Newton’s 3 rd Law Chapter 8

Recall this info… Newton’s 1 st – An object at rest tends to remain at rest and an object in motion tends to stay in motion in a straight line at a constant velocity, unless acted upon by a unbalanced force.

Recall this info too… Newton’s 2 nd – If an unbalanced for acts on an object the object will accelerate in the direction of the unbalanced force. – The equation that follows the law is: Force = Mass x Acceleration

Newton’s 3 rd Law Newton’s 3 rd – For every action, there is an equal but opposite reaction Forces always act in equal pairs. Example: – If object A exerts a force on object B, then object B exerts a force of the same size on object A

Action and Reaction Forces Examples: 1) When you: Pull your backpack -you lift your backpack -your backpack pulls back 2) When you: Jump -you push down on the ground -the ground pushes up on you 3) When you: Walk -you push back on the ground -the ground pushes back on you 4) When you: Throw a baseball -you push forward on the ball -the ball pushes back on your hand AKA: Action Reaction Force Pairs

Newton’s 3 rd Example BOAT KE Y Action Force Reaction Force

Action Reaction Explained Action reaction forces act on DIFFERENT objects! NOT THE SAME The ACTION force acts on the block & The REACTION forces acts on the woman To test this push the side of your hand on the desk.

Newton’s 3 rd and Movement So if you push something and it pushes back with equal force how does anything move? 1.The objects have different masses 2.The forces are exerted on the different objects NOT the same. 3.Also there may be other forces acting such as friction!

MOMENTUM Momentum: Equal to the mass of an object multiplied by its velocity Formula: Momentum = mass x velocity – An object’s momentum depends on both its mass and velocity Train has a large momentum because it has a large mass Bullet has a large momentum because it has a large velocity

The Law of Conservation of Momentum The Law of Conservation of Momentum: The total momentum of any group of objects remains the same unless outside forces act on the objects. Ex. Baseball bat hitting baseball

Momentum Questions Why is it harder to stop a truck moving at 50km/hr than the same truck moving at 10 km/hr? How can a 75kg defensive back in football stop a 100 kg fullback? Explain why an ocean liner slowly docking and a bullet rapidly fired both have a large momentum.