Newton’s 1st Law of Motion and Inertia


Newton’s First Law:

An object at rest or in constant motion will remain at rest or in constant motion unless acted on by an unbalanced force.

This law is often called The law of Inertia

In short, Inertia means that an object will keep doing what it is already doing. If an object is at rest, it will remain at rest. If it is moving, it will keep moving. This will remain true as long as no external forces act on the object. Inertia is the property all objects have (due to their mass) that causes them to resist any attempt to change their state of uniform motion.


Some other observations caused by the inertia of an object:

As an object becomes heavier:

  • It will be harder to stop moving
  • It will be harder to change its direction
  • It will be harder to start it moving if it is stationary

Observing the effects of inertia:

  • As an aircraft accelerates for takeoff you feel as though you are being pushed back in your seat. The inertia of your body (which was initially stationary) and the forward motion of the plane make it ‘feel’ like you are being pushed back.
  • When standing on a train that suddenly decelerates as it approaches a station you feel like your body is being thrown forward. The inertia of your body (which was initially moving) keeps you moving forward as the train stops.
  • When you are in a car that goes around a corner you feel like you are being pushed to the side of the car. The inertia of your body (moving straight at a constant speed) makes it feel like you are being pushed as the car changes its direction.
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