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What is Motion?

Motion is the area of physics that studies how things move. It was developed about 300 years ago by Isaac Newton. He discovered that all objects move according to three basic laws. Newton’s Laws can explain almost all of physics. When we look at the other branches of classical physics, they all can be explained using these laws of motion.

The first law is the Law of Inertia. It says that all things will move in straight lines with the same speed or if they’re not moving, they stay still. Think of a hockey puck on ice. If you don’t touch it, it will sit there forever. When you push it, it will slide along the ice in a straight line – until it hits something to make it stop or turn.

The second law is the Law of Forces. If something is not moving in a straight line, a force must be acting on it to change its direction. A force is a push or a pull. In the hockey puck example, if you want the puck to start moving, you have to push it. This is a force. If you want the puck to turn, you have to hit it sideways; it won’t curve by itself. Another force is friction, which slows things down by rubbing. Try pushing the puck on cement. It doesn’t move very far because cement is rough. Friction between the puck and the cement slow the puck down. If something has a lot of inertia, it takes a large amount of force to move it. If it has not much inertia, it takes only a small force to move it.

The third law is the Law of Rockets: for every action, there is an equal and opposite reaction. This means if you push on something, it pushes back on you the same amount. To demonstrate this, stand with your feet together holding a heavy rock. Now throw the rock away from you as hard as you can and try not to move your feet. It’s not so easy! When you push the rock away from you, at the same time the rock pushes you away from it with the same force.

These three laws help physicists to understand how most everything in the world moves. They were first used to find out how the planets move around the sun. Today they can help us to understand how to make rockets work that take us to those planets (among many other things).

Wonders of Physics
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This page updated on August 29, 2008