F.5 phy circular motion

The motion of the sticky tape is given by,

T.cos(theta) + Ff = mR.w^2
where T os the tension in the threadc, (theta) is the angle between the thread and the horizontal, R is the radius of the circular motion, w is the angular speed.
Ff is the friction acts on the thread by the rim of the glass tube

Since, T = Mg, R = (L+a).cos(theta), where a is the radius of the sticky tape
w = 2.pi/(t/20) = 40.pi/t
thus, Mg.cos(theta) + Ff = m.Lcos(theta).[40.pi/t]^2
i.e. Mg +Ff = 1600.(pi)^2.m(L+a)/t^2
Mg = 1600(pi)^2.m(L+a)/t^2 - Ff

The constant b is thus the frictional force Ff