Rigid rod

1. Jan 13, 2010

zhenyazh

hi,
the question is in the image attached.
i just don't get the correct answer.
what i did was count the total torque in respect to the pivot and equal it to i*alfa.
maybe i am getting the signs and direction wrong or maybe i am misplacing the masses.
can some one please write the equation so i could compare?

thanks
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

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2. Jan 13, 2010

rock.freak667

You need to find the torque produced when the rod moves through the given angle. Do you know the formula τ=rFsinθ ? (τ=torque)

3. Jan 13, 2010

zhenyazh

this is exactly the point.
i know what i should write. but i guess i may be making a mistakes with the minuses or pluses
or with the sizes of angles...
this is why it could be great to see how it should be and from that understand where i was wrong

4. Jan 13, 2010

rock.freak667

Draw the rod at the angle of 51.1 and then find the perpendicular distance of the force from the pivot point. Multiply those two and then you can equate that to $I \alpha$