GREPhysics.NET: GR9277 #41

GR9277 #41

Problem

GREPhysics.NET Official Solution

\prob{41}
A cylinder with moment of inertia $4kgm^2$ about a fixed axis initially rotates at 80 radians per second about this axis. A constant torque is applied to slow it down to 40 radians per second.

The kinetic energy lost by the cylinder is

80 J
800 J
4000 J
9600 J
19,200 J

Mechanics $\Rightarrow$ }Energy

The kinetic energy is related to the inertia I and angular velocity $\omega$ by $K = \frac{1}{2}I\omega^2$ .

The problem supplies $I=4kgm^2$ so one needs not calculate the moment of inertia. The angular velocity starts at $80 rad/s$ and ends at $40 rad/s$ .

Thus, the kinetic energy lost $\Delta K = \frac{1}{2}I(\omega_f^2-\omega_0^2)=\frac{1}{2}(4)(40^2-80^2)=2(1600-6400)=-9600J$ , as in choice (D).

Alternate Solutions

Ryry013
2019-10-06 09:10:04

A slightly faster way to do this problem is to know that if omega halves, then kinetic energy will decrease by 1/4. Then energy LOST is just (0.5)(4)(80^2) * (3/4) = (0.5)(6400)(3/4) = 9600J

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Comments

Ryry013
2019-10-06 09:10:04

A slightly faster way to do this problem is to know that if omega halves, then kinetic energy will decrease by 1/4. Then energy LOST is just (0.5)(4)(80^2) * (3/4) = (0.5)(6400)(3/4) = 9600J

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mnky9800n
2013-09-24 13:05:47

I also like to think in terms of the potential energy of a 100kg person who is about to fall 10 meters. That is mgh~(100 kg)(10 m/s^2)(10 m) = 10000 J. Thus A and B are probably too small and E is probably too big.

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aloha
2008-10-10 03:21:51

Maybe it's a silly question, but how can we know that I=4?
Thanks :)

Monk
2008-10-13 20:11:46

it tells you in the question...it is 4 kg m^2

kg m^2 is the units not a formula

tensordyne
2008-11-04 13:16:28

I am with Monk, from reading the problem it seems like they are saying $I = 4 k g m^2$ is a formula in k (proportionality factor maybe), g ( $9.8 m/s$ ) and mass m instead of as units because there is no grouping of the units, such as in say $I = 4 kg \cdot m^2$ , which would have made the use of units much more manifest.

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Maybe it's a silly question, but how can we know that I=4? Thanks :)

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