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# Rotational Motion Web Assign (Physics C)

## Question

Hey guys, I have a question.

I'm having trouble figuring out part B of question 16 on the web assign:

Two objects are attached to ropes that are attached to wheels on a common axle as shown in Figure 9-55. The two wheels are glued together so that they form a single object. The total moment of inertia of the object is 39 kg·m2. The radii of the wheels are R1 = 1.1 m and R2 = 0.3 m.

(a) If m1 = 24 kg, find m2 such that there is no angular acceleration of the wheels.

( If 12 kg is gently added to the top of m1, find the angular acceleration of the wheels.

Find the tension in the rope holding m1.

Find the tension in the rope holding m2.

Ok, so - I found the weight needed to counteract the weight on the bigger wheel. In my example, you would need a mass of 88 kg on the smaller wheel. If you add 12 kg to the first weight, isnt that only mass that would change the net torque? So couldnt you find the angular acceleration by :

$\tau _{net} = (12kg)(g)(r_{1}) = I \alpha$

When I get:

$\alpha = (\frac{(12)(10)(1.1)}{39})= 3.38 rad/s^2$

(the 39 is the Moment of intertia the problem is giving me.)

Where am I going wrong?

## Recommended Posts

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Keep in mind that you have two torques still on the wheel, so:

${\tau _{net}} = {T_1}{R_1} - {T_2}{R_2} = I\alpha$.

Combine that with Newton's 2nd Law equations for both mass 1 and mass 2 and you'll have a system of equations you can solve to obtain your answer. :banghead)

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Yes, so wouldnt i do:

[(36)(g)(1.1)-(88)(g)(.3)]/39 because i get the same answer when i do that.

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oh wait, you mean T1 and T2 as in tension?

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Exactly... T1 as in the tension due to mass 1, and T2 is the tension due to mass 2. Also, don't forget that you need to use 9.8 for g in the WebAssign problems.

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