On which principle does seismograph work?
A) Principle of inertia
B) Principle of conservation of momentum
C) Principle of gravitational force.
D) None of the above.

Hint: Essentially seismograph was developed by a Chinese astronomer and mathematicians Chang Heng. It was invented in 132 A.D. It is used for measuring the earthquake. We might have heard of earthquakes having a Richter scale; this device also works like a seismograph.

Complete solution:
A seismograph is a device for measuring movements on the planet earth it consists of a sensor known as ground motion detection sensor, this sensor is known as seismometer it is coupled with a recording system so that movement in the Earth’s tectonics plate can be saved or recorded. In seismograph the relative motion of the pendulum and that of ground is recorded. It is equally possible to take the ratio between the deflection of the pendulum and the velocity of the ground. The ratio between the deflection of the pendulum and the velocity of the ground is called the velocity sensitivity of the ground. To get a correct reading of seismograph one has to negate its free oscillations otherwise it will mask the recordings that are taken during an earthquake. In other words, the pendulum has to stop free oscillations due to inertia and it does oscillations in the first place due to inertia.

Option (A) is correct. The principle of seismograph works on the principle of inertia.

Note: Here we need to define seismograph and its working, also we need to explain about how inertia plays a role in operating a pendulum and how it also assists in the free movement of the pendulum which indeed is taken care of.