Torque for rotating a shaft of unbalanced weight

[sekar_507]

1. Hi all. Can anyone please help me find the torque required for rotating a shaft which has two masses 1000 Kg each at 120 Degree angles. Please refer to the attachment for clarity.

Homework Equations

The Attempt at a Solution

 

[rude man]

I think you need the lengths of the two bars.

 

[sekar_507]

The bars are 500mm long each. How do I calculate the torque?

 

[rude man]

Torque = force times length.
So, what do you think you should use for length and force?

You sure the bars are of equal length? Looks like bar 1 is longer than bar 2.

 

[sekar_507]

should i use formula torque = I * Angular velocity for calculating the torque for rotating the shaft? Please clarify

 

[rude man]

No, since that's the formula for angular momentum, not torque.

 

[sekar_507]

ok then. since both the masses are at 120 deg angle. what is the total mass i should i consider. should i add 1000+1000 = 2000 Kg or should i calculate the resultant mass? can u help me in finding the resultant mass?

 

[rude man]

You never answered my question: are you sure the masses are of equal length?

 

[sekar_507]

Yes. Both the masses are of equal length.

 

[rude man]

OK, (though I'm still wondering why the drawing shows them as clearly of different lengths).

How about writing an expression for the torque about the axis of rotation as a function of angular position? You can call zero angle at any position you like.

Take one bar at a time. What's the torque gravity applies to one of the bars as a function of its angular position?