1.6Chapter 1: Energy

Planning: pendulum motion — Alhussan Physics - Stage 7 Revision Notes & Topical Questions

5 min read 9 parts Lesson 6 of 8 1 quiz

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Lesson summary

1.6 Planning: pendulum motion — Investigating Variables — Alhussan International Academy — Physics (Stages 7–9) notes: Identify a testable hypothesis, decide how many measurements to take and present results clearly in an investigation of a pendulum.Pendulum = an object that swin…

Variables that might affect the period
Actual effects
Students' hypothesis
Prediction
Independent variable
Results table

Part 1 of 7How a pendulum moves

How a pendulum moves

Pendulum

Pendulum an object that swings backwards and forwards, like a child on a swing or a car on a theme-park ride.

Parts

Parts a simple pendulum is a string with a small mass, called the bob, on the end.

Period

Period the time a pendulum takes to swing backwards and forwards once, from one end to the other and back.

Positions

Positions the pendulum is stationary at the ends of the swing (points A and B) and moves fastest in the middle (point C).

Amplitude

Amplitude the maximum displacement from the midpoint.

Energy

Energy at the highest point GPE is a maximum, and as it swings down GPE is transferred to KE, which is greatest at the lowest point.
A small amount of energy is dissipated each swing by air resistance and friction at the pivot, so the swings gradually get smaller.

Part 2 of 7Asking a scientific question

Asking a scientific question

Question

Aditi and Diya decided to investigate what factors affect the period of the pendulum.

Variables that might affect the period

Variables that might affect the period the length of the pendulum, the mass of the bob and the distance the bob is pulled back.

Actual effects

Actual effects for a simple pendulum only the length changes the period significantly.
A longer pendulum has a longer period.
The mass has no effect.
The amplitude has little effect for small angles up to about 20 degrees.

Part 3 of 7A testable hypothesis

Testable hypothesis

Testable hypothesis a hypothesis is testable if you can collect data to check a prediction based on it.

Students' hypothesis

Students' hypothesis how long the pendulum takes to swing depends on the length of the string,
because the energy stores and transfers depend on the height of the bob at the middle and end of its swing.

Prediction

Prediction a prediction based on it, for example that changing the length changes the period, can be checked by collecting data.

Part 4 of 7Aditi's plan

Independent variable

Independent variable the length of the string, starting at 10 cm and going up by 5 cm until 50 cm.

Dependent variable

Dependent variable the time to swing 10 times, divided by 10 to give the period.
Timing many swings reduces the effect of reaction time.

Controls

Controls the mass of the bob and the distance it is pulled back.

Results table

Results table it has columns for length and time.

Part 5 of 7Diya's plan

Independent variable

Independent variable the length of the pendulum, using lengths between 20 cm and 100 cm.

Dependent variable

Dependent variable the time for one swing.

Repeats

Repeats she timed each length three times and found the average time, which makes her results more accurate.

Results table

Results table it has columns for length (cm), three times (s) and average time (s).

Comparison

Comparison each student identified the variable to change and the variable to measure and explained how to control the other variables.

Part 6 of 7Carrying out and presenting results

Measuring time

Measuring time use a stopclock or stopwatch, and repeat measurements because short times are difficult to measure accurately.

Repeats

Repeats always repeat measurements to reduce the effect of errors.

Control the set-up

Control the set-up measure the length from the pivot to the centre of the bob, control the starting angle with a protractor and use a clamp stand with the pendulum fixed securely.

Table

Table present results in a table with a heading and units at the top of each column.

Graph

Graph plot length on the x-axis and period on the y-axis.
The line curves upward, showing the period increases with length but not in direct proportion.
A graph of period squared against length is a straight line.

Evaluation

Evaluation identify anomalous results, calculate means, and discuss human error in timing and release angle.
Use a light gate or video analysis to improve precision.

Part 7 of 7Why pendulums matter

Clocks

Clocks pendulums keep regular time,
- so they were used in clocks for centuries,
- and Galileo noticed the property by watching a swinging lamp in a cathedral.

Seconds pendulum

Seconds pendulum it has a period of 2 seconds and a length of about one metre.

Gravity

Gravity the period depends on length and gravitational field strength, so a pendulum can detect small differences in gravity at different places.

Energy top-up

Energy top-up a pendulum clock is topped up by a falling weight or a wound spring to replace the energy lost in each swing.

Key terms

Term	Meaning
Pendulum	a mass hanging from a pivot that swings freely.
Period	the time for one complete swing.
Amplitude	the maximum displacement from the midpoint.
Pivot	the fixed point about which something swings.
Bob	the mass on the end of a pendulum.
Testable hypothesis	a hypothesis from which a prediction can be checked with data.

Avoid these mistakes

Measuring the string only, not to the centre of the bob — measure from the pivot to the centre of the bob.

Changing both mass and length — change only one variable at a time.

Using a large amplitude — use small angles of about 20 degrees or less.

Saying that heavier bobs swing faster — the mass does not change the period.

Last updated: 13 October 2026