# Homework Help: Pulleys, torque and tension

1. Apr 11, 2010

### PhrozenN

A 1.2 kg mass and a 7.1 kg mass are connected by a massless string over a pulley that is in the shape of a bicycle wheel (all mass is concentrated at the rim) having a radius 0.25 m and mass 4.6 kg. Determine the time for the masses to move 2.6 m starting from rest.

A.
2.714 sec
B.
0.223 sec
C.
1.55 sec
D.
1.68 sec
E.
1.077 sec

T1 = 7.1kg, T2 = 1.2kg

T1 - m1g = -m1a
T2 - m2g = m2a
T1R - T2R = I(alpha)

a=R(alpha)

I was just wondering if I set these equations properly. I substituted and got an answer similar to E except its off a little even though I didn't round. Thanks in adv!

2. Apr 11, 2010

### rl.bhat

Hi PhrozenN, welcome to PF.
Unless you show your calculations we are not in a position to point out your mistakes.
So please show your calculations. By the way, which expression you have used for the moment of inertia for the pulley?

3. Apr 11, 2010

### PhrozenN

I = mR^2 since its a rim-like thingy

T1R - T2R = I(a/R)
m1(g-a)R - m2(g+a)R = mRa
a = 4.44 m/s^2

x = vt + 0.5at^2
t = 1.082

4. Apr 11, 2010

### rl.bhat

Check the calculation of a. I am getting the correct answer.

5. Apr 11, 2010

### PhrozenN

Oh, I did the calculations wrong
I got a = 4.48 so now it's 1.077

The equations are set properly right? Thanks btw