Newton’s Third Law Chapter 12.3 Notes

Newton’s Third Law Newton’s third law states that for every action force, there is an equal and opposite reaction force Newton’s third law is also called the law of action and reaction When one object exerts a force on a second object, the second object exerts a force equal in size and opposite in direction on the first object

Forces Always Occur in Pairs Action and reaction forces are applied to different objects, and they are always equal and opposite Action and reaction forces exist even when there is no motion You are sitting on a stool. Your weight pushes down on the stool; this is the action force. The stool pushing back up with a force equal to your weight is the reaction force.

Forces in Force Pairs Newton’s third law says that forces happen in pairs—although the forces are equal and opposite, they do not cancel each other because they act on different objects For example: a swimmer’s hands and feet exert an action force on the water, the water exerts the reaction force on the swimmer’s hands and feet. Action and reaction forces occur at the same time, but never act on the same object

Equal forces don’t always have equal effects When a soccer ball is dropped, the force of gravity pulls it towards the Earth The force of gravity is the action force that the Earth exerts on the ball The reaction force is that gravity also pulls the Earth towards the soccer ball

Momentum Momentum is a property for all moving objects For movement along a straight line, momentum is calculated by multiplying an object’s mass and velocity momentum=mass x velocity p=mv The SI unit for momentum is kg·m/s

Properties of Momentum For a given velocity, objects with a greater mass will also have a greater momentum Moving at the same speed, a tractor-trailer truck will have a greater momentum than a sports car For equal masses, the object that is moving faster has a greater momentum If an object is not moving, then its momentum is zero

Conservation of Momentum Law of Conservation of Momentum states that the total amount of momentum in an isolated system is conserved In other words, the total amount of momentum of two or more objects after a collision is the same as it was before the collision Example: imagine two cars of different masses moving with different velocities collide head on The momentum of the cars after the collision is equal to the amount of momentum before the collision momentum

Conserving Momentum Imagine hitting a billiard ball with a cue ball such that the billiard ball starts moving During a collision with a billiard ball, the cue ball transfers some of its momentum to the billiard ball Anytime two or more objects interact, they may exchange momentum, but the total momentum of the systems always stays the same The Law of conservation of momentum is explained by Newton’s third law—in a game of pool, the cue ball hits a billiard ball with a force (the action force), and the billiard ball exerts an equal but opposite force on the cue ball (the reaction force)