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# Newton's Laws of Motion

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### Inertia: Newton's First Law of Motion

Newton's First Law of Motion, also known as the Law of Inertia, states that an object's velocity will not change unless it is acted on by an outside force. This means that an object at rest will stay at rest until a force causes it to move. Likewise, an object in motion will stay in motion until a force acts on it and causes its velocity to change.

You can do a simple inertia experiment to demonstrate this. You'll need a hard-boiled egg and a raw egg for this activity. First, spin the hard-boiled egg on its side. When it's going fast, gently put your fingers down on it to stop it and then move your hand off immediately when it stops. Next, spin the raw egg. Stop it in the same way you did with the hard-boiled egg. After you let go, what happens? The egg should start to turn again. This is because the motion of the liquid within the egg is still going; the force you exerted was not enough to stop both the inertia of the shell and the inertia of the liquid inside of it. If you held the egg longer, enough force would have been exerted to stop the egg completely.

The results of the experiment fit in with the Law of Inertia: an object will continue to remain in one state until sufficient outside force acts upon it, either to put it in motion or to bring it to rest.

The greater mass or velocity an object has, the greater its inertia. You can test this the next time you're at the grocery store. It takes a strong push to get a loaded shopping cart moving, but once it gathers speed it keeps going even if you let go of the handle. When you stop a moving cart full of groceries, it takes much more force to stop it than an empty cart (one with less mass). Likewise, it takes more force to brake a fast-moving bike than a slow one (one with less velocity), even though the mass of each is equal.

For further thought: Why do wheels and tops eventually stop spinning, without appearing to be touched by a force?

### Newton's Second Law of Motion

Newton's Second Law of Motion states that "when an object is acted on by an outside force, the strength of the force equals the mass of the object times the resulting acceleration". In other words, the formula to use in calculating force is force=mass x acceleration. Opposing forces such as friction can be added or subtracted from the total to find the amount of force that was really used in a situation.

You can demonstrate this principle by dropping a rock or marble and a wadded-up piece of paper at the same time. They fall at an equal rate—their acceleration is constant due to the force of gravity acting on them. However, the rock has a much greater force of impact when it hits the ground, because of its greater mass. If you drop the two objects into a dish of sand or flour, you can see how different the force of impact for each object was, based on the crater made in the sand by each one.

Another way to show this is two push off two toy cars or roller skates of equal mass at the same time, giving one of them a harder push than the other. The mass is equal in both, but the acceleration is greater in the one that you exerted greater force on.

• Check out our dynamic carts to gain a better understanding of Newton's Second Law.

### Newton's Third Law of Motion

Stated simply, Newton's Third Law of Motion says that "for every action, there is an equal and opposite reaction." Use a pair of roller skates and a ball to show how this works. What happens when you're standing still in skates and then throw a ball hard? The force of throwing the ball pushes your skates (and you) in the other direction.

You can also demonstrate this using Newton's Cradle. This apparatus consists of steel balls suspended on a frame. When the ball on one end is pulled back and then let go, it swings into the other balls. The ball on the opposite end then swings up with an equal force to the first ball, as shown in the illustration on the right. The force of the first ball causes and equal and opposite reaction in the ball at the other end.

For further thought: Thrust is an important result of Newton's Third Law. How does this work in a rocket? Read more about rockets and rocketry.

• Check out our Newton's Cradle for a classic demonstration of Newton's Third Law of Motion.
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