Rotation about a fixed axis


by roam
Tags: axis, fixed, rotation
roam
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#1
Jun17-10, 07:30 PM
P: 884
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 [tex]\mu[/tex] kg/m and its rotational inertia about this pivot is [tex]0.093 \mu[/tex] 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: [tex] \tau =Mg \left( \frac{L}{2} \right)[/tex]

Angular accleration and torque: [tex]\sum \tau = I \alpha[/tex]

3. The attempt at a solution

Since [tex]\mu = \frac{M}{L}[/tex], and L=0.4m I get

[tex]\tau = 0.4 \mu g \frac{0.4}{2} = 0.78 \mu[/tex]

[tex]\alpha = \frac{\tau}{I} = \frac{0.78 \mu}{0.093 \mu} = 8.43[/tex]

But my answer is wrong. The correct answer must be 10.5 rad/s2. Could anyone please help me? :(
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Doc Al
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#2
Jun17-10, 07:36 PM
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Quote Quote by roam View Post
An expression for the magnitude of the torque due to gravitational force about an axis through the pivot: [tex] \tau =Mg \left( \frac{L}{2} \right)[/tex]
Why L/2?

Since [tex]\mu = \frac{M}{L}[/tex], 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?
roam
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#3
Jun18-10, 07:04 AM
P: 884
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 [tex]\alpha = 31.6[/tex]...

inky
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#4
Jun18-10, 07:49 AM
P: 101

Rotation about a fixed axis


Typing error

torque=I*alpha
torque=Fr
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#5
Jun18-10, 07:50 AM
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Quote Quote by roam View Post
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?
inky
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#6
Jun18-10, 07:53 AM
P: 101
Quote Quote by Doc Al View Post
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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#7
Jun18-10, 08:48 AM
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Quote Quote by inky View Post
distance=0.1 cm
The question was meant for the OP, of course.
(And your units are off.)
inky
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#8
Jun18-10, 05:36 PM
P: 101
Quote Quote by Doc Al View Post
The question was meant for the OP, of course.
(And your units are off.)
I am sorry for my wrong units. Actually I should write r=0.1 m. Thanks so much.


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