# Weight causing a wheel to rotate

1. Dec 7, 2011

### JamesGold

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

2. Relevant equations

I_solidcylinder = .5mr^2
I_thinring = mr^2
torque = I*Ω (where Ω = angular acceleration)
change in theta = .5(Ω)(time^2)

3. The attempt at a solution

Attempted to solve for angular acceleration:

(2.5)(9.8)(0.1) = (3*0.25^2 + 0.5*4*0.1^2)Ω

Ω = 11.8 rad/s/s

which I then plugged into the rotational kinematics formula:

Δ∅ = 0.5*11.8*0.5^2

which gave me a wrong answer. Where'd I go wrong?

2. Dec 7, 2011

### grzz

The torque causing angular acceleration is provided by the tension in the string.
But is this tension equal to the weight of the hanging mass?
Note that the hanging mass is accelerating downwards.

3. Dec 7, 2011

### JamesGold

I don't see why it wouldn't be. Can you explain why it isn't?

4. Dec 7, 2011

### JHamm

What if you had two blocks connected by a string and dropped them over a balcony, would the string have a tension equal to the weight of the lower block?

5. Dec 7, 2011

### grzz

Is the hanging mass accelerating downwards?

6. Dec 7, 2011

### JamesGold

I'm not sure. Wouldn't it depend on the relative weights of the two blocks?

Yes.

7. Dec 7, 2011

### grzz

So if the hanging mass is accelerating which is the greater force, the tension upwards or the weight of the mass downwards?

8. Dec 7, 2011

### JamesGold

The weight of the mass downwards.

9. Dec 7, 2011

### grzz

Correct.

10. Dec 7, 2011

### JamesGold

So mg - T = ma?

11. Dec 7, 2011

### grzz

Hence the tension is not equal to the weight.

The torque is provided by the tension and this tension is still unknown.

But one can obtain another equation so that one can find the tension.

12. Dec 7, 2011

### grzz

exactly.

13. Dec 7, 2011

### JamesGold

Then how do you find the acceleration of the block?

14. Dec 7, 2011

### grzz

The linear acceleration is related to the angular acceleration.

Note that

for distance there is, x = rθ
hence for velocity there is, v = rω
and for acceleration there is, a = rα

15. Dec 7, 2011

### grzz

I used α instead of Ω for angular acc

16. Dec 7, 2011

### JamesGold

Right... but without knowing the tension of the string, I don't see how we can calculate the angular acceleration of the wheel or the acceleration of the block.

17. Dec 7, 2011

### grzz

We have two equations:

1.....torque eqn for rotational acc

2.....f = ma for linear acc

get linear acc 'a' in terms of the ang acc

and then eliminate tension from these two equations.

18. Dec 7, 2011

### JamesGold

Okay, so I have now:

String tension * moment arm = I of system * ang acc

[ (2.50)(9.80) - (2.50)(0.1)α ] * 0.1 = (0.02 + 0.1875) * α

Is this correct?

19. Dec 7, 2011

### grzz

Yes.

20. Dec 7, 2011

### JamesGold

Got the right answer. I can't thank you enough, selfless sir.