What does Newton's first law of motion state?

What does Newton's first law of motion state?

• A. An object accelerates unless acted on by a net external force
• B. An object at rest stays at rest and an object in motion stays in motion unless acted on by a net external force
• C. For every action there is an equal and opposite reaction
• D. Energy cannot be created or destroyed

Answer: (B)

Inertia—the tendency of objects to keep doing what they’re doing—underlies Newton’s first law. It says that an object at rest stays at rest and an object in motion stays in motion unless acted on by a net external force. In other words, if the total force on an object is zero, its velocity won’t change: a stationary object remains stationary, and a moving object keeps moving in a straight line at the same speed. Think of a hockey puck sliding on ice: if friction is minimal, it keeps gliding at roughly the same speed and direction until someone or something applies a force to slow it down or change its path. Only when a net external force acts does the motion change. That’s why the other statements aren’t describing this law. An idea that objects must always accelerate unless a force acts would contradict the fact that no force means no acceleration. The statement about action and reaction describes a different principle—Newton’s third law. And energy conservation is about energy accounting, not how motion changes.

Inertia—the tendency of objects to keep doing what they’re doing—underlies Newton’s first law. It says that an object at rest stays at rest and an object in motion stays in motion unless acted on by a net external force. In other words, if the total force on an object is zero, its velocity won’t change: a stationary object remains stationary, and a moving object keeps moving in a straight line at the same speed.

Think of a hockey puck sliding on ice: if friction is minimal, it keeps gliding at roughly the same speed and direction until someone or something applies a force to slow it down or change its path. Only when a net external force acts does the motion change.

That’s why the other statements aren’t describing this law. An idea that objects must always accelerate unless a force acts would contradict the fact that no force means no acceleration. The statement about action and reaction describes a different principle—Newton’s third law. And energy conservation is about energy accounting, not how motion changes.