Angular velocity, can someone help?

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Homework Help Overview

The discussion revolves around a problem involving a laboratory centrifuge, specifically focusing on calculating the average angular acceleration given the rotor radius, initial angular velocity in rpm, and the number of revolutions during deceleration. The subject area pertains to rotational motion and angular kinematics.

Discussion Character

  • Exploratory, Assumption checking, Conceptual clarification

Approaches and Questions Raised

  • Participants discuss converting rpm to radians per second as a starting point. There are attempts to apply equations related to constant angular acceleration, with some questioning the relevance of the radius in the calculations. Others raise concerns about the units used in the calculations.

Discussion Status

The discussion is ongoing, with participants exploring different interpretations of the problem. Some guidance has been offered regarding the use of standard equations for angular motion, but there is no explicit consensus on the role of the radius or the correctness of the calculations presented.

Contextual Notes

Participants note that the radius may not be necessary for the calculations at hand, suggesting that it could be a source of confusion. There is also mention of the need for clarity regarding the units used in the calculations.

knowledgerich
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Homework Statement


a laboratory centrifuge has rotor radius (from center of rotor to sample) of 12.0 cm and typically spins at a rate of 2000 rpm. When switched off, it turns through 50.0 revolutions in coming to rest. Find the value of its average angular acceleration.

Homework Equations


The Attempt at a Solution


so far I only turned the rpm to radians/s which is the angular velocity but I wasn't sure what to do next
 
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welcome to pf!

hi knowledgerich! welcome to pf! :smile:
knowledgerich said:
a laboratory centrifuge has rotor radius (from center of rotor to sample) of 12.0 cm and typically spins at a rate of 2000 rpm. When switched off, it turns through 50.0 revolutions in coming to rest. Find the value of its average angular acceleration.

so far I only turned the rpm to radians/s which is the angular velocity but I wasn't sure what to do next

you have initial speed, final speed, and total distance, and the question asks for acceleration

sooo … just use one of the standard constant acceleration equations (but using angle instead of distance) :wink:
 
is the radius used in this equation? I came up with (-33^2 rad/s)/(2*100π) = -3.5 rad/s^2
 
knowledgerich said:
is the radius used in this equation? I came up with (-33^2 rad/s)/(2*100π) = -3.5 rad/s^2
No, I think the radius was given you to confuse you. (There ought to be more questions like that IMO.)
What are the units of the '33'?
 
knowledgerich said:
is the radius used in this equation?

the radius would only be needed if they gave you the tangential (linear) speed instead of the angular speed …

eg if they said a wire was coming off the wheel at so-many metres per second :wink:
 

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