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PhysicsClassical Physics

What Is Newton's First Law? Choose the Correct Statement

Newton's first law says that zero net external force preserves an object's state of rest or constant-velocity motion; among the four choices, statement B is correct.

Question

What does Newton's first law state?

A. A body at rest tends to remain at rest and a body in motion tends to remain in motion at a constant acceleration unless acted on by a net external force.

B. A body at rest tends to remain at rest and a body in motion tends to remain in motion at a constant velocity unless acted on by a net external force.

C. The rate of change of momentum of a body is directly proportional to the external force applied to the body.

D. The rate of change of momentum of a body is inversely proportional to the external force applied to the body.

Answer

Correct answer: B. Newton's first law says that an object remains at rest, or continues moving in a straight line at constant velocity, unless a net external force changes that state of motion. In compact form,

F⃗net=0⟹v⃗=constant.\vec F_{\text{net}}=0 \quad\Longrightarrow\quad \vec v=\text{constant}.

Option A is incorrect because zero net force gives zero acceleration, not constant nonzero acceleration. Option C describes the momentum form of Newton's second law, F⃗net=dp⃗/dt\vec F_{\text{net}}=d\vec p/dt. Option D is not a correct statement of Newton's laws.

The phrase net external force matters. Several forces may act on an object and cancel. In that case the object can remain at rest or keep moving at constant velocity.

Evidence boundary

This answer evaluates the four options in OpenStax Physics 4.2, Check Your Understanding 5. It assumes an inertial reference frame. Constant velocity means both constant speed and constant direction; the answer does not claim that no individual forces act, only that their vector sum is zero.

Sources

These references support the concepts and methods used in the explanation above.