Power and torque what did i do wrong?

  • Thread starter Thread starter ahello888a
  • Start date Start date
  • Tags Tags
    Power Torque
Click For Summary

Homework Help Overview

The problem involves a rotating uniform cylindrical platform that decelerates from a specified speed when the motor is disconnected. Participants are tasked with estimating the power output required to maintain the platform's speed before disconnection, using principles of work and energy in rotational motion.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning, Problem interpretation

Approaches and Questions Raised

  • Participants discuss the calculation of work done by friction and its relation to the power output of the motor. There are attempts to clarify the conversion of units from revolutions per second to radians per second. Questions arise regarding the correctness of the initial calculations and the method used to determine power.

Discussion Status

The discussion is ongoing, with participants questioning the accuracy of the calculations and the approach taken. Some suggest that the method may not account for the changing power due to the deceleration of the platform, indicating a need for further exploration of instantaneous power in rotational dynamics.

Contextual Notes

Participants note the importance of unit conversion and the implications of using average versus instantaneous power in their calculations. There is an acknowledgment of the complexity involved in the problem setup, particularly regarding the dynamics of the system during deceleration.

ahello888a
Messages
23
Reaction score
0
power and torque what did i do wrong?

Homework Statement


A rotating uniform cylindrical platform of mass 230 kg and radius 5.6 m slows down from 3.8 rev/s to rest in 16 s when the driving motor is disconnected. Estimate the power output of the motor (\rm hp) required to maintain a steady speed of 3.8 rev/s


Homework Equations


Wfriction=change in KE = -.5*I*w^2
power=work/time


The Attempt at a Solution


So I was was able to get the amount of work that the friction force does and it was -.5 (.5 * MR^2)w^2 = 45053.39J and that work done by friction is the same amount of work needed for the motor to output. So the power of the motor needed is 45053.39/16s = 2690.8Watts and to convert that to HP I just divide by 746W and got 3.6HP. is that right?
 
Physics news on Phys.org


I fear the calculation for energy is not correct, Please recalculate. (Don't forget to convert rev/s into radians/s)
 


is it 86.12HP? I guess I did forget to do that conversion, but is the method correct?
 


HI ahello888a,

ahello888a said:

Homework Statement


A rotating uniform cylindrical platform of mass 230 kg and radius 5.6 m slows down from 3.8 rev/s to rest in 16 s when the driving motor is disconnected. Estimate the power output of the motor (\rm hp) required to maintain a steady speed of 3.8 rev/s


Homework Equations


Wfriction=change in KE = -.5*I*w^2
power=work/time


The Attempt at a Solution


So I was was able to get the amount of work that the friction force does and it was -.5 (.5 * MR^2)w^2 = 45053.39J and that work done by friction is the same amount of work needed for the motor to output. So the power of the motor needed is 45053.39/16s = 2690.8Watts and to convert that to HP I just divide by 746W and got 3.6HP. is that right?

I don't believe this is the correct approach. This power that you are finding here is the average power from the frictional torque during the entire slowing down process. But while the speed is decreasing, the power from friction is also decreasing.


So here you need the power at the beginning of the slowing down process. What is the formula for the instantaneous power (for the rotational case)?
 

Similar threads

What is the torque exerted by the flywheel on the machine?
  • · Replies 5 ·
Replies
5
Views
4K
How Do You Calculate Power from Torque and Angular Velocity?
  • · Replies 3 ·
Replies
3
Views
2K
How Do You Calculate Torque and Power for Rotational Motion in Space?
  • · Replies 4 ·
Replies
4
Views
1K
How do I calculate wheel radius using below data?
  • · Replies 1 ·
Replies
1
Views
1K
How many crates can you load in a minute with different average power outputs?
  • · Replies 1 ·
Replies
1
Views
2K
What is the power input of a motor pulling up an elevator?
  • · Replies 3 ·
Replies
3
Views
3K
Calculation of torque to stop a wheel.
  • · Replies 4 ·
Replies
4
Views
6K
Calculating Power from Torque and Rotor Speed
  • · Replies 5 ·
Replies
5
Views
3K
Calculating Power Transferred in a 2-Pulley System
  • · Replies 9 ·
Replies
9
Views
3K
How Is the Average Power Output Calculated for an Airplane Propeller?
  • · Replies 1 ·
Replies
1
Views
1K