RP03 - Determination of g

• What is 'g'?

  The Gravitational Field Strength
• What is meant by 'free-fall'?

  • An object is said to be in free-fall if the only force acting on it is gravity
  • This means that any resistive forces that are acting can be considered to be negligible
• What graph can be plotted from experimental data to determine 'g'?

  • A graph of 2h/t against t can be plotted
  • The value of 'g' is equal to the gradient of the line plotted
• Describe how an electromagnet system can be used to determine 'g'

  • A magnetic ball bearing can be released by an electromagnet clamped at a known height
  • The timing system starts when the electromagnet is switched off and the timer is stopped when the ball lands on the finish pad
• When using a clamp stand in this experiment, what safety precaution should be taken?

  The clamp stand should have a counterweight or a G-clamp attached to its base to provide a moment to prevent the clamp stand from toppling over
• What safety precaution should be taken when using an electromagnet?

  • Electromagnets heat up over time
  • To reduce this heating effect, you should switch it off when not in use
• Suggest how light-gates could be positioned to ensure that the ball falls directly through them

  • A plumb line could be used to demonstrate the expected path of the ball
  • This allows the light-gates to be positioned in appropriate places so that the ball will fall through them
• Why is it better to use a small ball bearing over a larger ball?

  • The smaller the ball, the smaller the effects of air resistance
  • In the case of a small ball bearing, these effects can be considered to be negligible
• Why should there be a gap between the release position and the first light gate?


  • There should be a gap to ensure that the time over which the ball is passing through the light gate is negligible
  • The ball is moving sufficiently quick enough
• Explain why this experiment would not be valid if the air resistance acting on the ball wasn't negligible

  • The ball wouldn't be in free fall since the acceleration would not be purely due to the force of gravity
  • The acceleration would also be variable since air resistance increases with speed, and so the uniform acceleration equations couldn't be used
• Suggest why your obtained value of 'g' may not be the same as the accepted value

  • Delays in the timing equipment (if using a stop clock, this will be human reaction time)
  • Resistive forces may be significant
  • Errors in height measurements, such as measuring from different positions on the ball each time
• Would you expect your value of 'g' to be greater or lower than the accepted value?

  • You will most likely obtain a value that is lower than the accepted value
  • due to air resistance reducing the downwards resultant force acting on the object
• How could your results be improved?

  • You should take repeated readings at each height and then calculate the mean time taken
  • You should also ensure that height measurements are taken from the same position on the ball each time
• What is the advantage of using light gates over a stop clock in this experiment?

  • Using light gates should result in a lower uncertainty in your time measurements
  • A stop clock will involve human reaction times and therefore an associated error
• What is the minimum number of repeated readings you should take in this experiment?

  • You should take at least 3 repeat readings at each height
  • This allows for anomalous results to be more easily identified
• gradient derivation:

  s = ut + 1/2 at^2

  2s = 2ut + at^2

  2s/t = 2u + at

  where:

  2s/t = y

  2u = c

  t = x

  a = m