Newton’s Third Law Physical Science

The heavy weight champion can hit the massive bag with considerable force.

But with the same punch he can only exert a tiny force on the tissue paper in midair. Why is this? Give more examples with the punching the wall/walking on the floor/holding books over my head/tug of war (the force of the rope is the same, but the force exerted on the ground is more for the side that wins)

Newton’s 3rd Law For every action there is an equal and opposite reaction These forces are equal in magnitude and opposite in direction and comprise a single interaction. Action and reaction means that the forces act in pairs. Either force can be the action or reaction Write down just 1st bullet point/ Note this is not cause and effect, this is at the exact same time. Can also say for every force there is an equal and opposite force.

Forces come in pairs Object A exerts a force on B Object B exerts a force on A There are two objects in an interaction There has to be two objects involved for their to an action/reaction pairs. Ask, in this picture, what is the action/reaction pair. Or what are the force pairs? Say the objects and in which direction they are going.

Action and Reaction Forces Don’t Cancel Newton’s Third Law 3 Action and Reaction Forces Don’t Cancel You constantly use action-reaction force pairs as you move about. When you jump, you push down on the ground. The ground then pushes up on you. It is this upward force that pushes you into the air.

Action and Reaction Pair

Action and Reaction Pair?

Action and Reaction on Different Masses Newton’s 2nd law requires us not only to look at force but also mass The force exerted against the recoiling rifle is just as great as the force that drives the bullet. Why then, does the bullet accelerate more than the rifle? Write 1st bullet point. 100N = 100N Fbullet = Fgun mA = Ma 2 x 50 = 25 x 4

Examples While driving down the road, a firefly strikes the windshield of a bus and makes a quite obvious mess in front of the face of the driver. This is a clear case of Newton's third law of motion. The firefly hit the bus and the bus hits the firefly. Which of the two forces is greater: the force on the firefly or the force on the bus? The force is the same for both. Bug has smaller mass and unable to withstand acceleration Why does the bug get smooshed? Because it has less mass it has a greater change in acceleration. The bus has a large mass so it’s acceleration is basically unaffected.

Examples Many people are familiar with the fact that a rifle recoils when fired. This recoil is the result of action-reaction force pairs. A gunpowder explosion creates hot gases that expand outward allowing the rifle to push forward on the bullet. Consistent with Newton's third law of motion, the bullet pushes backwards upon the rifle. The acceleration of the recoiling rifle is ... a. greater than the acceleration of the bullet. b. smaller than the acceleration of the bullet. c. the same size as the acceleration of the bullet

The force on the gun equals the force of the bullet The force on the gun equals the force of the bullet. Acceleration depends on both force and mass. The bullet has a greater acceleration due to the fact that it has a smaller mass This helps explain the previous slide.

Which exerts more force, the Earth pulling on moon or moon pulling on the Earth? The pull are the same but in opposite direction