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Newton’s First Law: How Inertia Works

In physics, Newton’s laws explain what happens with forces and motion, and his first law states, “An object continues in a state of rest, or in a state of motion at a constant velocity along a straight line, unless compelled to change that state by a net force.” Translation? If you don’t apply a net, or unbalanced, force to an object at rest or in motion, it will stay at rest or in that same motion along a straight line. Forever.

For example, when scoring a hockey goal, the hockey puck slides toward the open goal in a straight line because the ice it slides on is nearly frictionless. If you’re lucky, the puck won’t come into contact with the opposing goalie’s stick, which would cause it to change its motion.

Newton’s first law may not seem very intuitive because most things don’t seem to continue moving in straight lines forever. Left to themselves, most moving things come to a halt. The idea that the natural tendency of an object in motion is to come to a halt was Aristotle’s, and it was accepted wisdom for 2,000 years. It took the tremendous insight of Newton to see that the natural state of motion is actually to continue in a straight line at constant velocity. Only when acted on by a force does the motion change.

In everyday life, objects don’t coast around in straight lines at constant velocity. This is because most objects around you are subject to friction forces. So, for example, when you slide a coffee mug across your desk, it slows and comes to a stop (or spills over). That’s not to say Newton’s first law is invalid, just that friction provides a force to change the mug’s motion to stop it.

What Newton’s first law really says is that the only way to get something to change its motion is to use force. It also says that an object in motion tends to stay in motion, which introduces the idea of inertia.

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