CIE A Level Physics (9702) 2019-2021

Revision Notes

3.1.8 Acceleration of Free Fall Experiment

Acceleration of Free Fall Experiment

  • A common experiment to determine acceleration of a falling object which can be carried out in the lab


  • Metre rule, ball bearing, electromagnet, electronic timer, trapdoor

Measuring g, downloadable AS & A Level Physics revision notes

Apparatus used to measure g


  • When the current to the magnet switches off, the ball drops and the timer starts
  • When the ball hits the trapdoor, the timer stops
  • The reading on the timer indicates the time it takes for the ball to fall a distance, h
  • This procedure is repeated several times for different values of h, in order to reduce random error
  • The distance, h, can be measured using a metre rule as it would be preferable to use for distances between 20 cm – 1 m

Analysing data

  • To find g , use the same steps as in the problem solving section
  • The known quantities are
    • Displacement s = h
    • Time taken = t
    • Initial velocity u = 0
    • Acceleration a = g
  • The equation that links these quantities is
    • s = ut + ½ at2
    • h = ½ gt2
  • Using this equation, deduce g from the gradient of the graph of h against t2

Sources of error

  • Systematic error: residue magnetism after the electromagnet is switched off may cause the time to be recorded as longer than it should be
  • Random error: large uncertainty in distance from using a metre rule with a precision of 1mm, or from parallax error

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