# Determining the length of a Torsional Pendulum

Currently working on this question, however I'm not sure how to solve it.

As a pendulum swings in simple harmonic motion at the surface of the Earth, the angle the pendulum makes relative to its equilibrium position is given by $\theta(t) = (0.140 \:\mathrm{rad})\cos(5.72t)$ where t is in seconds. What is the length of this pendulum?

a) 0.140 m

b) 0.250 m

c) 0.300 m

d) 0.439 m

e) 0.801 m

The equation provided indicates that this is a torsional pendulum, correct?

Therefore the length of the string with torsion supporting the disc at the bottom does not matter at all? If so, why?

• It sounds to me like a plain-old pendulum. It is swinging at a frequency of 5.72 radians/second or about .9 swings per second, and it started from an extreme position. It's maximum angle of swing is .14 radian or about 8 degrees. – Mike Dunlavey May 2 '13 at 1:04
• I've edited the question to follow our homework policy better. – Manishearth May 2 '13 at 23:23