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Angular speed question please help.

  • Thread starter jj8890
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1. Homework Statement
As a result of friction, the angular speed of a wheel changes with time according to
d θ/d t = ω_0 e^(−σ t) ,
where ω0 and σ are constants.
initial angular speed = 5.45 rad/s
σ =.23913
Determine the number of revolutions the wheel makes after 2.33 s .
Answer in units of rev.


3. The Attempt at a Solution

I know I have to take the integral which is:
θ = - w_0/(σ *e^(-σ *t))
My numbers are initial velocity= 5.45 rad/sec, sigma= .023913, and time=2.33 seconds. so when I solved I got -13.0589 radians and divided it by 2pi to get revolutions, which gave me -2.07839. I thought the answer would be 2.07839 because revolutions can't be negative but it is not right, should i use the negative or am i doing something wrong?
 

Answers and Replies

tiny-tim
Science Advisor
Homework Helper
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Determine the number of revolutions the wheel makes after 2.33 s .
Answer in units of rev.

{snip} … so when I solved I got -13.0589 radians and divided it by 2pi to get revolutions, which gave me -2.07839. I thought the answer would be 2.07839 because revolutions can't be negative but it is not right, should i use the negative or am i doing something wrong?
Hi jj! :smile:

I'm with you on this: the question is badly worded. :frown:

The wheel makes 2.07839 revolutions clockwise!

However, that's clearly not the answer they want - if this is a computer thing where you're only allowed to put in a number, then you must put in the number they want - this isn't a constitutional rights case!

But if you're allowed a full answer, write something like "-2.07839 revolutions in the direction of increasing θ (2.07839 revolutions in the direction of decreasing θ)" - with the answer they want first. :smile:
 

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