Rotation with pulleys

  • #1

Homework Statement


A block has a mass of 500 g, another block has a mass of 460 g. Both are attached to a pulley with a frictionless wire that does not slip. The pulley, which is mounted in horizontal frictionless bearings, has a radius of 5.00 cm. When released from rest, the heavier block falls 75.0 cm in 5.00 seconds. What is the magnitude of the blocks' accleration? What is the magnitude of the pulley's angular acceleration?

Fig: http://i212.photobucket.com/albums/cc150/deadfishfactory/1.jpg


Homework Equations


Torque = Moment of Inertia * Angular Acceleration
Force = Mass * Acceleration
Acceleration = Angular Acceleration / Radius
Moment of Inertia = (1/2)MR^2

The Attempt at a Solution


T=tension; W=weight; A=accleration; R=radius; I=moment of inertia
M(1) = left box; M(2) = right box; M(3) = pulley

A(1) = A(2) = a/R = A

1. T(1) - W(1) = m(1) * A; T(1) = W(1) + M(1) * A
2. T(2) - W(2) = -m(2) * A; T(2) = W(2) - M(2) * A
3. T(1)R - T(2)R = I * A / R
T(1) - T(2) = M(3) * A
W(1) + M(1) * A - W(2) + M(2) * A = M(3) * A
Solved for A.

Well, I started plugging things in and stuff, but I can't solve the problem because the mass of the pulley is unknown. I'm pretty sure I did it wrong so can someone point out my mistake or show me how to do it if I did it completely wrong? Thanks!

answer: a) 6.00 cm/s^2 b) 1.20 rad/s^2
 

Answers and Replies

  • #2
rl.bhat
Homework Helper
4,433
8
Using kinematic formula, find the acceleration of the falling body. Using this value find the angular acceleration of the pully. In this problem mass of any object is not needed
 
Last edited:
  • #3
Still having trouble finding the acceleration of the blocks.

I found the angular acceleration of the pulleys by using the constant acceleration equation like you said. I got 1.22 rad/s^2, which is close enough. I can't figure out how to find the acceleration of the blocks since I used gravity for the block's acceleration to find the angular acceleration. My physics skills aren't too good, can you explain it to me very precisely since I'm kinda slow.
 
  • #4
169
0
Heavier block falls 75.0 cm in 5.00 sec. (The system was released from rest.) Can you find A from the basic equations of Kinematics. (Remember those s-u-v-a-t equations??)
 
  • #5
Ohhhh. Pfft. When you said "heavier block falls 75.0 cm in 5.00 sec," it immediately smacked into me. Haha. I'm such an idiot for not noticing. Anyways, I got the answer. Thank you very much!
 

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