Superphysics Superphysics

The Laws of Motion

by Isaac Newton Icon
2 minutes  • 367 words
Table of contents

Law 1

Every body perseveres in its state of rest, or uniform motion unless it is compelled to change that state by external forces impressed on it.

  • PROJECTILES persevere in their motions so far as they are not retarded by air resistance, or impelled downwards by gravity.
  • A spinning top does not stop unless it is resisted by air.
  • Planets and comets retain their motion as there is less resistance in outer space

Law 2

The change of motion is proportional to the external motive force impressed. It follows the direction of that force.

If a 1x force generates 1x motion, a 2x force will generate 2x the motion, a 3x force will generate 3x the motion, whether that force be impressed at once, or gradually and successively.

If the body is already moving, then this motion is added to it if it is in the same direction, and deducted if in the opposite direction. If obliquely joined, then a new motion will be produced, compounded from the determination of both.

Law 3

For every action there is always opposed an equal reaction. The mutual actions of two bodies on each other are always equal, and directed to contrary parts.

Whatever draws or presses another is as much drawn or pressed by that other.

If you press a stone with your finger, the finger is also pressed by the stone.

If a horse draws a stone tied to a rope, the horse will be equally drawn back towards the stone.

  • This is because the rope will try to relax or unbend itself by drawing the horse as much towards the stone, as it does the stone towards the horse.
  • It will try to obstruct the progress of the one as much as it advances that of the other.

If a body impinges on another, and by its force changes the motion of that other, then that body will change its own motion. The changes made by these actions are equal, not in the velocities but in the motions of bodies.

The motions are equally changed, the changes of the velocities made towards contrary parts are reciprocally prpportional to the bodies.

This law takes place also in attractions.

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