Which law states that for every action there is an equal and opposite reaction?

• A. Newton's First Law
• B. Conservation of Energy
• C. Newton's Second Law
• D. Newton's Third Law

Answer: (D)

The key idea here is that forces come in pairs when two objects interact. For every force one object exerts on another, the second object exerts an equal force back on the first, but in the opposite direction. These action–reaction forces act on different objects, so they don’t cancel each other out even though they are equal in strength.

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. Or imagine a rocket: it pushes exhaust gases backward, and the gases push the rocket forward with an equal and opposite force. In swimming, your push on the water creates a backward force on the water and a forward force on you, propelling you ahead.

This law describes the nature of interactions itself, and it works alongside the other laws by explaining why forces exist in pairs during any contact or interaction.