Question

Why is clock’s pendulum made of invar?
A. It is lightweight
B. It is easily available
C. Its efficient coefficient of linear expansion is low
D. It is cheaply available

Answer 1

Hint: Invar is a nickel-iron alloy which is known for its low coefficient of thermal expansion. Time period of a pendulum depends on the length of the string.
Thermal expansion is the property of matter to change its shape and dimensions as a consequence of change in temperature.

Complete step-by-step answer:
Matter has a tendency to change its shape, length, area, volume, and density in response to change in temperature. This property has been named as thermal expansion.
A pendulum in a clock is a timekeeping element. It swings to and fro in a precise time interval dependent on its length. Time period for a pendulum of length $l$ can be written as:
$T=2\pi \sqrt{\dfrac{l}{g}}$
Since this length of pendulum depends on temperature, the time period changes with change in temperature, which will result in inaccurate time in the clock. To overcome this problem, we need a material, the length of which does not depend on temperature i.e. it has a low coefficient of thermal expansion.
Invar is a nickel-iron alloy which is known for its low coefficient of thermal expansion. Since, its length remains almost constant for temperature change, therefore it is used to make the clock's pendulum.

So, the correct answer is “Option C”.

Note: Matter has a tendency to change its shape, length, area, volume, and density in response to change in temperature. This property has been named as thermal expansion.
Pendulum clocks must be stationary to operate; any accelerations will affect the motion of the pendulum which will cause inaccuracies, so other mechanisms must be used in portable clocks.