Experiment to measure the acceleration of an object down a slope

Equations of motion experiment using light gates, a timer and a trolley and mask.An experiment to measure the acceleration of an object down a slope

The acceleration of an object down a slope can be measured using the set-up shown above. This uses the following equipment:

  • two lght gates
  • timer
  • trolley carrying a mask (piece of card) of a known length

Set up apparatus or similar as in photograph so the trolley runs down the slope and the mask on the trolley cuts the beams of both light gates.

Measurements required:

  • length of mask (in metres)
  • time to cut first light gate t_1(in seconds)
  • time between light gates t_2(in seconds)
  • time to cut second light gate t_3(in seconds)

What are the advantages of repeating the experiment at least five times?

An average value can be determined from the five results. A number of results lets you work out the approximate random mean uncertainty. This can help indicate how confident you can be in the result.


The length of the mask can affect uncertainty of the result, what would be the advantage of a longer mask?

A longer mask will ensure a longer time and less reading uncertainly in the time.(e.g. if time is measured to ± 0.01 seconds and the time with a short mask is 0.05 s this will be 20% uncertainty. A longer mask with a time of 0.20 s will have a reading uncertainty of 5%)


Note that each light gate gives a time for the whole mask to pass through it. Dividing the time by the length of the mask gives an average velocity.

This average velocity is the 'instantaneous' velocity of the trolley measured by the light gate. This is only valid at the instant the mid-point of the mask passes through the light gate.

A barrier on wheels approaches a beam, then blocks it, moving at measure velocity, and finally moves past the beam.

If the mask length is too long it will maximize an error inherent in this method. Each light gate measures the total time for the mask to pass through the light gate. The longer the mask the longer this time will be.

How could this be avoided?

The time for the trolley to move between the two light gates includes the end of the time the mask takes to pass through the first light gate and the start of the time it takes to pass through the second light gate. So the time between light gates should include half of these times.

If a timer device with direct acceleration display is used it will calculate acceleration with this correction.

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