An experimental measurement is always affected by uncertainty (error), that may depend on the method used to measure, or on the instrument's precision, etc.[br]This example illustrates the propagation of error when we sum or multiply the lengths of two objects [i]a[/i], [i]b[/i].[br][br]Enter the [i]mean value[/i] and the [i]absolute error[/i] of each length in the related input fields, then select the operation to view.[br][br]Move the sliders to explore the range of each length and observe how the sum and multiplication are affected.[br]Click the button to reset the lengths to their exact measure (i.e. error=0).[br][br]The unit of measurement has been intentionally omitted - a formally correct way to write a measurement is [math]a = (4.5 \pm 0.1) m[/math]