Inertia — Science definition

Defined in 2 courses:

Where it is taught

In use

Newton's first law is the law of inertia: an object at rest stays at rest, and an object already moving keeps moving at the same speed and in the same straight-line direction, unless an unbalanced force acts on it.

Newton's Three Laws of Motion

Newton's first law, the law of inertia, states that an object keeps doing whatever it is already doing — sitting still or gliding at constant velocity in a straight line — unless a nonzero net force acts on it.

Newton's Three Laws of Motion

Questions using inertia

Problem 1

Zero net force allows rest OR constant velocityZero net force allows rest OR constant velocityNet force = 0: cart at rest0 s · 1 s · 2 sSame position each secondNet force = 0: cart at constant rightward velocity0 s1 s2 s
Equal-time snapshots can show no position change or equal changes in position. Both are consistent with zero net force in an inertial reference frame. The moving cart does not need a forward net force to keep its constant velocity. On a real surface, rolling resistance can slow it unless balanced. The rest snapshots overlap intentionally; no extra force called inertia should be drawn.
For this unit’s illustrated case, why is inertia not an extra force on a force diagram?
Answer and explanation

Inertia describes how mass resists acceleration. It is not an interaction from another object and should not be added as a force arrow.

A released toy cart slows due to interactions such as rolling resistance, not because motion uses up a stored force. A passenger’s motion changes when a seat or restraint exerts force; connect the law to interactions rather than calling inertia a pushing force.

Inertia describes how mass resists acceleration. It is not an interaction from another object and should not be added as a force arrow.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.

Problem 2

Which property of an object is the direct measure of its inertia?
  1. Its mass
  2. Its weight
  3. Its acceleration
  4. Its speed
Answer and explanation

Its mass

Mass measures inertia — the more mass an object has, the more it resists a change in its motion.

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