Average angular acceleration back for more

[nutster]

Hey again. It's that time of week, and once again I'm stumped. There are three problems, but we'll take them one at a time. The first is as follows:

A certain CD has a playing time of 68 minutes. When the music starts, the CD is rotating at an angular speed of 4.8x10^2 revolutions per minute (rpm). At the end of the music, the CD is rotating at 2.1x10^2 rpm. Find the magnitude of the average angular acceleration of the CD. Express your answer in rad/s^2.


What I've tried is to take the difference of the angular displacement (each divided by 60 to change to seconds), multiplied this by 2pi to change to radians, and divide that difference by 68 minutes (4080 seconds). The answer I get is -0.0069 rad/s^2...but this is wrong

Here's the 'layout':

((210rpm-480rpm)/60s)*2pi)/(4080s)=-0.0069

What have I done wrong??

Thanks in advance.

 

[TenaliRaman]

The only problem is that u have got a wrong sign, otherwise everything else is fine.

-- AI

 

[HallsofIvy]

Apparently, you did the arithmetic wrong! You analysis looks good but according to my calculator, ((210- 480)/60)(*2 pi)/4080)= -0.01206., not -0.0069.

 

[Doc Al]

Looks OK to me. Why do you think it's wrong?

 

[nutster]

Tenali had it correct...the sign was backwards. I'm still not sure why that is, but it was correct. On to my second, relatively unrelated question:

A 65.0 kg person, running horizontally with a velocity of +4.06 m/s, jumps onto a 10.4 kg sled that is initially at rest.
(a) Ignoring the effects of friction during the collision, find the velocity of the sled and person as they move away.

Which I figured out was 3.5 m/s...now my question:

(b) The sled and person coast 30.0 m on level snow before coming to rest. What is the coefficient of kinetic friction between the sled and the snow?

Any suggestions on how to go about solving this?

 

[arildno]

Well, if Tenali is right, then the answer is wrong.
The disc is SLOWING DOWN from 480rpm to 210rpm.

 

[nutster]

That's what I thought...as the work I did shows. Down with WebAssign! Any thoughts on the second problem?

 

[nutster]

Better idea...I'll post it to a new thread.

 

[TenaliRaman]

Honest! i did not set this question!

-- AI