**Question 8:** Give the drawbacks to use the period of a pendulum as a time standard.

Answer

If ‘l’ is the length of a pendulum, the time period ‘T’ is given by

….. (A)

Since 2π is a constant term therefore, T∝ l/g.

So time period of a pendulum would have a number of drawbacks if used as a time standard.

(1) Material objects expand on heating and contract on cooling. With the increase in temperature (for example in summer or by any other reason), the length of the pendulum will increase. As there is a direct relationship between T and l, therefore, the time period of the pendulum will also increase. Similarly, in winters (or by any other reason) when the temperature falls down, the length of the pendulum will contract, thereby decreasing the time period of the pendulum.

(2) Again the time period of the pendulum has an inverse relation with the gravitational acceleration g. Since ‘g’ has different values at different altitudes (altitude is the height of a point above the sea level); ‘g’ decreases as we go above the sea level. Therefore, the value of ‘T’ will increase as we go to high altitude zones and will decrease as we come down.

(3) Air resistance affects the motion of the bob of the pendulum. If the speed of air changes, it will change the speed of the bob of the pendulum and hence the time period.

Due to these major drawbacks, pendulum cannot be a good choice to be used as a time standard.

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