Question

A lift having a simple pendulum attached with its ceiling is moving upward with constant acceleration 'a'. What will be the tension in the string of pendulum with respect to a boy inside the lift and a boy standing on earth, mass of bob of simple pendulum is m.

Answer 1

F.B.D. of bob (with respect to ground)
T - mg = ma
T - mg + ma ......... (i)
With respect to boy inside the life, the acceleration of bob is zero.
So he will write above equation in this manner.
T - mg = m(0).
$\therefore$ T - mg
He will tell the value of tension in string is mg. But this is wrong. To correct his result, he makes a free body diagram in this manner, and uses Newton's second law.
T = mg + ma ....... (ii)
By using this extra force, equation (i) and (ii) give the same result. This extra force is called pseudo force. This pseudo force is used when a problem is solved with a accelerating frame (non-inertial).