Air friction vs viscous damping

[stevenz]

Advertisement - Members don't see this ad

This comes from question 64 in the waves/vibrations section of TBR physics 1. It asks:

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

a) it will experience an increase in period
b) it will experience a decrease in frequency
c) the pendulum will dissipate energy faster
d) the pendulum will dissipate energy slower

answer: d
my answer c

Their explanation: heating the air causes it to be less dense, reducing their air friction

My explanation heating the air causes it to become more viscous, increasing the viscous damping

Does any one have any insight on this topic? I'm not sure how air friction would be different from air damping - in fact I had assumed they were essentially the same thing. Are they not?

 

[MangoPlant]

The pendulum dissipates energy by colliding with air molecules in its path. When the air is heated up, the particles move further apart (less dense) and the pendulum encounters less air molecules in its path. Less air molecules encountered = less energy dissipated.

 

[stevenz]

Could you not just as well reason that heating the air causes the air molecules to gain momentum, so that when they collide with the pendulum, they cause the pendulum to lose more energy? It would seem just as reasonable to me to assume that the viscous damping is greater as a result of..well, higher viscosity.

 

[Eastfolding]

Could you not just as well reason that heating the air causes the air molecules to gain momentum, so that when they collide with the pendulum, they cause the pendulum to lose more energy? It would seem just as reasonable to me to assume that the viscous damping is greater as a result of..well, higher viscosity.

You should know why this is wrong if you're a fourth year physics student.

 

[stevenz]

You should know why this is wrong if you're a fourth year physics student.

I'd like to hear your explanation first.