UNIVERSITY PHYSICS （20 MARKS）

DAMON

2013-10-20 10:38 pm

A high-speed flywheel in a motor is spinning at 450rpm rpm when a power failure suddenly occurs. The flywheel has mass 45.0kg kg and diameter 74.0cm cm . The power is off for 34.0s 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 170 complete revolutions.

1 At what rate is the flywheel spinning when the power comes back on?
（in angular velocity)

2 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?

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

更新1:

ANS incorrect..... and the first one should be in angular velocity..

更新2:

SOR,, the first one is correct but the other two incorrect,,,

回答 (1)

天同

2013-10-21 2:35 am

✔ 最佳答案

1. Use equation: s = (1/2).(u+v)t
with s = 170 rev., u = 450 rpm, t = 34 s
hence, 170 = (1/2)(450 + v).(34/60)
v = 150 rpm

2. Deceleration of flywheel = -(170-450)/(34/60) rev/min^2
= -494.1 rev/min^2
Use equation: v = u + at
with v = 0 rpm, u = 450 rpm, a = -494.1 rev/min^2, t =?
0 = 450 + (-494.1)t
t = 0.9107 min = 54.64 s

3. Use equation: s = (1/2).(u+v)t
s = (1/2).(450).(0.9107) rev = 205 rev

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