Lesson 15: Newton's 1st Law (Inertia)

Newton’s Laws of Motion, as written in his book the Principia, are actually very difficult to read.

He actually wrote the laws in a specific order for a specific reason.

The 1st Law (The Law of Inertia)

Every body continues in a state of rest or uniform velocity in a straight line,
unless an external force acts on it.

Every body…”
Means any physical object in the universe that has mass. It can be here on the Earth, on the moon, floating in space, wherever.

“…continues in a state of rest or uniform velocity in a straight line…”
If it is sitting still, it will stay that way. If it is moving, it will keep on moving forever at that velocity in a straight line.

“…unless an external force acts on it…”
Unless something else pushes it.

It is important that you understand the idea of “external forces”.

An object resisting a change in its “state of motion” (stopped or moving in a straight line) is something that Newton called inertia.

Example 1: A hockey puck will keep moving in the same direction at (almost) the same speed unless someone stops it or changes its direction. This would be done by applying a force.

Example 2: A book sitting on a desk won’t start to move all on its own. A force needs to be applied to it.

Newton’s 1st Law goes against what Aristotle said, and is basically what Galileo had said a few years earlier.