Rotational Motion

by simplygenuine07
Tags: motion, rotational
simplygenuine07 is offline
Jul8-07, 03:11 AM
P: 2
1. The problem statement, all variables and given/known data

A block (mass = 2.4 kg) is hanging from a massless cord that is wrapped around a pulley (moment of inertia = 1.5 x 10-3 kgm2), as the figure shows. Initially the pulley is prevented from rotating and the block is stationary. Then, the pulley is allowed to rotate as the block falls. The cord does not slip relative to the pulley as the block falls. Assume that the radius of the cord around the pulley remains constant at a value of 0.032 m during the block's descent.
Find (a) the angular acceleration of the pulley and (b) the tension in the cord.

2. Relevant equations

Newtons Second law and Newtons second law of rotation
F=ma and Torque=Ialpha

3. The attempt at a solution
I tried using this equation, but i get the wrong answer no matter what I do.
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benzun_1999 is offline
Jul8-07, 05:05 AM
P: 258
i think it must be


because the tension is opposite to the gravitational pull......
and where is the pic???
huyen_vyvy is offline
Jul8-07, 07:28 AM
P: 25
ma=mg-T => a=g-T/m (1)
also, a=alpha*radius of pulley and alpha= Torque/inertia = T*radius of pulley/inertia of pulley
=> a=radius^2*T/inertia (2)
from 1 and 2, hopefully we can find the answer. I'm not sure why they mention the radius of the cord?
Tell me if it works out, I didn't have time to actually solve it myself.

simplygenuine07 is offline
Jul8-07, 01:25 PM
P: 2

Rotational Motion

Yes thankyou, that helped alot!!

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