Action & Reaction Forces

Who was Isaac Newton? Sir Isaac Newton (1643-1727), an English scientist and mathematician famous for his discovery of the law of gravity, also discovered the three laws of motion. He published them in his book Mathematic Principles of Natural Philosophy in 1687. Today these laws are known as Newton’s Laws of Motion and describe the motion of all objects we experience in our everyday lives. 2

Newton’s Third Law States: “For every action there is an equal and opposite reaction.”

In Other Words…. When one object exerts a force on a second object, the second object exerts an equal but opposite force on the first.

What does this mean? This means that when you push on a wall, the wall pushes back on you with a force equal in strength to the force you exerted. The forces exerted by two objects on each other are often called action-reaction force pairs. 5

What is Force? Force is any push or pull, and they can be balanced or unbalanced. Balanced results in no change in speed or direction, while unbalanced forces result in a change in speed and/or direction.

Action Reaction Forces You constantly use action-reaction force pairs as you move about. For instance, 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.

Some Examples from Real Life When you walk forward, you push backward on the ground. Your shoe pushes Earth backward, and Earth pushes your shoe forward. 8

More Examples from Real Life When a bird flies, its wings push downward and backward direction. With Newton’s third law, the air pushes back on the bird in the opposite directions—upward and forward. This force keeps a bird in the air and propels it forward.

If forces are opposite and equal, then why do we experience movement? Earth has much more mass compared to you so it does not move when you exert a force on it but you move when it exerts a force on you. If you exert a force on something that has less mass than you do, like a soccer ball, it will move and you will not. 10

Forces Act in Pairs Both objects that a force is exerted on will accelerate. The amount of acceleration depends on the mass of the object. Small mass  more acceleration Large mass  less acceleration

Action and Reaction Forces act on Different Objects How can a horse pull a cart if the cart is pulling back on the horse with an equal but opposite force? Aren’t these “balanced forces” resulting in no acceleration? NO!! Forces are equal and opposite but act on different objects.

Action and Reaction Forces act on Different Objects The hammer exerts a force on the nail to the right. The nail exerts an equal but opposite force on the hammer to the left.

How Does a Rocket Use Newton’s Third Law? A rocket contains fuel that, when ignited, produces a hot gas. The gas molecules exert a force on the inside walls of the rocket, and the walls exert a force that pushes back on the gas. The gas is pushed out of the bottom of the rocket. This downward push is the action force and the reaction force is the upward push on the rocket engine by the gas molecules.

How Does a Rocket Use Newton’s Third Law? The rocket exerts a downward force on the exhaust gases. (FR) The gases exert an equal but opposite upward force on the rocket. (FG) FG FR

Try the next set of questions! Comprehension Check Describe Newton’s Third Law in your own words. Do action and reaction forces act on the same object or on different objects? Explain your answer. Kent Swim has rowed his boat within three feet of the dock. Kent decides to jump onto the dock and turn around and dock his boat. Explain to Kent why this strategy is not a good strategy. Try the next set of questions!

A truck driving down the road runs into a bug flying the other way. a) Which object has the greater force exerted on it? Both objects have the same small force exerted on them during the collision according to Newton’s third law. b) Which object will change its motion the most? The bug squishes easily! The bug obviously changes its motion significantly more than the truck because of its much lower inertia mass. Click to see answer. Click to see answer.

A boxer hits a piece of paper. Which object has the greater force exerted on it? Both objects have the same small force exerted on them during the collision according to Newton’s third law. b) Which object will change its motion the most? The paper obviously changes its motion significantly more than the glove because of its much lower mass. Click to see answer. Click to see answer.

A cannonball is fired from a cannon. Which object has the greater force exerted on it? Both objects have the same force exerted on them during the collision according to Newton’s third law. b) Which object will change its motion the most? The cannonball obviously changes its motion significantly more than the actual cannon because of its much lower mass. Click to see answer. Click to see answer.