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Newton's First Law of Motion - aka Law of Inertia

Written by tutor R Bruce N.

So what is inertia? Inertia is the tendency all material has to take the low energy route and not change its state. If something is moving it will do so in a straight line and never stop unless something gets in the way. That's an outside force - could be a push, pull, obstacle, friction, air resistance, gravity, etc.

Newton's Law of Inertia

Think of a locomotive.
What would it take to get it moving?
A BIG force, right?

What would it take to stop it?

The locomotive will only turn from a straight line if something pushes, or pulls it to the side. Tracks force trains into curves. If it jumped the tracks on a high-speed curve – what would happen?

It would go straight in a line, wouldn't it?

And if the train is not moving, then it will stay put until some force comes along and makes it move.

Think about a ball on the floor. Could it move if nobody kicks it (no outside force) to make it move?

If it were on a perfect floor and there were no resistance of any kind – no friction, etc, then once moving, the ball would never stop.

Think of a puck on ice, or on a carpet. Why does it slide easily and far on ice, but not slide at all on a carpet?

Friction is barely present on ice, but very present on a carpet.

Think of a boat moving along – it needs an engine to keep moving. Cut the engine and it stops. Why? Water is a kind of resistance, or drag.

Keep the engine off and the boat goes nowhere. Unless a current of water or wind moves the boat. Right?

Remove the rudder from the boat – it goes in a straight line.

In short:
No external force = no acceleration = no change in velocity = no change in direction.
External force = acceleration = change in velocity (higher or lower), or a change in direction.

This leads us to Newton's 2nd Law of Motion.

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