Rotational kinetic energy question

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Homework Help Overview

The problem involves a car utilizing energy from a rotating flywheel, with specific parameters including the flywheel's radius, mass, and rotational speed. The goal is to determine the kinetic energy stored in the flywheel and how long it can power the car compared to a specified motor power.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning

Approaches and Questions Raised

  • Participants discuss the formula for kinetic energy and the necessary parameters, such as the moment of inertia and angular velocity. Questions arise about calculating angular velocity and the moment of inertia for the flywheel.

Discussion Status

Some participants have made attempts to calculate angular velocity and kinetic energy, while others are seeking clarification on the moment of inertia. There is an exchange of ideas regarding the conversion of horsepower to joules per second, indicating a collaborative effort to explore the problem further.

Contextual Notes

Participants are working under the constraints of a homework assignment, which may limit the information they can use or the methods they can apply. There is a focus on understanding the underlying physics concepts rather than providing direct solutions.

physicsbro
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Homework Statement


A car is designed to get its energy from a rotating flywheel with a radius of 1.90 m and a mass of 487 kg. Before a trip, the flywheel is attached to an electric motor, which brings the flywheel's rotational speed up to 5150 revolutions per minute. Find the kinetic energy stored in the flywheel.If the flywheel is to supply energy to the car as would a 11.3 hp motor, how long could the car run before the flywheel would have to be brought back up to speed?


Homework Equations


KE=(1/2)Iw^2


The Attempt at a Solution

 
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physicsbro said:

Homework Statement


A car is designed to get its energy from a rotating flywheel with a radius of 1.90 m and a mass of 487 kg. Before a trip, the flywheel is attached to an electric motor, which brings the flywheel's rotational speed up to 5150 revolutions per minute. Find the kinetic energy stored in the flywheel.If the flywheel is to supply energy to the car as would a 11.3 hp motor, how long could the car run before the flywheel would have to be brought back up to speed?


Homework Equations


KE=(1/2)Iw^2


The Attempt at a Solution




so the ke of the flywheel is?
 
I don't know.
 
physicsbro said:
I don't know.

[tex]E_k=\frac{1}{2} I \omega^2[/tex]


do you know ω? Do you know what I is for a flywheel?
 
Welcome to PF!

Hi physicsbro! Welcome to PF! :smile:

(have an omega: ω and try using the X2 tag just above the Reply box :wink:)
physicsbro said:
… Find the kinetic energy stored in the flywheel.

KE=(1/2)Iw^2

You need to learn the moment of inertia of the simple shapes …

see eg http://en.wikipedia.org/wiki/List_of_moments_of_inertia" :wink:
 
Last edited by a moderator:
I'm not sure if this is right, but this is what i got for ω. i thought it would be 10300π/60s or 539.3 rad/s. I don't what I is for a flywheel, i want to say (1/2)mr^2.
 
I found the KE to be 1.28E+08 J, any hints for the second part of the question?
 
What is 11.3 hp in joules per second?
 
Thanks, i got it.
 

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