Newton's Third Law

Newton's third law of motion is well known but often misunderstood. Let's all say it together - for every action there is an equal and opposite reaction. What does this mean? Consider dropping a bowling ball on your foot. You can feel the force of the ball striking your toe! But the ball has a very rapid change in acceleration - it comes to a stop. A force must be applied by your toe to the ball in order for that to happen and it is the same force that the ball applies to your foot - just in the opposite direction. Thus we state Newton's third law of motion:

When one object exerts a force on a second object, the second object exerts a force on the first object equal in magnitude to the first force but applied in the opposite direction.

When a rocket takes off, its forward motion is not due to the gases pushing against the ground or the atmosphere. Rockets move forward because a mass of hot gas with very high velocity is being ejected from the rocket with a force in the opposite direction of the rocket's flight. This is an example of the third law. Another example comes from pulling on two spring scales connected together. Trying to pull on one scale harder than on the other is impossible - both will register the same amount of force although they are being pulled in opposite directions.

Often it is helpful to see what someone else has to say on the topic. For more on Newton's Third Law:


For Practice Problems, Try:

From the University of Oregon, Go to this page and select and appropriate problem (don't worry - you won't be able to do all of them yet):

Giancoli Multiple Choice Practice Questions (Select "Practice Problems) and try some.