Angular Deceleration Homework: 1200 Revs in 40s

Click For Summary

Homework Help Overview

The problem involves a circular saw blade that completes 1200 revolutions in 40 seconds while decelerating to a stop. The subject area pertains to angular motion and kinematics, specifically focusing on angular deceleration and initial angular speed.

Discussion Character

  • Mixed

Approaches and Questions Raised

  • Participants discuss the need for kinematic equations that incorporate time and angular displacement. There are attempts to clarify the relationship between average angular velocity and initial angular speed. Some participants express confusion about the definitions of angular displacement and the implications of constant deceleration.

Discussion Status

There is ongoing exploration of the problem, with various participants suggesting different approaches to relate known variables to unknowns. Some guidance has been offered regarding the use of kinematic equations, but there remains uncertainty about the initial conditions and the uniqueness of the solution.

Contextual Notes

Participants note that the problem does not provide explicit information about the initial angular speed, leading to questions about the sufficiency of the given data to arrive at a unique solution. The discussion reflects a mix of interpretations regarding the physics involved.

  • #31
Yeah I edited that out. Anyway, you're only wasting your time with this problem.

Who is? Why is it a waste of time?


why...i still don't have the answer, and I'm sure there is one...

There is.
 
Physics news on Phys.org
  • #32
hage567, I'll give you all my goods if you find me an answer. :wink:
 
  • #33
hage567 said:
Who is? Why is it a waste of time?




There is.

alright hage, your really helping me out a lot here, but i am still stuck, I'm not sure where to go from here...i've got two variables and two unknowns, so i try plugging one equation into another, but i keep getting stuck with nothing left.
 
  • #34
i did end up with 60/89 or .674 to be my angular velocity...does that make sense.?..i really don't want to write down all my steps.
 
  • #35
xXmarkXx, how to convince you... look if you chose the initial velocity arbitrarily, you could very well chose an acceleration that will make it stop in 1200 revolutions and 40 seconds. This analogous to stopping a linearly moving body on a distance of 1200*2pi and on spam of 40 seconds. For any initial velocity, there is a different acceleration that does that.
 
  • #36
xXmarkXx said:
i did end up with 60/89 or .674 to be my angular velocity...does that make sense.?..i really don't want to write down all my steps.

No. The initial velocity is approximately 188 and 0.674 < 188. 188 is inferior to the initial velocity since it is the average during deceleration. This makes 0.674 way off.
 
Last edited:
  • #37
Werg22, I will send you my solution by PM. Please let me know what you don't like about my method.
 
  • #38
No problem.
 
  • #39
hage567 said:
Werg22, I will send you my solution by PM. Please let me know what you don't like about my method.


do you have any other suggestions for me?

allright, so you posted 2400pie=1/2alpha40^2 + w040
and i used -w0=alpha*40

when i plug that into solve for alpha, i keep getting stuck.
 
  • #40
i have now arrived at 40pie revolutions/sec as the angular velocity. agree?? disagree??
 
  • #41
xXmarkXx said:
i have now arrived at 40pie revolutions/sec as the angular velocity. agree?? disagree??

It is best to make sure everything is expressed in RADIANS, not revolutions, before you start. Angular speed is expressed as rad/s.

So I disagree.
 
  • #42
hage567 said:
It is best to make sure everything is expressed in RADIANS, not revolutions, before you start. Angular speed is expressed as rad/s.

So I disagree.


ok, i ended up at 40pie=w0 i just labeled it incorrectly. But you still disagree?? if so, could you point me in the correct direction.
 
  • #43
hage567, wait up, you're solution is wrong all around, I'm writing you why.
 
  • #44
anybody still willing to help me out?
 
  • #45
hage, where did you get 1400*2pie...is it not 1200 revolutions? in 40 sec? so wouldn't it be 1200*2pie?
 
  • #46
xXmarkXx said:
hage, where did you get 1400*2pie...is it not 1200 revolutions? in 40 sec? so wouldn't it be 1200*2pie?

I don't think I ever said that.
 
  • #47
Werg22 said:
hage567, I believe you're wrong. Angular velocity and acceleration are the equivalent of linear acceleration and velocity along the circumference of the unit circle. This means that if we represent the angle covered by a revolving body on a straight line, we would have case of linear acceleration. For example, if we have the number of revolutions per sec before deceleration, k we would have an angular velocity of v = 2{\pi}k. If the body came to a stop in 40 sec, the angle covered is 1400*2{\pi}

The angular acceleration, a, would then depend on of this velocity since we have by linear acceleration,

1400*2{\pi} = \frac{1}{2}a 40^{2} + v*40

Obviously we see that the acceleration is dependent on the choice of the initial velocity.



where did the 1400*2pie come from??
 
  • #48
Ok, hage, I have to apologize sincerely. Your solution was right, and I was completely wrong. I had disregarded that the fact that it comes to a stop dictates the solution. Sorry buddy, my bad. xXmarkXx, just set two equations.

0 = v_{i} + 40a

and

1200*2{\pi} = \frac{1}{2}a40^{2} + 40v_{i}

You have two equations and two variables, and you solve. The 1200*2pi comes from the fact that at each revolution, the disk covers 2pi radians. So for 1200 revolutions, it will cover 1200*2pi radians. Once again, sorry hage, I got cocky and stupid.
 
  • #49
right, i end up with the angular acceleration to be just pie.. vi=-40a, then i plugged that into the other equation and solved for a. which is pie. Is that what you guys got??
 
  • #50
No hard feelings, Werg22. :smile:
 
  • #51
and now i used pie=a and solved for the angular velocity, which i had gotten before to be equal to 40*pie...agreed??
 
Last edited:
  • #52
my fault, i checked it over again and now i got -3pie = angular acceleration. Would you guys at least tell me if i am on the right track or not?
 
  • #53
xXmarkXx said:
my fault, i checked it over again and now i got -3pie = angular acceleration. Would you guys at least tell me if i am on the right track or not?

Yes, you appear to be on the right track.
 
  • #54
and then i got 120pie = w0
how bout that? correct o mundo? Thanks hage and werg for the help and being patient with me. I'm a slow learner.
 
  • #55
Looks good to me.

You're welcome. :smile:
 

Similar threads

[Mechanics] Friction decelerating a helicopter rotor assembly
  • · Replies 1 ·
Replies
1
Views
2K
I please Angular deceleration
  • · Replies 4 ·
Replies
4
Views
2K
How to Determine Angular Deceleration and Initial Speed of a Coasting Saw Blade?
  • · Replies 1 ·
Replies
1
Views
3K
Angular Acceleration Homework: 962 Revolutions in 10 Secs
  • · Replies 1 ·
Replies
1
Views
4K
Finding angular acceleration from deceleration
  • · Replies 2 ·
Replies
2
Views
2K
Rotation w/constant angular acceleration
  • · Replies 2 ·
Replies
2
Views
3K
Conservation of Angular momentum problem
  • · Replies 46 ·
2
Replies
46
Views
5K
Angular Deceleration Calculation for Medical Lab Centrifuge
  • · Replies 3 ·
Replies
3
Views
2K
Calculate the magnitude of angular acceleration
  • · Replies 5 ·
Replies
5
Views
3K
Angular Deceleration of a Bicycle Wheel
  • · Replies 5 ·
Replies
5
Views
12K