.15 A high-speed flywheel in a motor is spinning at 450 rpm when a power failure

Question

.15

A high-speed flywheel in a motor is spinning at 450 rpm when a power failure suddenly occurs. The flywheel has mass 39.0 kg and diameter 80.0 cm . The power is off for 35.0 s and during this time the flywheel slows due to friction in its axle bearings. During the time the power is off, the flywheel makes 150 complete revolutions.

Part A

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

1.328

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Part 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?

36.6

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Part C

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

153

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.15

A high-speed flywheel in a motor is spinning at 450 rpm when a power failure suddenly occurs. The flywheel has mass 39.0 kg and diameter 80.0 cm . The power is off for 35.0 s and during this time the flywheel slows due to friction in its axle bearings. During the time the power is off, the flywheel makes 150 complete revolutions.

Part A

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

=

1.328

rad/s

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Incorrect; Try Again; 4 attempts remaining

Part 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?

t =

36.6

s

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Part C

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

N =

153

rev

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Correct

Explanation / Answer

A)
d =150 revolution = 150*2*pi rad = 942.5 rad
wi=450 rpm = 450*2pi / 60 = 47.12 rad/s
t = 35 s
use:
d=wi*t+ 0.5*a*t^2
942.5 = 47.12*35 + 0.5*a*(35)^2
a= -1.154 rad/s^2

Now use:
wf = wi + a*t
= 47.12 + (-1.154)*35
= 6.74 rad/s
Answer: 6.74 rad/s
B)
a as alredy found above = -1.154 rad/s^2
Here wf= 0
use:
wf = wi+a*t
0= 47.12 + (-1.154)*t
t= 40.8 s
Answer: 40.8 s
c)
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