This is Newton’s 3rd Law in his own words

“For every action, there is an equal and opposite re-action.”
This is one of the simpler laws that Sir Isaac Newton made. It means that for any force, there is an equal force of the same type pushing back on the object. It is called an action-reaction pair. This confuses a lot of people at first. For example, if a huge truck smashes into a stationary car, one would think that the truck hit the car with more force than the car hit the truck with. This is because the car wasn’t moving. However, at the instant that they collided, they both hit each other with the same amount of force, according to Newton’s 3rd Law. A confused student would argue that the truck hit the car with more force, because the car would go sliding away with the impact, and the truck wouldn’t move that much. However, with these situations, we must not pay any attention to the after-effects. The actual reason why the stationary car was effected more by the collision was because of inertia. Inertia is the resistance of change. A larger mass object has more inertia. And because the truck has more inertia, it will want to keep moving at its original speed. Even though the car pushed back on the truck the same amount, the truck will push the car farther forward then the car pushed the truck back.

external image newton_third_law_2.gif
As you can see in this example that no matter if you are pulling against a wall or a very strong cave man, the force on each side of the spring is the same.

You can easily find an action-reaction pair by remembering these steps:
  • The pair always uses the same type of force (ex: normal force, tension force, etc.)
  • Call the first object ‘x’. The force will be on x, by the other object, which we’ll call ‘y’.
  • The other force will be on y, and by x.

Try to find the action-reaction force in this situation.
Q. A box is sitting on a shelf.
A. The Box applies a normal force on the shelf. The shelf applies a normal force on the box.

How about this one?
Q. A rocket lifts off earth and enters the atmosphere.
A. The fuel applies a force on the space ship. The space shuttle puts a normal force on the fuel.