Uncertainty and Absolute Error

• What is the equation for percentage error?

  percentage error = (uncertainty ÷ measured value) x 100
• The height of a person is 150cm. The uncertainty of the instrument used is: ±0.3m.

  Calculate the percentage uncertainty

  1. 0.3m = 30cm
  2. 30/150 x 100 = 20%
• An object is 0.3m in length. This was measured using a 30cm ruler with an uncertainty of ±0.5cm

  Remember! When using a ruler there is a chance of uncertainty at both ends:

  1. 0.5 x 2 = 1cm = 0.001m
  2. (0.001/0.3) x 100 = 0.333%
• A ball has a mass of 500g with an uncertainty of ±5g. The volume of the ball is 0.4m^3 with an uncertainty of ±0.02m^3.

  Calculate the overall percentage uncertainty

  1. Mass of ball: (5/500) x 100 = 1%
  2. Volume of ball: (0.02/0.4) x 100 = 5%
  3. 1 + 5 = 6%
• How do you work out Absolute error?

  You half the resolution of the finest scale given. E.g. this ruler has a resolution of 0.1cm and so an absolute error of 0.05cm.