Newton’s Third Law of Motion Chapter 6

Forces and Interaction When you push on a wall, it pushes back on you Most people don’t realize that walls can push on you every bit as you can push on walls In its simplest sense, a force is a push or a pull A closer examination shows that a force is not a thing in itself, but a part of an interaction between one thing and another You and the wall Hammer and nail

Newton’s Third Law of Motion Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object For every action there exists an equal opposing reaction One force is called the action force, the other is called the reaction force They are partners in a single interaction in which neither force can exist without the other The two forces are equal in magnitude and opposite in direction

Forces Exist in Pairs When we walk… When we drive… When we swim… We push against the floor with our feet The floor pushes back on our feet When we drive… The tires push against the road The road pushes back on the tires When we swim… We push the water backward The water pushes us forward

Questions? Does a stick of dynamite contain force? A motor scooter accelerates along the road. Strictly speaking, what is the force that moves the scooter Earth and the moon are connected by a gravitational force. Is Earth pulling on the moon or is the moon pulling on Earth?

Identifying Action and Reaction Pairs Identifying reaction pairs is not always obvious What are the action and reaction forces in the case of a falling boulder? Earth is pulled up by the boulder with just as much force as the boulder is pulled down by Earth We know the boulder falls to Earth Can we also say that the Earth, in turn, falls to the boulder? Forces are equal in magnitude and opposite in direction However, the masses are different Remember, acceleration is inversely proportional to mass, so the acceleration of Earth is infinitesimally small

Questions We know that Earth pulls on the moon. Does the moon also pull on Earth? If so, which pull is stronger? What happens when you step off a curb onto a street? Can you identify the action and reaction forces of an object falling in the vacuum of outer space?

Do Action and Reaction Forces Cancel? No If two forces act on the same body in equal and opposite directions, they cancel Internal to the system Two feet kick a football with equal force from opposite sides of the ball If one body exerts a force on a second body, the force is independent from the force exerted back on it from that body External from the system One foot kicks a football and the football exerts the same force back on the foot

Question Suppose a friend who hears about Newton’s third law says that you can’t move a football by kicking it because the reaction force by the kicked ball would be equal and opposite to your kicking force. He says that the net force would be zero, so no matter how hard you kick, the ball won’t move. What would you say to your friend? You can’t cancel a force on a football with a force on your foot. Period.