Core practical 1: acceleration of free fall

A typical method

• An electromagnet holds a small steel ball. Switching it off releases the ball and starts an electronic timer.
• The ball falls through height h and opens a trapdoor (or passes a light gate), which stops the timer.
• Measure h from the bottom of the ball with a metre rule, using a set square to avoid parallax.
• Repeat each height several times and take a mean; use at least five different heights.

Analysis

From rest, h = (1/2)gt². Plot h (y-axis) against t² (x-axis): a straight line through the origin with gradient g/2, so g = 2 x gradient. With a light gate measuring speed v after a fall h, v² = 2gh, so a graph of v² against h has gradient 2g.

Errors and improvements

• Electronic timing removes human reaction time.
• A small, dense ball makes air resistance negligible.
• A delay in the magnet releasing the ball adds the same time to every reading: a systematic error that makes each calculated g too small. Plotting sqrt(h) against t gives a straight line whose intercept shows the delay, and g = 2 x gradient² is still accurate.
• Repeating and averaging reduces random errors.

Worked example

A graph of h against t² has gradient 4.90 m/s². Find g.

1. h = (1/2)gt², so gradient = g/2
2. g = 2 x 4.90

Answer: g = 9.80 m/s²

Check yourself

1. Which graph gives a straight line through the origin in this experiment?
   • h against t
   • t against 1/h
   • h against t²
   • h² against t
   Show answer

   h against t². h = (1/2)g t², so h is proportional to t².

2. A graph of h against t² has a gradient of 4.85 m/s². Find g in m/s².
   Show answer

   9.70 m/s². g = 2 x gradient = 9.70 m/s².

3. A ball falls 1.20 m from rest in 0.495 s. Find g in m/s², to 3 s.f.
   Show answer

   9.79 m/s². g = 2h / t² = 2.40 / 0.2450 = 9.79 m/s².

4. A ball is released from rest and passes a light gate 0.80 m lower at 3.95 m/s. Find g in m/s², to 3 s.f.
   Show answer

   9.75 m/s². v² = 2gh, so g = 3.95² / 1.60 = 9.75 m/s².

5. Why is an electronic timer better than a stopwatch here?
   • It makes the ball fall faster
   • It removes air resistance
   • It measures the height more accurately
   • It removes human reaction time from the readings
   Show answer

   It removes human reaction time from the readings. Fall times are under a second, so reaction time would be a large fraction of each reading.

6. The electromagnet releases the ball slightly after the timer starts. What kind of error is this?
   • A random error
   • A systematic error
   • A zero error in the metre rule
   • Parallax error
   Show answer

   A systematic error. Every time reading is too long by about the same amount, which is systematic.

7. Why is a small, dense ball used?
   • So it is easier to see
   • So it reaches terminal velocity quickly
   • So it falls more slowly
   • So air resistance is negligible compared with its weight
   Show answer

   So air resistance is negligible compared with its weight. A dense ball has a large weight for its size, so drag has little effect.

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Sources

• Pearson Edexcel International Advanced Subsidiary/Advanced Level in Physics specification (XPH11/YPH11), Issue 3
• Pearson qualifications: International Advanced Levels Physics (2018)
• Cognito: Edexcel International A level Physics specification and core practicals
• Pearson Edexcel IAL Physics Unit 1 (WPH11) question paper, January 2023: list of data (g = 9.81 m/s²)

Last updated 7 November 2026. Spotted a mistake? Tell us.