# Rotation about a fixed axis

by roam
Tags: axis, fixed, rotation
 P: 916 1. The problem statement, all variables and given/known data A uniform metre-rule is pivoted to rotate about a horizontal axisthrough the 40-cm mark. The stick has a mass/unit length of $$\mu$$ kg/m and its rotational inertia about this pivot is $$0.093 \mu$$ kg/m2. It is released from rest in a horizontal position. What is the magnitude of the angular acceleration of the rod? 2. Relevant equations An expression for the magnitude of the torque due to gravitational force about an axis through the pivot: $$\tau =Mg \left( \frac{L}{2} \right)$$ Angular accleration and torque: $$\sum \tau = I \alpha$$ 3. The attempt at a solution Since $$\mu = \frac{M}{L}$$, and L=0.4m I get $$\tau = 0.4 \mu g \frac{0.4}{2} = 0.78 \mu$$ $$\alpha = \frac{\tau}{I} = \frac{0.78 \mu}{0.093 \mu} = 8.43$$ But my answer is wrong. The correct answer must be 10.5 rad/s2. Could anyone please help me? :(
Mentor
P: 41,588
 Quote by roam An expression for the magnitude of the torque due to gravitational force about an axis through the pivot: $$\tau =Mg \left( \frac{L}{2} \right)$$
Why L/2?

 Since $$\mu = \frac{M}{L}$$, and L=0.4m I get
L = 1 m. (It's a meterstick.)

What force provides the torque? How far does that force act from the pivot?
P: 916
 Why L/2?
Because the gravitational force on the stick acts at its center of mass.

Since the it is pivoted at the 40 cm mark, should I use 60/2 (since the mass is uniformly distributed)?

But this didn't work because I ended up with $$\alpha = 31.6$$...

 P: 101 Rotation about a fixed axis Typing error torque=I*alpha torque=Fr
Mentor
P: 41,588
 Quote by roam Because the gravitational force on the stick acts at its center of mass.
True, but to calculate the torque due to the weight you need its distance from the pivot. What is that distance?
P: 101
 Quote by Doc Al True, but to calculate the torque due to the weight you need its distance from the pivot. What is that distance?
distance=0.1 cm
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P: 41,588
 Quote by inky distance=0.1 cm
The question was meant for the OP, of course.