Waves and momentum

[sillyjoe]

Advertisement - Members don't see this ad

Why do waves transfer momentum? I was under the assumption that they only could transfer energy? Where is the mass coming from?

(See question 636 in EK 1001 physics for the source of my confusion)

Thanks!

 

[DrknoSDN]

First, physical waves are different from electromagnetic waves.

In a physical wave molecules are actually colliding with other molecules to propagate the wave.
In a sound wave energy is transferred into velocity of air molecules in every direction. The moving molecules keep moving until hitting a new molecule that was further away. This transfers kinetic energy into the new molecule thus transferring momentum (mv) assuming mass of molecules is constant.

EM waves shouldn't transfer momentum if they are massless.

 

[sillyjoe]

Thanks. So when doing these questions I should separate the physical waves from EM waves. Physical waves will be imparting momentum. What will the EM waves impart? Just energy?

 

[DrknoSDN]

Thanks. So when doing these questions I should separate the physical waves from EM waves. Physical waves will be imparting momentum. What will the EM waves impart? Just energy?

I don't believe for the MCAT you need to worry about EM wave momentum, as it is non-standard.

There are theoretical space ships that use giant collecting parachutes directed at the sun that gain momentum by reflecting the radiation from the sun. Also solar radiation exerts a small force on things that it hits, but I think that is relativistic physics. I'm not sure if MCAT would test that material.

For more information:
http://en.wikipedia.org/wiki/Radiation_pressure
http://farside.ph.utexas.edu/teaching/em/lectures/node90.html

 

[sillyjoe]

Were you a physics major? You really seem to know your physical sciences...

 

[DrknoSDN]

Bio. Physics is just interesting.

Also I want to change my suggestion. Don't ignore EM momentum. I don't think anything is off limits for MCAT physics and you should spend as much time as you can becoming familiar with obscure concepts. Based on how the physical science section of the MCAT is testing people recently, they are throwing extremely challenging passages at people and just giving generous curves. (Reference May8, June5, official threads)

The real exam surprised me with how "tricky" the problems were. AAMC 3-9 were truly simplistic by comparison.
Even with the increased difficulty people are still reporting scores very close to their AAMC averages so the standardization process seems to be working.

http://forums.studentdoctor.net/threads/the-official-may-8-2014-mcat-thread.1047972/page-7
http://forums.studentdoctor.net/threads/the-official-june-5-2014-mcat-thread.1067761/page-4

 

[sillyjoe]

When did you take your MCAT?

 

[DrknoSDN]

June 5th.

I appreciate the more difficult exam. I never liked how general test taking errors (misreads, etc) were a significant obstacle at higher scores.
With a more challenging test everyone is making more errors so those one or two questions wont make or break your overall score.