Newton's first law — 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 newton's first law

Problem 1

Newton's first law says an object keeps its motion unless acted on by
  1. gravity only
  2. an unbalanced force
  3. friction only
  4. its own mass
Answer and explanation

an unbalanced force

Without a net force, velocity does not change — this is inertia.

Problem 2

Explain, using Newton's first law, why a passenger's head snaps backward when a car is rear-ended and lurches forward.
Answer and explanation

Before the impact the passenger's head is at rest. When the car and seat are suddenly pushed forward, the head tends to stay where it was because of inertia (Newton's first law), so it lags behind the moving body and appears to snap backward. A headrest supplies the force needed to accelerate the head along with the body, preventing whiplash.

Problem 3

State Newton's first law of motion and give one everyday example of it.
Answer and explanation

Newton's first law says an object at rest stays at rest, and an object in motion stays in motion at constant velocity, unless a nonzero net force acts on it. Example: a coffee cup slides forward on the dashboard when the car brakes, because the cup tends to keep moving even though the car slows down.

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