# Pendulum Question

The pendulum consists of a uniform disk with radius r = 10 cm and mass 900 g attached to a uniform rod with length L = 500 mm and mass 100 g.

(a) Calculate the rotational inertia of the pendulum about the pivot.
kgm2

(b) What is the distance between the pivot and the center of mass of the pendulum?
m

(c) Calculate the period of oscillation.
s

First, since this seems like a simple pendulum, I calculated the time of oscillation using the equation T = 2pi SqRt(l/g)
T = 2pi SwRt(0.5/9.81)
T = 1.4185 s which was incorrect.

Once I had the time of oscillation, I was plannning on finding the center of mass using the equation I = T^2MgR/4pi^2

Can someone please help me find the time for oscilation and tell me if my second thoughts are correct.

## Answers and Replies

Doc Al
Mentor
physical pendulum, not simple pendulum

First, since this seems like a simple pendulum, I calculated the time of oscillation using the equation T = 2pi SqRt(l/g)
It's not a simple pendulum, but what is called a physical pendulum. There's a reason that parts (a) & (b) come first--those quantities are needed to find the period of a physical pendulum.