(adsbygoogle = window.adsbygoogle || []).push({}); 3 Questions about Rotational Motion; Test Tomorrow!! Please Help!

Problem 1

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

A uniform disk of radius .12m and mass 5kg is pivoted so that it rotates freely about its axis. A string wrapped around the disk is pulled with a force of 20N.

a. What is the torque exerted on the disk?

b. What is the angular acceleration of the disk?

c. If the disk starts from rest, what is the angular velocity after 3s?

2. Relevant equations

τ = rFsinθ

I=1/2MR^2 for a uniform disk about its center

T=I(alpha)

3. The attempt at a solution

a. τ = rFsinθ

= (.12)(20)(sin 90)

= 2.4 NM

b. T=I(alpha)

T=.5MR^2(alpha)

2.4=.5(5)(.12^2)(alpha)

alpha=66.66 rad/s^2

c. wf-wi=(alpha)(t)

wf=(66.66)(3)

=199.98

Is that right??

Problem 2

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

A pulley of radius R1 and rotational inertia I1 is mounted on an axle with negligible friction. A light cord passing over the pulley has two blocks of mass m attached to either end, as shown above. Assume that the cord does not slip on the pulley. Determine the answers to parts (a) and (b) in terms of m, R1, I1, and fundamental constants.

a. Determine the tension T in the cord.

b. One block is now removed from the right and hung on the left. When the system is released from rest, the three blocks on the left accelerate downward with an acceleration g/3 . Determine the following.

i. The tension T3 in the section of cord supporting the three blocks on the left

ii. The tension Tl in the section of cord supporting the single block on the right

iii. The rotational inertia I1 of the pulley

Here's the picture:

http://courses.ncssm.edu/aphys/problems/AP%20Problems/ap160809.doc [Broken] 1st problem.

2. Relevant equations

I am very confused. I do not know how to answer this problem. I assume I must relate the tension to the torque so:

τ = rFsinθ

=(Tension)(radius)

But how would I know that the tension is the force I should use? What about the force of gravity on the two masses? How do you do this???

Problem 3

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

A light string that is attached to a large block of mass 4m passes over a pulley with negligible rotational inertia and is wrapped around a vertical pole of radius r. The system is released from rest, and as the block descends the string unwinds and the vertical pole with its attached apparatus rotates. The apparatus consists of a horizontal rod of length 2L, with a small block of mass m attached at each end. The rotational inertia of the pole and the rod are negligible. A) Determine the rotational inertia of the rod-and-block apparatus attached to the top of the pole. B) Determine the downward acceleration of the large block. C) When the large block has descended a distance D, how does the instantaneous total kinetic energy of the three blocks compare with the value 4mgD? (greater, equal, less)

Here's the picture:

http://www.physics4students.com/pages/apc/apcaday/apcaday_040.doc [Broken]

1st problem

2. Relevant equations

W-T=4ma

Once again, I just don't know how to approach this problem. I'm sorry.

Could someone just walk me through the initial steps? I need to get the answer myself.

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# Homework Help: 3 Questions about Rotational Motion; Test Tomorrow

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