physics problem

2007-10-25 11:07 pm

Hybrid cars are becoming popular due to increasing gasoline costs. You are considering buying one but are skeptical of the manufacturer’s claims. One model uses a ring-shaped flywheel to store energy. Supposedly it can store 10MJ of energy and supply 25 kW of power for about 400 seconds. You find these data on the flywheel: mass of 48kg, rotation speed of 30,000 rpm, and diameter of 41.1 cm. Are the specs correct?

回答 (1)

魏王將張遼

2007-10-25 11:25 pm

✔ 最佳答案

This is the problem dealing with the rotational kinetics.
First of all, a ring-shaped (uniform) has its moment of inertia about its central axis given by MR2, where M and R are its mass and radius respectively.
Therefore, the flywheel's M.I. is 48 × (0.2055)2 = 2.027 kg m2
Then its rotational K.E. is given by 0.5Iω2 where I and ω are its M.I. and angular speed respectively.
Now, its angular speed is:
ω = (30000 × 2π)/60 = 1000π rad/s
Therefore, rotational k.e. is:
0.5Iω2 = 0.5 × 2.027 × (1000π)2
= 10 MJ (approx.)
Therefore the specs are correct.

參考: My physics knowledge

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