Project to Determine if a Swing Moves Faster with a Lighter Weight on It

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Author: Janice VanCleave

What You Need to Know

A pendulum is a weight hung so that it swings about a fixed point. The hanging weight of a pendulum is called the bob. Frequency is the number of swings that a pendulum makes per second. Potential energy is the energy a stationary (nonmoving) object has because of its position or condition. Kinetic energy is the energy an object has because it is moving.

How Does a Pendulum Work?

A simple pendulum is made of a weight at the end of a material, such as a string, a rope, or a rod. When a pendulum hangs vertically, it is said to be at its resting point. In the diagram on the left, the bob (the metal washer) is pulled to side A. When the bob is released, gravity causes the pendulum to swing back and forth between positions A and C. The transfer between potential and kinetic energy keeps the pendulum swinging, and the friction between the pendulum and the air causes it to slow and finally stop. One back-and- forth movement from A to C is equal to one swing. Frequency is measured in swings per time. The formula to use to calculate frequency is F = swings/time. For example, if the pendulum makes 10 swings in 5 seconds, its frequency is:

F = 10 swings/5 seconds

   = 2 swings/second

This is read as 2 swings per second.

How Does a Pendulum Work?

What Does This Have to Do with How Fast a Swing Moves?

A swing is an example of a pendulum. The bob for this pendulum is the seat and the person on it. So the weight of the bob changes with the weight of the person sitting on the seat. When the swing is pulled to one side and then released, gravity pulls it down and it continues to move back and forth for a while. The swing is falling because of the pull of gravity, and the ropes attached to it cause it to move in a curved path. If the size of the bob doesn't change, and it is pulled to the side the same distance each time, will changing its weight affect its frequency?

What Does This Have to Do with How Fast a Swing Moves?

Fun Fact

One of the largest pendulum clocks in the world is 97 feet (29 m) tall, which is about as high as a 5- or 6-story building. While this pendulum clock is very large, it looks small compared to the building to which it is attached, the 810-foot (243 m) twin tower railroad station in Tokyo, Japan.

Real-Life Science Challenge

Two things needed to make a clock include a way to mark off equal amounts of time and a way of keeping track of and displaying the amount of time. Around 1582, Galileo Galilei (1564–1642) observed that, as it moves, a pendulum marks off equal amounts of time. But it was the Dutch scientist Christiaan Huygens (1629–1695) who, in 1656, built the first pendulum clock. His first clock, with an error of less than 1 minute per day, was the first clock to keep such accurate time. A pendulum clock works because the swinging pendulum moves gears inside the clock that move the hands of the clock.


Now, start experimenting with pendulums and weight. Start with a small pendulum and then compare your results with real-life kids on swings.


  • Pieces of string with different numbers of washers attached to them can be used as small pendulums.
  • Be sure to vary only the weight. Keep the string length and how far you displace the pendulum the same for each experiment.
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