The Professional Development Service for Teachers is funded by the Department of Education and Skills under the National Development Plan

.     

T.H.

Measurement of g, the acceleration due to gravity, using a free-fall apparatus.

PDST Resources for Leaving Certificate Physics 2

.     

Apparatus

PDST Resources for Leaving Certificate Physics 3

.     

Method

Set up the apparatus as in the photo. Measure and record the distance s from

the bottom of the ball bearing to the metal plate.

Release the ball bearing. This starts the scaler-timer.

When the ball bearing strikes the plate the scaler-timer stops.

Record the time t on the clock.

PDST Resources for Leaving Certificate Physics 4

.     

Here is a close-up of the release mechanism. Repeat the release of the ball bearing three times and take the minimum value for t as the

clock sometimes starts too early.

PDST Resources for Leaving Certificate Physics 5

.     

Graph s against t 2. Draw a best straight line. Find the slope, m, of the graph. Calculate a value for g using g = 2m.

Note: We graph t2 against s as the units of acceleration are m s-2, which makes it easier to remember how to draw a suitable graph.

PDST Resources for Leaving Certificate Physics 6

.     

Use the following table to draw a suitable graph and hence find a value for g.

PDST Resources for Leaving Certificate Physics 7

.     

Note: Don’t forget to draw a new table, with s in metres and t 2 in seconds squared.

PDST Resources for Leaving Certificate Physics 8

.     

Draw the best straight line and choose two non-data points.

PDST Resources for Leaving Certificate Physics 9

.     

Find the slope m and calculate g = 2m

PDST Resources for Leaving Certificate Physics 10

.     

This is reasonably close to the true value, g = 9.8 m s-2

PDST Resources for Leaving Certificate Physics 11

.     

Sources of Error

Take the readings from the metre stick at eye level to avoid the error of parallax.

The release mechanism may start the clock too soon giving too high a value for t.Take a number of readings for t and choose the minimum value .

The distance s must be measured from the bottom of the ball bearing to the top of

the shock plate.