AAMC Phys QPack Q74 and Q75

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AAMC Phys QPack Q74 and Q75

Post by dtufano » Sun Jan 27, 2019 9:48 pm

I have a general question, but it relates to the #74 in AAMC's Phys QPack. In the explanation, they completely ignore the cos(theta) value in the work function. Is this because 1) we neglect cos(theta) in pulley systems or 2) cos(theta) = cos(0) = 1, because the direction of displacement is upwards, and there is no movement in the x direction?

For Q75, I understand why the speed of sound remains the same when reflected. However I'm confused as to why the other factors change when reflected. Can someone please explain this? Thanks!
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Re: AAMC Phys QPack Q74 and Q75

Post by NS_Tutor_Will » Mon Jan 28, 2019 8:39 am

Great questions!

For Q74 -- You are exactly right. Since the pulley system brings everything into the y-direction, there is no x-component of the work vector. The force applied is at an angle, but the work done is not!

For Q75 -- The key here is that we are dealing with an "imperfect reflector" (per the question stem). That means that we will be losing some energy as the light ways reflect of the surface. Remember, speed of sound comes from the medium (which isn't changing, so it stays the same). As energy is lost, the intensity, frequency, and wavelength would all change (because they are related to energy). Speed is independent of energy and so will stay the same.

Hope this helps!
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