2.4 - Choosing Measuring Equipment

Choosing equipment to measure distance and time

Speed is calculated from a measured distance and time:

$$\text{Speed}=\frac{\text{distance}}{\text{time}}$$

Measuring distance

• Use an instrument longer than the distance being measured to avoid moving it along.
• Under a metre (lab experiments) - A ruler or metre stick.
• Larger distances (e.g. walking speed over several metres) - A tape measure or trundle wheel.

Measuring time

• Longer intervals (more than about 5 seconds) - A stopwatch is suitable.
• Very short intervals (e.g. a trolley passing a point) - Light gates connected to a data logger are best.
• A stopwatch involves human reaction time, which introduces a significant percentage error into short time measurements.

Video analysis as an alternative

1. Record the motion on video alongside a metre ruler and view it frame by frame.
2. Count how many frames the object takes to travel a set distance on the ruler:

$$\text{Time taken}=\frac{\text{number of frames}}{\text{frame rate (fps)}}$$

Speed is then calculated from distance ÷ time.