- #1
dolpho
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Homework Statement
A 2.85 kg bucket is attached to a disk-shaped pulley of radius .121 m and mass .742 kg. If the bucket is allowed to fall, what is the linear acceleration, angular acceleration, and how far does it drop in 1.5 seconds.
I really only need help with the first part since the other two are pretty easy and depend on the first answer.
Homework Equations
t - mg = ma
alpha = a / r
The Attempt at a Solution
I'm a bit confused on which signs I should be using. This was my first try and I'd love some input to see where I went wrong.
T - mg = ma
Since we don't know or a we need to use the torque produced by the pulley.
torque = TR = I alpha
T= .5Mr^2 alpha / R
alpha = a / R
So then T = .5Ma ----> we plug this into T in the original equation
.5Ma - mg = ma,
.5(.742)(a) - (2.85)(9.8) = 2.85a
.371a - 27.93 = 2.85a
-27.93 = 2.85a - .371a
= -27.93 = 2.479a
a = - 11.26
Now I'm pretty sure I messed up on the signs somewhere. Wouldn't it all depend on how you set up the diagrams? So in this case the tension is pointing upwards so it's positive, and MG is - so it's negative. Also, how would you know if the pulley is going clockwise or counter clockwise?
Would love any clarification of how to set up the signs. Thanks guys!