Forces do not occur alone. There is always an interaction between forces.
Newton’s 3 rd Law If you try to push a heavy object it will not move. It is pushing back on you with as much force as you are pushing on it. Even if you were moving the heavy object, it would still be pushing back on you with as much force as you are pushing on it. This is Newton’s 3rd Law.
Newton’s 3 rd Law Newton’s 3rd Law states that whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object.
Newton’s 3 rd Law Or, in simpler terms, for every action, there is an equal and opposite reaction.
Check for Understanding When you jump up, what pulls you to the ground? The Earth/gravity
Check for Understanding Do you pull back up on the Earth? Yes. The force you pull up on the Earth is the same as the force the Earth pulls on you. If you weigh 150 lbs then the Earth pulls you down with 150 lbs of force. You pull up on the Earth with 150 lbs of force! Forces ALWAYS come in pairs that are equal and opposite.
Check for Understanding Why don’t you notice the movement of the Earth when you pull up on it as you are jumping? The Earth has sooooooo much mass that the force you pull on the Earth with makes it accelerate only a teensy-tinsey bit.
F = ma Force on you = Force on Earth m a = m a The forces are the same but because the masses are different, the accelerations are different.
Check for Understanding 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?
Check for Understanding Trick Question! Each force is the same size. Do the bug and the windshield have the same acceleration? No, the bug has much less mass so it has a much bigger acceleration (or deceleration in this case). That’s why the bug splats.
Check for Understanding 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 which 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.
Check for Understanding The force on the rifle equals the force on the bullet. Yet, acceleration depends on both force and mass. The bullet has a greater acceleration due to the fact that it has a smaller mass. Remember: acceleration and mass are inversely proportional.
Action-Reaction Pairs Forces always come in pairs - known as "action-reaction force pairs."
Action-Reaction Pairs The baseball forces the bat to the left; the bat forces the ball to the right. Together, these two forces exerted upon two different objects form the action-reaction force pair. Note that in the description of the two forces, the nouns in the sentence describing the forces simply switch places.
Action-Reaction Pairs Identify the reaction: Baseball pushes glove leftwards. The glove pushes the baseball rightward.