A grandfather clock is "losing" time because its pendulum moves too slowly. Assume that the pendulum is a massive bob at the end of a string.
The motion of this pendulum can be sped up by:

a) decreasing the mass of the bob.
b) increasing the mass of the bob.
c) lengthening the string.
d) shortening the string.

d) shortening the string

Explanation:

Time period of a pendulum clock is dependent on two factors namely:length and acceleration due to gravity.

When a clock loses time, the time period of the pendulum clock increases.

This however can be corrected by decreasing the length of the pendulum.The time period of the pendulum clock is not dependent on the mass of the bob. The time period of the pendulum clock can be corrected only by changing the length of the pendulum string.

The motion of this pendulum can be sped up by shortening the string. (d)

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