TBR Periodic Motion and Wave question

[zoner]

in Vibrational Motion passage in the handout

question #26

A pendulum may undergo dampened harmonic oscillation in the presence of a resistive force. When the gas through which the pendulum travels is heated:

A) & B) The pendulum will experience an increase in period or dcrease in frequency.

D) The pendulum will dissipate its energy more slowly.

The right answer was D. I understand why it is D, but why can't it also be either A or B? I know that pendulum depends on G and L, but if you can imagine doing the experiment in the water where the surrounding density is much higher than air, the period will definitely increase and the frequency decrease due to the resistance of water. And relating this to the question, since heating the air makes it more dense it will provide more resistance to the motion of the pendulum. No?

Thanks

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[PeterPesto]

Actually heating the fluid / air makes it less dense.

Remember that when studying mechanical waves / sound.

 

[Rabolisk]

It only makes sense that heating a gas will make it less dense. First, going from solid to gas generally decreases density. Second, per the ideal gas law, volume increases when temperature increases.

 

[zoner]

oh yeah.. i thought that hot air sinks and cold air rises. also doesn't hot water sink and cold water rises and thats why the lake freezes from top down?

 

[zoner]

Actually heating the fluid / air makes it less dense.

Remember that when studying mechanical waves / sound.

Well, but what if you think of it as because you are heating up the air molecules, they are traveling faster, so more chance they will have to run into the moving pendulum, thus create more resistance?

 

[PeterPesto]

Whether or not something sinks depends on its density. If hot air sinks, doesn't that imply that hot air is more dense than cold air? No, because hot air is less dense than cold air therefore it rises.

Water is the exception. I believe it is most dense between 1-4 degrees but you don't need to know this for the MCAT.

 

[BerkReviewTeach]

Well, but what if you think of it as because you are heating up the air molecules, they are traveling faster, so more chance they will have to run into the moving pendulum, thus create more resistance?

There's also more chance they might run into the back of the pendulum and help push it along. The great collision rates balance out on the opposite sides of the ball. For drag on a slow moving object like a pendulum, the density of the air is key factor when thinking about resistance.

 

[zoner]

oh yeah......