Question

A uniform rod of length l and mass M is pivoted at the centre. Its two ends are attached to two ends are attached to two springs of equal spring constant k. the springs are fixed to rigid ruppot as shown in figure and the rod is free to oscillate in the horizontal plane. the rod is gently pushed through a small angle `theta` in one direction and relased. the frequency of oscillation is
A. `(1)/(2pi)sqrt((2k)/(6M))`
B. `(1)/(2pi)sqrt((k)/(M))`
C. `(1)/(2pi)sqrt((6k)/(M))`
D. `(1)/(2pi)sqrt((24k)/(M))`

Answer 1

Correct Answer - C

If the rod is rotated through an angle `theta`, extension in one spring=compression in the other spring,
i.e., `x=l theta//2`
therefore, force acting on each of
the ends of the rod,
`F=kx=k(ltheta//2)`
Restoring torque on the rod,
`tau=-Fl=-k(ltheta//2)l=-kl^(2)(theta//2)`
As `tau=Ialpha=(Ml^(2)//12)alpha`,
`-kl^(2)(theta//2)=(Ml^(2)//12)alpha`
or `alpha=-(6k)/(M)theta`, i.e., `alpha proptheta`
thus, the motion of the rod is simple harmonic with
`omega^(2)=(6k)/(M)` or `omega=sqrt((6k)/(M))` or `upsilon=(1)/(2pi)sqrt((6k)/(M))`