Which statement best describes Newton's First Law of Motion?

Which statement best describes Newton's First Law of Motion?

• A. For every action there is an equal and opposite reaction.
• B. A body at rest tends to stay at rest and a body in motion tends to stay in motion unless acted upon by an outside force.
• C. Energy cannot be created or destroyed.
• D. Force equals mass times acceleration.

Answer: (B)

Inertia is the tendency of a body to resist changes to its motion. That means a body at rest stays at rest and a body in motion keeps moving in the same straight path and at the same speed unless something outside acts to change it. The amount of resistance to changing motion grows with mass, so a heavier object needs a bigger net external force to alter its motion. When the net external force is zero, the object's velocity remains constant, which is exactly what this statement describes. The other ideas described by the other statements cover different principles: action-reaction forces between two bodies (Newton’s third law), conservation of energy, and how force, mass, and acceleration relate (Newton’s second law). Only this inertia-based description captures the persistence of motion or rest in the absence of a net external influence.

Inertia is the tendency of a body to resist changes to its motion. That means a body at rest stays at rest and a body in motion keeps moving in the same straight path and at the same speed unless something outside acts to change it. The amount of resistance to changing motion grows with mass, so a heavier object needs a bigger net external force to alter its motion. When the net external force is zero, the object's velocity remains constant, which is exactly what this statement describes.

The other ideas described by the other statements cover different principles: action-reaction forces between two bodies (Newton’s third law), conservation of energy, and how force, mass, and acceleration relate (Newton’s second law). Only this inertia-based description captures the persistence of motion or rest in the absence of a net external influence.