Newton’s first law of motion (the law of inertia) states that an object at rest stays at rest, and an object in motion continues moving in a straight line at constant speed unless acted on by an unbalanced external force.

Core idea

  • The law describes how objects behave when the net force on them is zero: their motion does not change.
  • “No change in motion” means no change in speed or direction, so velocity stays constant (including the special case of zero velocity, i.e., rest).

Inertia

  • Inertia is an object’s tendency to resist any change in its state of motion.
  • Heavier objects (greater mass) generally have more inertia, so they are harder to start, stop, or turn than lighter ones.

Everyday examples

  • A book on a table stays where it is until you push it; your push is the unbalanced force that changes its motion.
  • A puck on nearly frictionless ice keeps sliding in a straight line at almost constant speed because there is very little force (like friction) to slow it.

Information gathered from public forums or data available on the internet and portrayed here.