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Calculating amount of revolutions

by smpolisetti
Tags: beginning, high speed drill, revolutions, rpm
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smpolisetti
#1
Nov17-10, 09:29 PM
P: 10
1. The problem statement, all variables and given/known data
A high-speed drill reaches 2760 rpm in 0.260 s. Through how many revolutions does the drill turn during this first 0.260 s?



2. The attempt at a solution

UPDATED:

Here's what I have right now

2760 rpm * (2n/1 rev) * (60 s / 1 min) = 1040495.49 rad/s

1040495.49 rad/s * 0.260 s = 270,528.83 radians

270,528.83 radians * (1 rev / 2n) = 43,056 revolutions

Is that right? I haven't put the answer in because I have a limited amount of tries but I want to make sure I did it right.
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rock.freak667
#2
Nov17-10, 09:33 PM
HW Helper
P: 6,202
Quote Quote by smpolisetti View Post
To calculate the amount of revolutions I divided rpm to convert it by seconds and then multiplied by 0.260 seconds but that's wrong.

when you converted rpm you got rad/s. So multiplying that by 0.26s will give you the radians it moved.

now you know that 2π rad = 1 rev.

You need to do another conversion to get the revolutions.
smpolisetti
#3
Nov17-10, 09:37 PM
P: 10
I divided 11.96 by 2pi and got 1.90 revolutions, but the computer program says that's wrong. What's my mistake?

americanforest
#4
Nov17-10, 09:39 PM
P: 218
Calculating amount of revolutions

You know the final (and initial) angular velocity and the time it took to get there. With this you can get the angular acceleration. Given that, you can find how many revolutions it traversed in the given time.
rl.bhat
#5
Nov17-10, 09:44 PM
HW Helper
P: 4,433
Hi smpolisetti, welcome to PF.
in the problem. initial angular velocity is zero and final angular velocity = 2760*2π/60 rad./s.
Find the angular acceleration using ω = ωο + α*t.
Then find the angular displacement using θ = ωο*t + 1/2*α*t^2
smpolisetti
#6
Nov17-10, 09:44 PM
P: 10
I know that the acceleration is 1110 rad/s/s but I don't know how to get the amount of revolutions from that
rock.freak667
#7
Nov17-10, 09:49 PM
HW Helper
P: 6,202
Quote Quote by smpolisetti View Post
I know that the acceleration is 1110 rad/s/s but I don't know how to get the amount of revolutions from that
no no 2760 rpm you have.

1 rpm = 2π/60 rad/s

you do not need angular acceleration.

Convert the rpm to rad/s and then multiply by the 0.26 sec.
smpolisetti
#8
Nov17-10, 10:07 PM
P: 10
Here's what I have

2760 rpm * (2n/1 rev) * (60 s / 1 min) = 1040495.49 rad/s

1040495.49 rad/s * 0.260 s = 270,528.83 radians

270,528.83 radians * (1 rev / 2n) = 43,056 revolutions

Is that right? I haven't put the answer in because I have a limited amount of tries but I want to make sure I did it right.
rl.bhat
#9
Nov17-10, 10:22 PM
HW Helper
P: 4,433
Angular acceleration = 1100 rad/s/s.

θ = ωο*t + 1/2*α*t^2

θ = 1/2*1100*(0.26)^2

find θ and then find n.
smpolisetti
#10
Nov17-10, 10:35 PM
P: 10
Thanks so much!!


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