Page 54 - Physics - XI

1. First of all, fi nd out the Vernier constant (or least count) and zero error of the Vernier Calliper and
determine the mean diameter and hence mean radius of the spherical bob.

2. Tie the hook of the bob of mass m (say 50 g) to the cotton thread about 150 cm long and the other end
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of the thread is passed through the two half pieces of a split cork as shown in Fig. 8.2.
3. Put ink marks on the thread at distances of 120 cm from the centre of gravity of the bob including
the height of the hook as shown in Fig. 8.2. This distance gives the eff ective length (L) of the simple
pendulum (L = l' + h + r).
4. Take a clamp stand and place it on the table in such a way so that the bob of mass m is about 2 cm
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above the ground. Mark two lines with a piece of chalk just below the bob, one parallel to the edge
of the table and the other perpendicular to it. Mark two points B and C on either side of the point of
interaction O and at a distance of about 5 cm as shown in Fig. 8.1.

5. Now, put 120 cm mark at the lower surface of the cork by pulling the thread through the cork pieces.
Adjust the clamp so that the bob is about 2 cm above the ground.

6. Find out the least count of the stopwatch and bring its hands at zero position.
7. Now, displace the bob at mass m to one side of its rest point O and release the bob gently so that the
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bob starts vibrating in a vertical plane about point O. Make sure that the bob does not spin about its
own axis while vibrating.

8. When amplitude of the vibration of the bob becomes small, start the stopwatch when the bob just
crosses its mean position O (say from left to right) and count it as zero vibration. When the bob again
passes through O in the same direction (from left to right), i.e., O to B, B to C, and back to O one
vibration is just completed.
9. In this way count 20 vibrations. When 20 vibration is just completed, stop the stopwatch. Note the
th
reading of the stopwatch. This gives the time taken for 20 vibrations. Repeat this observation by taking
the time for 20 vibrations once again at the same eff ective length of the pendulum. Take the mean of
these two readings.
10. Now, replace the bob of mass m with another bob of mass m of same size (say 75 g) and repeat the
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2
steps 4, 5, 6, 7, 8, and 9 keeping the eff ective length (L) remains same (120 cm).
11. Repeat the step 10 with at least three another bobs of diff erent masses, but same size keeping eff ective
length of the simple pendulum remains same, i.e. L = 120 cm in all cases and note the observations.

Observations

Vernier constant of the Vernier Callipers = (V.C.) = ___________ cm
Zero error of Vernier Callipers (e)
(i) = ___________ cm (ii) = ___________ cm (iii) = ___________ cm

Mean zero correction = (– e) = ___________ cm
Observed diameter of all the bobs of diff erent masses = d = ___________ cm
0
Corrected diameter of all the bobs of diff erent masses = d = d + (– e) = ................ cm
0
d
Radius of all the bobs of diff erent masses = r = 2 = ___________ cm

Mass of the bob m = ___________ g
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