push back against the ground with a force greater than the cart's resisting force, then the horse will accelerate.

Acceleration will occur if one force pair (push of ground/push of horse)

is greater than another force pair (friction/pull of cart).

 

What forces act on the cart? The horse pulls it forward, and there is a backward force from the ground: friction. If the horses' pull exceeds the friction of the cart, it will accelerate.

 

Can you explain this?

To summarize…How far and if an object moves depends on the net force.

MomentumThe momentum of an object is its mass times its

velocity.

P=mv

The unit is kg.m/s

Momentum explained

Law of Conservation of Momentum

Momentum can be transferred from one object to another.

The results of a collision depends on the momentum of each object. For example, if two hockey pucks are speeding towards each other with the same speed, the total momentum is zero.

Force and changing momentum

F= (mvf – mvi)/t

mvf = final momentum

mvi= initial momentum

An explanation:When you catch a ball, your hand exerts a force

on the ball that stops it. The force you exert is equal to the force the ball exerts on your hand. This force depends on the mass and initial velocity of the ball and how long it takes it to stop.