Newton's Third Law
   

Here is the traditional version of Isaac Newton's Third Law of Motion: "For every action there is an equal and opposite reaction."

I don’t really like this very common phrasing, because it is vague. Also, it sounds contradictory: how can two things be both equal and opposite? I mention this phrasing only because you may have heard it before. By action and reaction, what is meant are forces.The forces have equal magnitudes but opposite directions. For these reasons, I offer this alternate phrasing:

Every force is an interaction between two objects. Both objects feel the same magnitude of force, but in opposite directions.

Let's call the two objects A and B. Then the force exerted on object B by object A is equal in magnitude to, but opposite in direction, the force exerted on object A by object B.           

It is convenient to call one of these the "action" and the other the "reaction", but which is which is arbitrary. Together, they are called an action-reaction pair.

A common source of confusion is: if these two forces are equal magnitude but in opposite directions, don't they always "cancel out", creating a net force of zero? The answer is No, because these two forces are acting on different objects.

EXAMPLES

  • A hammer hits a nail, driving the nail downwards into a piece of wood. The “reaction” is the force of the nail pushing upwards on the hammer, which stops the hammer.
  • You are standing on the floor. The floor is pushing up on you (holding you up against gravity). The “reaction” force is that you are pressing down on the floor.
  • Earth pulls down on you (with gravity). You pull up on the Earth.
  • When a gun or cannon is fired, hot exploding gases push the bullet or shell forward. The gun itself is pushed backwards. This is called "recoil". Here's a video showing where you don't want to stand when a cannon is fired.
  • Rockets work by exploding fuel in a combustion chamber, and pushing it backwards. The exiting mass pushes forward on the body of the rocket. This is thrust. Rockets sitting on the launch pad are typically 90% fuel. All of that is thrown downward at high speed, pushing the rocket upwards. Here are a few cool videos of rockets in action.
  • Jet engines, propellers, helicopter rotors and airplane wings also produce thrust by pushing gas away. Airplane propellers and jet engines push gases backwards, thrusting the airplane forward. Boat propellers push water backwards, and therefore push the boat forward. Helicopter rotors and airplane wings both push air downwards, pushing the craft upwards (a force called "lift"). Here's a video showing what it's like behind the propeller of a small radio-controlled electric airplane.

 

Activities & Practice
to do as you read

 

 

 

 

 

 

 

1. Draw a free-body diagram that shows the forces acting on a hammer and a nail, at the moment the hammer strikes the nail.

2. Draw a free-body diagram showing the forces acting on the Earth and Moon due to gravity.

3. Draw a free-body diagram showing the forces involved when you are standing on the ground.