As disc like reel with massless thread unrolls itself while falling vertically downwards the acceleration of its fall is
a)	g	b)		c)	Zero	d)

(d)

Let  be acceleration of fall of the thread, then net force acting downwards, balances the force due to tension  in the thread.

                                      …(i)

Also torque (also known as moment or couple acts on the system).

From Eq. (i),

                          …(ii)

Let  is moment of inertia of reel and  the angular acceleration, then torque is

                                                  …(iii)

where,

                         …(iv)

Equating Eqs. (ii) and (iv), we get