CIE A Level Physics复习笔记2.1.8 Acceleration of Free Fall Experiment

• 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 + 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