SCM 542: Forces, Work, Energy and Momentum
Lesson 3: Newton's Third Law of Motion
If you think about applying a force to an object, such as pushing a block forward with your hand, you realize that the block "pushes back" because you feel the block against you hand. The harder you push the block, the more you feel it, so it seems reasonable that the block's force on your hand is equal in magnitude but opposite in direction to the force your hand exerts on the block. So of the forces are equal and opposite, isn't the next force zero...and if so why does the block move? Seems like Newton's First law would say if it starts at rest, it will remain at rest.
OH, NO, A TRICK QUESTION!!!!
The answer lies in analyzing the sources and targets of the individual forces. To do this we use free-body diagrams in which we look at each body separately, defining the forces on each. So if you consider the only the block by itself, there is only one force acting upon it: the force your hand is applying, so it will accelerate according to Newton's Second Law.
In the case of your hand, your arm muscles are applying a force to it which is greater than the force exerted on it by the block, giving a net force toward the block.
Another example of this "pairing" of forces occurs when you sit down on a chair. Your body exerts a force on the chair, but since you don't fall down, the chair must be exerting a (Normal) force on you to keep up. But why don't you move? If you analyze your situation with a free body diagram, you find there are two forces acting on you: gravity is pulling you down, the chair's force is pushing you up. Since you aren't drifting upwards or falling downwards, these two forces must be balanced for a net zero force.
So what about the chair? It's not moving, but your body is pushing down on it with a force equal to your weight? What's pushing back? The floor exerts an upward normal force on the chair that exactly balances your downward force on the chair.
If you look at what we've been analyzing, you see that forces always occur in pairs of equal magnitude and opposite direction. But the paired forces do not act on the same object. Rather, the force exerted by object 1 on object 2 is equal to but opposite of the force exerted by object 2 on object 1.
This last statement is Newton's Third Law of Motion, the topic of this lesson.
The elements of this lesson are contained in the following links. Proceed to each element below in the order listed. You may want to bookmark this page so you can find these elements quickly in the future.
When you're finished the elements of this lesson, go the the "Assignments" section of the course for the assignments for Lesson 3.
© 2002 Barry L Lutz
