Measuring the Acceleration Due to Gravity Using Light Gates


Free fall apparatus consisting of two sets of light gates, ball bearing and batteries, timer.



1. The apparatus should be mostly set up already as shown in the diagram. However, you will need to connect the timer to the light gates and align these gates. If the set up is correct, then as the falling mass passes through the first gate the timer should start and when the mass passes through the second gate the timer should stop. In each case the mass interrupts the light beam at the gates. the timer will read time,to ± 0.01 second. The upper gate is fixed in position a distancefrom the top of the apparatus.

2. The lower light gate should be positioned initially about 55 cm below the upper gate, measure this distance,

3. Allow the mass to fall from the top of the guide tube and so obtain a time for the fall between the two light gates. Repeat twice more and so obtain a mean value ofto three significant figures.

4. Repeat stage 4 for seven further different distances of fall,between 55 cm and 120 cm.

Theory, Graph and Calculation:

Throughout the fall through distance,the acceleration is assumed to be constant and equal to the acceleration due to gravity. The velocity at the first light gate equalsand that at the second equalswith time of fall equal to



and sowhich we can write as

This equation has formso if a graph ofis plotted on the- axis againston the- axis it should be a straight line of gradient

Plot this graph and hence determine

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