Going Further 7.1: Newton’s Third Law
Our daily experiences might lead us to think that forces are always applied by one object on another; for example, a horse pulls a carriage, a person pushes a grocery cart, or a hammer hits a nail. It took Sir Isaac Newton to realize that things are not so simple, and not so one-sided. True, if a hammer strikes a nail, the hammer exerts a force on the nail (thereby driving it into a board). Yet, the nail must also exert a force on the hammer since the hammer’s state of motion is changed, and according to the first law, this requires a net (outside) force. This is the essence of Newton’s third law: For every action there is an equal and opposite reaction. However, it is important to understand that the action and the reaction are acting on different objects.
Try this: Press the side of your hand against the edge of a table. Notice how your hand becomes distorted. Clearly, a force is being exerted on it. You can see the edge of the table pressing into your hand and feel the table exerting a force on your hand. Now press harder. The harder you press, the harder the table pushes back on your hand. Remember this important point: You can only feel the forces being exerted on you, not the forces you exert on something else. So, it is the force the table is exerting on you that you see and feel in your hand.
Action–reaction pairs like the force of the hand on the table and the force of the table on hand are all around us. If you are reading this while sitting in a chair, you can feel the force the chair is exerting upward on you while you exert a force downward on it. And if you are reading on a computer monitor that sits on the table in front of you, then there is a balanced action–reaction pair between the monitor and the table, as well as between the table and the floor, and so on for all the objects you see around you. Each of these is an action–reaction pair. Such action–reaction pairs are the domain of Newton’s third law.
Not every equal and opposite pair of forces is a third law pair, however. In the third law, two objects are involved with the two forces. For example, a hand and a table, a table and a hand. We can have two forces of equal strength and opposite direction due to Newton’s second law, but these forces act on a single object. For example, if you hold a ball still in your hand, there is an upward force due to your hand on the ball that is exactly equal to the downward force of gravity from Earth on the ball. That is why the ball does not move. These two forces are acting on a single object, the ball, whose net force and acceleration are zero by Newton’s second law.
Newton's third law is also related to the concept of conservation of momentum. Momentum is defined as an object's mass times its velocity; because velocity is a vector, momentum is also a vector. Momentum is also related to force: the change in momentum over time is equal to the net force.
So, for two objects that are a third law pair, the change in momentum of one object will be equal in strength and opposite in direction to the change in momentum of the other. This principle is important in collisions. For example, if two billiard balls collide, they will bounce off of each other and travel in opposite directions. It is also important for propulsion. For example, a squid is a sea creature that takes in water; when it wants to move, it squirts water out of its body in one direction, and it moves in the opposite direction. As another example, if you are standing on a frozen pond having a snowball fight in the winter, if you throw a snowball, the snowball will move forward, but you will also slide backward a little. The velocity of the snowball will be greater than your velocity because you have a greater mass (unless you make a really big snowball).