Third law of newton

A good example of the third law of Newton is when you punch a wall (made with concrete blocks) real hard and you hurt your hand.

There are two things going on here although many people don't realize it.

#1:You punch the wall with a force

#2: The wall punches back at you with the same force

For many people #1 is clear yet #2 is not so clear.

Think of it this way. If #2 did not exist, then you would never hurt your hand.

Moreover, you actually did some damage to the wall as well although it could be insignificant. I think it depends on how strong you are.

The picture below illustrates what I just said.  

Newton's third law

You hit the wall with a force. This force is shown with the blue arrow.

The wall hits back with the same force. This force is shown with the red arrow. 

This is basically Newton's third law. It states that whenever an object exerts a force on a second object, the second object exerts a force that is equal and opposite on the first object.  

The force you exert on the wall could be called the action force and the force the wall exerts on your hand is called the reaction force.

The third law of newton is also known as the law of action and reaction

It does not matter which force is called the action force or the reaction force.

The important thing to keep in mind is that one does not exist without the other. Instead, they are coparts of a single interaction.

Other examples of the third law of Newton

a. Hitting a nail with a hammer: The hammer exerts a force on the nail and the nail also exerts the same force on the hammer.

b. Swimming: You push the water backward. The water pushes you forward.

Some action and reaction forces are not easy to identify as we shall see next.

c. What are the action and reaction forces when a car accelerates?

Most people will say that the engine is the action force. If that is true, then tell me why the car refuses to move when there is no friction or not enough friction such as when driving the car on an icy road?

In such situation, you can push the gas pedal as much as you like, the car will resist motion. 

In reality, the rotating tires will push back on the road. This is the action force. The road now pushes forward on the tires. This is the reaction force.

d. A ball is dropped from the third floor.

Action force: earth's gravity pulls down the ball.

Reaction force: Ball pulls the earth.

If you are having problems understanding this one, think of it as said below:

If X is pulling Y, Then Y is also pulling X.

Therefore, if the earth is pulling the ball, then the ball is also pulling the earth.

But I don't see the earth moving!

A different lesson will show you why you don't see the earth moving!

Know that It does move, but only a tiny distance.  

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