2Forces and Interaction When you push on a wall, it pushes back on youMost people don’t realize that walls can push on you every bit as you can push on wallsIn its simplest sense, a force is a push or a pullA closer examination shows that a force is not a thing in itself, but a part of an interaction between one thing and anotherYou and the wallHammer and nail
3Newton’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 objectFor every action there exists an equal opposing reactionOne force is called the action force, the other is called the reaction forceThey are partners in a single interaction in which neither force can exist without the otherThe two forces are equal in magnitude and opposite in direction
4Forces Exist in Pairs When we walk… When we drive… When we swim… We push against the floor with our feetThe floor pushes back on our feetWhen we drive…The tires push against the roadThe road pushes back on the tiresWhen we swim…We push the water backwardThe water pushes us forward
5Questions? Does a stick of dynamite contain force? A motor scooter accelerates along the road. Strictly speaking, what is the force that moves the scooterEarth and the moon are connected by a gravitational force. Is Earth pulling on the moon or is the moon pulling on Earth?
6Identifying Action and Reaction Pairs Identifying reaction pairs is not always obviousWhat 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 EarthWe know the boulder falls to EarthCan we also say that the Earth, in turn, falls to the boulder?Forces are equal in magnitude and opposite in directionHowever, the masses are differentRemember, acceleration is inversely proportional to mass, so the acceleration of Earth is infinitesimally small
7QuestionsWe 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?
8Do Action and Reaction Forces Cancel? NoIf two forces act on the same body in equal and opposite directions, they cancelInternal to the systemTwo feet kick a football with equal force from opposite sides of the ballIf one body exerts a force on a second body, the force is independent from the force exerted back on it from that bodyExternal from the systemOne foot kicks a football and the football exerts the same force back on the foot
9QuestionSuppose 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.