The Motion Gallery

The Motion GalleryA child and an adult are seated on a playground swingset. The chains from the support bar to the seats are the same length; the adult is much bigger than the child. Who swings back and forth faster?

Is it the heavier adult because gravity pulls on the adult with more force? Or is it the lighter child because the child is more responsive to gravity and moves faster? The correct answer is neither. Both the child and the adult swing at the same rate.

Scientifically, a swing is a pendulum -- just like the swinging weight in a grandfather clock. Except in extreme cases, the only thing that affects the swing speed of a pendulum is the distance from the pivot point to the suspended weight. Decreasing the length of a pendulum makes it swing faster and increasing that length makes it swing slower. Two playground swings of the same length swing at the same rate no matter what the weight on the seat.

Why does the size of the weight make no difference? All objects resist changes in their motion. The heavier an object, the more it resists changes in its motion. This is an idea called "inertia." When a pendulum weight ("bob") is moved to one side and released, it has inertia and will tend to stay where it was released. However, the force of gravity quickly overpowers the bob's inertia and pulls it back to its center of motion. If the amount of matter ("mass") in the bob is changed, its inertia will likewise change, but the pull of gravity will also change proportionally, resulting in unchanged behavior. The effects of inertia's resistance and gravity's pull cancel each other and the pendulum swings at the same rate regardless of the bob's mass.

Pendulums were first studied scientifically by Galileo about the year 1602. He also first demonstrated that objects of different masses fall at the same rate -- the balance between inertia and gravity.