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