Describe a range of laboratory methods for determining the speeds of…

The idea

Speed is distance travelled divided by time taken, so any laboratory method must measure both quantities for the same moving object. Light gates are a common choice: a card of measured length passes through a gate, which times how long the card blocks the beam, and speed is card length divided by that time. Two gates spaced a measured distance apart give an average speed between them. Other methods include a stopwatch and metre rule for a trolley on a runway, a ticker-tape timer that marks dots at fixed time intervals so the spacing shows distance per interval, and a motion sensor linked to a data logger that records position against time. Each method trades precision against simplicity, and choosing one depends on the object and the accuracy needed.

What earns the marks

• States that speed equals distance travelled divided by time taken, so both must be measured.
• Describes a light gate timing a card of known length to give speed as card length divided by blocking time.
• Explains that two light gates a measured distance apart give an average speed between the gates.
• Identifies a stopwatch and metre rule, ticker-tape timer or data-logging motion sensor as further laboratory methods.
• Recognises that light gates reduce human reaction-time error compared with hand timing.

Common mistakes and how to put them right

• Uses the length of the whole runway instead of the card length when calculating speed from one light gate.

  Put it right: Use the measured length of the object that breaks the beam.

• Forgets to convert centimetres to metres or milliseconds to seconds before dividing.

  Put it right: Convert all quantities to SI units first.

• Assumes a single light gate measures acceleration.

  Put it right: Use two gates or a data logger to show changing speed.

Exam tips

• Write the equation speed = distance ÷ time before substituting values so the method is clear.
• Check units carefully: convert cm to m and ms to s before calculating.
• When comparing methods, name the specific error each one reduces, such as reaction time for light gates.