# Pendulum: Gravity and Other Forces

Most recent answer: 05/08/2018

Q:
For a pendulum moving on a satellite , g is zero as both the satellite as well as the pendulum are attracted by the same force.Then , if we keep the same pendulum setup on a stand on the surface of the earth, bot the stand as well as the bob of the pendulum are attracted by the same force, why is g here, not zero(and thus time period of pendulum infinite ?) ?
- Rajesh P (age 16)
Delhi,India
A:

The key is the "normal" force from th earth on the stand of the pendulum.  Because the nromal force is just big enough to kep the stand from accelerating into the earth depsite gravity, its magnitude is set by the gravitational force on the pendulum.

That normal force isn't  there in space. If you were to put the pendulum on an accelerating  rocket that exerted the same force on the stand, the pendulum would behave just like it does on earth.

Mike W.

(published on 05/08/2018)

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