Page 264 - Physics Coursebook 2015 (A level)

Cambridge International AS Level Physics
252
Problem
Improvement
‘It was difficult to judge the end of the swing.’
‘Use a fiducial mark at the centre of the oscillation as the position to start and stop the stopwatch.’
‘Use an electronic timer placed at the centre of the oscillation to measure the time.’
‘Make a video of the oscillation with a timer in the background and play it back frame by frame.’
‘It was difficult to measure the length to the centre of gravity of the weight accurately.’
‘Use a longer string so any errors are less important.’
‘Measure the length to the top of the weight and use a micrometer to measure the diameter of the bob and add on half the diameter to the length of the string.’
pendulum bob
QUESTION
A
B
C
18
Hang a mass from a spring or from a rubber band. Use a stopwatch to time the mass as it oscillates up and down. Measure the time for just one oscillation, the time for 10 oscillations and the time for 20 oscillations. Repeat each reading several times. Use your readings to find the time for one complete oscillation and the uncertainty in each time. Draw up a table to show the problems of such measurements and how to reduce them.
Figure P1.16 One complete oscillation is either from A to C and then back to A, or from B to C then back to B, then to A and back to B, as shown.
fiducial mark. This can be a line on the bench underneath the bob at the centre of the swing, or it can be another object in the laboratory which appears to be in line with the bob when it hangs stationary, as seen from where you are standing. As long as you do not move your position, every time the bob passes this point it passes the centre.
Another way to reduce the uncertainty in the time
for one oscillation is to time more than one swing, as explained above in the section on percentage uncertainty.
A simple practical task is to test the hypothesis that the time for one oscillation T is related to the length l of a simple pendulum by the formula T 2 = kl, where k is a constant.
What difficulties would you face and what are possible improvements? Table P1.9 gives some possibilities.
Experiment 4: Using force meters
You need to be able to read instruments, estimating the uncertainty, looking for sources of error and trying to improve their use. One such instrument is a force meter or newton-meter, shown in Figure P1.17.
In this experiment, the block is pulled using the force meter to find the force F needed to make a block just start to move. An extra mass is added on top of the block to see whether the relationship F = km is obeyed, where m is the total mass of the block and k is a constant.
weight newton-meter block
Figure P1.17 A newton-meter, just before it pulls a block along the bench.
‘Taking readings for just two lengths was not enough.’
‘The oscillations died away too quickly.’
‘The times were too small to measure accurately, as my reaction time was a significant fraction of the total time.’
‘Use more than two lengths and plot a graph of the average time squared against the length of the string.’
‘Use a heavier mass which swings longer.’
‘Use longer strings.’
‘Time 20 rather than 10 oscillations.’
Table P1.9 Suggestions for improving Experiment 3.
0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10