KinexiaLEARN PHYSICS

Pendulum Period Lab

Galileo timed pendulum swings with his own pulse and stumbled onto one of physics' most useful regularities. With a phone stopwatch you can do better: measure exactly what controls a pendulum's period, and rule out what does not.

Safety:

Tie the weight securely so it cannot fly off. Keep the swing small and clear of faces, screens, and breakable objects.

Materials

  • String or thread (about 1 m)
  • A small weight (metal nut, key, or small rock)
  • Tape or a binder clip
  • Phone stopwatch
  • Ruler or tape measure
  • Notebook for results

Procedure

1

Build the pendulum

Tie the weight firmly to one end of the string. Tape or clip the other end to a table edge or shelf so the weight hangs freely and can swing.

2

Measure the period at full length

Measure the string length from pivot to the weight's center. Pull the weight aside a small angle (under 15 degrees), release it, and time 10 complete back-and-forth swings. Divide by 10 — that is the period.

3

Shorten the string and repeat

Shorten the string to 75 cm, then 50 cm, then 25 cm. Time 10 swings at each length and record each period in your notebook.

4

Change the mass

Swap in a heavier and a lighter weight at the same string length. Time 10 swings for each and compare the periods.

5

Change the starting angle

At one fixed length, release from a small angle and then a larger one (about 30 degrees). Compare the periods.

What to observe

  • The period gets shorter as the string gets shorter.

  • Changing the mass does not change the period.

  • Small release angles give consistent periods; very large angles make the period slightly longer.

Why it happens

For small angles the period follows T=2πL/gT = 2\pi\sqrt{L/g}: it depends on the string length LL and gravity gg only. Mass cancels out because gravity pulls harder on a heavier bob but its greater inertia resists exactly as much — the two effects balance perfectly. This is why pendulum clocks keep time: the swing rate is built into the geometry, not the weight. Past about 15 degrees the small-angle approximation breaks down and the true period grows slightly.

Try variations

  • Time a playground swing — does the period still follow the square-root-of-length rule?

  • Try the experiment on video in slow motion and count frames for extra precision.

  • Predict the period for a 2 m string before measuring, then check.