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

Apparatus

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

Apparatus used to measure g

Method

• 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 + ? at²
  • h = ? 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