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Physics Class 2 Practice Set Questions #23,25,36
Fahad_5586
#1 Posted : Thursday, July 09, 2020 6:21:36 PM
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For Question 23:
Why is the answer 0.118 when the answer given in the solution is 1.1?
Also why are litres converted to millilitres when litres are the SI Unit?

For Question 25:
Is specific gravity always in comparison to water?

For Question 36:
Why is there more friction in the middle of a moving fluid in comparison to the edges of a moving fluid? Wouldn't there be more friction at the edges of a moving fluid as they are closer to the edges of the vessel, so they would collide with the vessel more resulting in increased friction?
INSTR_Katerina_102
#2 Posted : Saturday, July 18, 2020 7:20:02 PM
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Hi Fahad,


For Question 23:
Why is the answer 0.118 when the answer given in the solution is 1.1?
Also why are litres converted to millilitres when litres are the SI Unit?
'

Thanks for catching the typo, I will mention this, it seems the solution should be D.

The litres are converted into cubic metres because we need to cancel the units properly, we have the area of the tube in m^2 and therefore we need to have the volume in m^3 to properly cancel our units and get a velocity in m/s as designated in the answer.


For Question 25:
Is specific gravity always in comparison to water?


Yeah, if I recall canonically specific gravity is defined as the density substance/density standard, where for solids and liquids the standard is pretty much always water for the MCAT (because it has a density of 1 g/mL which is exceedingly convenient).

For Question 36:
Why is there more friction in the middle of a moving fluid in comparison to the edges of a moving fluid? Wouldn't there be more friction at the edges of a moving fluid as they are closer to the edges of the vessel, so they would collide with the vessel more resulting in increased friction?


I'm not 100% sure about this so you might want to ask this in office hours. Friction forces for fluids, or drag forces, aim to slow down fluid flow and are thus higher when a fluid is moving more quickly. Drag forces increase as you move faster (think about having a sunroof open on a car driving down a highway vs driving down a side street slowly). So in comparison to if the fluid was flowing faster near the edges (not in comparison to the fluid in the centre), friction is minimized if the fluid is flowing slower near the edges.

I hope this helps!

Katt
INSTR_Radhika_42
#3 Posted : Monday, July 20, 2020 11:46:21 AM
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Great questions! I just wanted to take a moment to add on to some of the information mentioned:

Question 25:
Yes, specific gravity (spgr) is always in relation to water. When this concept was founded, (reasonably) pure water was one of the most commonly available pure fluids. Scientists were looking for a way to relate the density of fluids to a standard and water seemed like a good choice. In medicine, you may be spgr in relation to urinary tests which compare solute content that thereby changes urine density.

Question 36:
This is a great question and should be thought of microscopically. So picture a circular cross-section of a tube with concentric layers of fluid. Now, think about the outermost layer - the layer in contact with the surface of the tube. The fluid molecules touch and slide past the inner surface of the piping; the fluid molecules are attracted to the surface of the material they are flowing within just as they are attracted to one another (- this is why we have viscosity). Generally, due to the attraction of the molecules to the surface of the pipe, we can expect friction which ultimately results in a slower speed. Now imagine the innermost layer of fluid, these molecules only interact with other fluid molecules - they don't touch the surface at all. As a result, there is reduced friction and a faster speed results.

Hope this helps!
INSTR_Faisal_57
#4 Posted : Monday, July 20, 2020 10:04:35 PM
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Hi Fahad,

Regarding specific gravity. While the above posts are correct that it is typical to use water as the standard for specific gravity, it is not necessarily the case. Specific gravity is simply a relative density concept: take the density of something relative to something else. That "something else" can anything.

Every now and then you will encounter a question where you're given a specific gravity that is not relative to water, just as I did when I wrote the MCAT.
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