What is Newton's third law of motion?

What is Newton's third law of motion?

• A. Every action has an equal and opposite reaction.
• B. An object at rest stays at rest unless acted upon.
• C. F = m a.
• D. Mass increases with speed.

Answer: (A)

When two objects interact, the forces they exert on each other come in pairs that are equal in strength and opposite in direction. This is the idea behind Newton's third law: every action force has a corresponding reaction force that acts on the other object in the opposite direction. These action-reaction forces are simultaneous and act on different objects, not on the same one. Think about pushing against a wall—your hand pushes on the wall and the wall pushes back on your hand with the same force in the opposite direction. A rocket moves forward by pushing exhaust gas backward; the gas pushes the rocket forward with an equal and opposite force. You can also feel a backward kick when a gun is fired because the gun and the bullet exert equal and opposite forces on each other. The other statements describe different ideas. One is about objects at rest or in motion staying that way unless acted on, which is inertia from Newton's first law. Another is F = m a, which is Newton's second law relating net force to acceleration. The idea that mass increases with speed is not a statement of Newton's third law in classical mechanics.

When two objects interact, the forces they exert on each other come in pairs that are equal in strength and opposite in direction. This is the idea behind Newton's third law: every action force has a corresponding reaction force that acts on the other object in the opposite direction. These action-reaction forces are simultaneous and act on different objects, not on the same one.

Think about pushing against a wall—your hand pushes on the wall and the wall pushes back on your hand with the same force in the opposite direction. A rocket moves forward by pushing exhaust gas backward; the gas pushes the rocket forward with an equal and opposite force. You can also feel a backward kick when a gun is fired because the gun and the bullet exert equal and opposite forces on each other.

The other statements describe different ideas. One is about objects at rest or in motion staying that way unless acted on, which is inertia from Newton's first law. Another is F = m a, which is Newton's second law relating net force to acceleration. The idea that mass increases with speed is not a statement of Newton's third law in classical mechanics.