Simple Pendulum

• A simple pendulum — bob tied to a string fixed at one point.
• Oscillation — one complete to-and-fro motion (centre → one side → other side → centre).
• Time period (T) — time for one oscillation; measured with stopwatch.
• NCERT Activity 13.4 — timing 20 oscillations and dividing by 20 for accuracy.
• Periodic motion — repeats at regular intervals (pendulum, swing, Earth rotation).
• Factors affecting period (intro): length of string; mass of bob has negligible effect at Class 7.

Level 1 — Core idea

1. Bob — heavy small object (metal ball); string — inextensible thread.

2. Mean position — lowest point when at rest vertically.

3. Amplitude — maximum displacement from mean (keep small for simple pendulum).

Level 2 — Process and representation

1. Time period formula — not derived at Class 7; T measured experimentally.

2. Diagram (text) — fixed support, string length L, bob swings left and right.

3. NCERT method — time 20 oscillations, T = total time / 20.

Level 3 — Applications and NCERT links

1. Errors — reaction time reduced by timing many oscillations.

2. Real world — pendulum clocks (historical); temple bells swinging.

3. Galileo — observed lamp swing in church (historical note).

NCERT Activity: Finding time period of a simple pendulum

Materials: bob, thread, clamp stand, metre scale, stopwatch.
Step 1 — Fix thread of length 100 cm (or 1 m) to stand; attach bob.
Step 2 — Displace bob slightly (small angle) and release — do not push hard.
Step 3 — Start stopwatch at mean position when bob passes centre going one way.
Step 4 — Count 20 complete oscillations; stop watch — e.g. 40 s.
Step 5 — Time period T = 40 / 20 = 2 s per oscillation.
Step 6 — Repeat 3 times; average T values.
Step 7 — Change length to 50 cm; T decreases (shorter pendulum swings faster).
Conclusion: pendulum shows periodic motion; T measurable and reproducible.

• Define oscillation and time period.
• Why time 20 oscillations instead of 1?
• What is the mean position of a pendulum?
• Name two examples of periodic motion.
• If 15 oscillations take 30 s, find T.
• Define amplitude and time period.

Open the Practice tab for graded questions on Simple Pendulum.