Newton’s Third Law of Motion

If you throw a ball against a concrete wall, it bounces back towards your hand and you catch it again.

This is one possible way to demonstrate Newton’s Third Law of Motion, which can be summed up in these words “whenever a force is exerted on an object, the object exerts an equal amount of force in the opposite direction.” Or, “for every action, there is an equal and opposite reaction.”

If you push a door to open it, the door pushes back at you with an equal amount of force. If you jump into the air, gravity from the earth pulls you down; but at the same time, gravity from your body pulls the earth up.

When you push your computer’s keys to type, the keys push back with an equal amount of force.

So if everything pushes back with the same amount of force, you may ask yourself: how does anything ever move?

The answer: INERTIA.
Inertia is a body’s tendency to keep moving in the same direction or remain stationary unless acted upon by an external force.

When you throw your ball against a concrete wall, it exerts a force on the wall, and the wall exerts the same amount of force on the ball. So why does the ball bounce backward instead of pushing the concrete wall forward, or coming to a dead stop?

The wall, being of a much greater mass, has a much higher INERTIA, which is impossible to break with the force of a tiny rubber ball projected by human strength.

On the other hand, what happens if you throw the ball against a Pepsi tin? It won’t bounce back. Instead, it will propel the tin forward. This is because the tin, being of little mass, has very little inertia, and the mass of the ball, combined with its rate of travel, has enough energy to break the inertia of the Pepsi tin.

When you push a door open, why don’t you move backward instead of the door going forward, or why doesn’t the two of you get stuck in the same spot? The hinges which suspend the door require very little friction for movement. But the friction of your feet on the floor is much greater, and requires a lot more energy to create movement.

What would happen if you tried to push a door open wearing roller skates?

If you throw a ball upwards, the gravity from the earth pulls it down. At the same time gravity from the ball pulls the earth up. Why then does the ball move downward and the earth remains in the same position? Because the ball is infinitely smaller than the earth, and the amount of gravity it possesses is too small to move the earth. Therefore the ball moves downward.

If you try to drive a piece of stick into the ground using a hammer, the hammer bounces upward upon impact. This is because of Newton’s Third Law of Motion.

If you plunge off a large ship, the force you and the ship exert on each other won’t be able to move the ship backwards, but will propel you forward. What happens if you plunge off a small wooden boat?

Scientists use Newton’s third law of motion when searching the galaxy for planets. When planets revolve around a star, they create a gravitational pull on the star which changes the appearance of the stars.

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