Swing Time!

Pendulum clocks use swinging weights to measure time. They were invented in 1656 CE by the Dutch physicist Christiaan Huygens, and were a big improvement over the other kinds of clocks that were used at that time. You might even have a pendulum clock around your house - many decorative clocks, like grandfather or cuckoo clocks, often use pendulums. If you have to wind the clock, it probably contains a pendulum.

You can do some easy experiment with pendulums and investigate how they measure time.

What you'll need:
Piece of string (about 24 inches or 60 cm long)
Ruler or tape measure
2 heavy metal nuts (from a hardware store)
Tape
Watch with a second hand or other timer

To make your first pendulum, put one end of the string through the hole in the nut and tie a knot. Now you need to find a good spot for your pendulum. You can hang it from a table, shelf or other support. Check with an adult to make sure that it's OK to use tape on your support. Make sure that the pendulum can swing freely without hitting anything. Measure the length of the string. Pull the pendulum back to an angle of about 45^o (just estimate; don't worry about measuring) as shown below. Let the pendulum go and time how long it takes to return to the starting position. This time is called the period. If the time is too short, let the pendulum swing three times and divide the total time by three.

OK! Now let's see what effect changing the properties of the pendulum has on the period. First, pull the pendulum back to only about 30^o (see figure below) and check the period. Does the pendulum move faster, slower, or at the same speed? Try some other angles!


Next, remove the tape and shorten the string to one-half its original length. Measure the period at 45 and 30 degrees. What happens when you shorten the string?

Finally, untie the knot and add a second nut to the string. This makes the pendulum twice as heavy as it originally was. Does the weight affect the period?