Newton's First Law is best described by which statement?

Newton's First Law is best described by which statement?

• A. An object will always change its motion unless acted on by a force.
• B. An object at rest stays at rest and a moving object continues in its motion unless acted upon by an external force.
• C. For every action, there is an equal and opposite reaction.
• D. The net force equals mass times acceleration.

Answer: (B)

Inertia is the tendency of objects to resist changes in their motion. If there are no net external forces acting on an object, its velocity stays constant: a resting object stays at rest, and a moving object continues moving in the same direction at the same speed. Only a net external force can change that motion. That idea is captured by the statement that says an object at rest stays at rest and a moving object continues in its motion unless acted upon by an external force. In everyday life we do feel forces like friction and gravity, which can alter motion, but the principle describes what happens when those forces are absent or balance out. The other statements describe different ideas: one misstates how motion changes, and the others correspond to Newton’s second and third laws.

Inertia is the tendency of objects to resist changes in their motion. If there are no net external forces acting on an object, its velocity stays constant: a resting object stays at rest, and a moving object continues moving in the same direction at the same speed. Only a net external force can change that motion. That idea is captured by the statement that says an object at rest stays at rest and a moving object continues in its motion unless acted upon by an external force. In everyday life we do feel forces like friction and gravity, which can alter motion, but the principle describes what happens when those forces are absent or balance out. The other statements describe different ideas: one misstates how motion changes, and the others correspond to Newton’s second and third laws.