# At what rate is the flywheel spinning when the power comes back on?

1. Apr 16, 2007

### nguyenn2

Physic help urgent

A high-speed flywheel in a motor is spinning at 500 when a power failure suddenly occurs. The flywheel has mass 36.0 and diameter 75.0 . The power is off for 34.0 and during this time the flywheel slows due to friction in its axle bearings. During the time the power is off, the flywheel makes 180 complete revolutions.

a)At what rate is the flywheel spinning when the power comes back on?

b)How long after the beginning of the power failure would it have taken the flywheel to stop if the power had not come back on?

c)How many revolutions would the wheel have made during this time?

my work
avg speed = distance / time the time is 34 sec * ( 1 min / 60 sec ) = 0.566667 min

and avg speed = 180 rev / 0.56667 min = 317.65 rpm

the average speed is the average of the initial and final, so the final and initial together must equal 2 * 317.65 = 635.3 rpm

Since initial is 500, the final must be 135.3 rpm

The flywheel slowed from 500 to 135.3 rpm in 34 sec or:

(500 - 135.3) / 34 = 10.73 rpm every sec

: 135.3 ( 1 sec / 10.73 rpm) = 12.6 seconds to bring the wheel to rest.

the avg speed would be (1/2) ( 135.3 + 0 ) = 67.65 rpm

so the wheel would go 67.65 rpm * 12.6 sec ( 1 min / 60 sec) = 14.2 revoluti

2. Apr 16, 2007

### nguyenn2

im getting the wrong answers

3. Apr 16, 2007

### andrevdh

This problem should rather be attempted with the constant angular acceleration equations

http://www.saburchill.com/physics/chapters/0023.html" [Broken]

http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html" [Broken]

http://electron9.phys.utk.edu/phys135d/modules/m8/rotation.htm" [Broken]

Hint: Use the data from the off period to calculate the angular deceleration of the flywheel.

Last edited by a moderator: May 2, 2017
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