What does Newton's First Law state?

• A. An object at rest stays at rest, and an object in motion stays in motion unless acted upon by an external force
• B. F = ma
• C. For every action there is an equal and opposite reaction
• D. Energy is conserved in all processes

Answer: (A)

Inertia is the key idea: an object tends to keep doing what it’s already doing—stay at rest or keep moving at a steady speed in a straight line—unless a net external force acts on it to change that state. If there’s no net external force, velocity remains constant; a stationary object stays at rest, and a moving object continues moving with the same speed and direction. Forces like gravity, friction, or a push are external forces that can alter motion by creating a net force. The other statements describe different principles: F = ma is Newton’s Second Law, which links net force to acceleration; the equal-and-opposite reaction describes Newton’s Third Law; and energy conservation is a separate principle about how energy varies but is conserved in processes. Real-world examples: a book resting on a table stays there until you push it, and a puck slows on a rough surface because friction provides a net external force.