A pendulum is hanged in the car.
Car starts to move by constant acceleration
How to measure acceleration by angle the pendulum?

Asked Afshin

Answer

If the pendulum makes an angle θ with the vertical, then a = g tan θ

This is based on the idea of pseudo force experienced by a body in an accelerated frame of reference.

When a body is in an accelerated frame of reference, it feels as if it is acted upon by a force equal to its mass multiplied by the acceleration of the system. Please have a look at the diagram below.

Please feel free to ask further doubts on the topic via comments