Long thread, slow swing

The idea

The time period of a simple pendulum depends on its length, not on its bob's mass; at a given place a pendulum of a given length has a constant time period, which is used to measure time.

Heavy or long?

Pema wants her pendulum to swing more slowly. She can use a heavier bob or a longer thread.

Which change will make the time period longer?

• A longer thread
• A heavier bob
• Neither one changes it

Compare two pendulums

Try the same length with different mass. Then try different lengths. Press Release both each time.

Length matters, mass does not

Two pendulums of the same length swung together, even when one bob was heavy and one was light. A longer thread took more time for each oscillation. (The periods in the picture are examples.)

The time period of a simple pendulum depends on its length, not on its bob's mass. At a given place a pendulum of a given length has a constant time period, which is used to measure time.

• Change | Time period
• Longer thread | Becomes longer
• Heavier bob | Does not change
• Same length, same place | Constant

Notes

The time period of a simple pendulum depends on its length, not on its bob's mass. At a given place a pendulum of a given length has a constant time period, which is used to measure time.

Check yourself

What does the time period of a simple pendulum depend on?

Two pendulums have the same length. One has a heavy bob and one has a light bob. Their time periods are …

Which pendulum takes more time for one oscillation?

Why can a pendulum of a given length be used to measure time?

• The mass of its bob. The time period does not depend on the mass of the bob.
• Its length — correct. Yes!
• The colour of its bob. Colour has nothing to do with the time period.
• different, the heavy one is slower. The mass of the bob does not change the time period.
• different, the light one is slower. The mass does not change the time period. Same length means same time period.
• the same — correct. Yes!
• The one with the longer thread — correct. Yes!
• The one with the shorter thread. A shorter pendulum has a shorter time period.
• The one with the heavier bob. The mass does not matter. Length does.
• Its time period keeps changing. If the time period kept changing, it could not be used to measure time.
• Its time period is constant at a place — correct. Yes!
• It makes a loud sound. The sound is not the reason. Its time period is constant at a place.