Which law does this pendulum demonstration illustrate?

• A. Newton's second law of motion
• B. Newton's first law of motion
• C. Hooke's law
• D. Law of conservation of energy

Answer: (B)

Inertia governs what you see in a pendulum. Newton's first law says an object in motion stays in motion in a straight line unless a net external force acts on it, and an object at rest stays at rest. When the pendulum is released, the bob has inertia and would tend to continue moving straight ahead. Gravity pulls on it, and the string tension provides the inward force needed to keep it moving along a curved path. The combination of gravity and tension continually changes the bob’s direction and speed, showing how motion changes only because a net external force acts on the mass. The other ideas don’t fit this demonstration as directly: Hooke's law describes springs, not a swinging pendulum; energy conservation describes how energy shifts between kinetic and potential, which is at play but isn’t the primary demonstration here; Newton's second law connects force, mass, and acceleration, but the key point illustrated by the pendulum’s ongoing change in motion is inertia interacting with an external force, i.e., the first law.