AAMC CBT 11 question 31

[agill786]

Hi,
I had a question with regards to this problem as I don't quite see why we can't assign g as being -10m/s^2? The Wg is going down and the Air resistance is going up Wa, so why isn't Wa positive? Also, could someone help me with finding the work done by Air Resistance, should it not be the same?

Thanks in advance!

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[gunj122]

i was confused by this one too. i also dont understand how the absolute values of Wg and Wa arent equal. What does Wa equate to?

 

[JeetKuneDo]

i was confused by this one too. i also dont understand how the absolute values of Wg and Wa arent equal. What does Wa equate to?

Work Air resistance. I guessed on this one. My reasoning was since the object is falling down, the work by gravity is greater than the work of the air resistance pushing it up? Not sure if that's the right reasoning though.

 

[knightstale4]

Not sure if you still need this, but maybe others can benefit from it. When the object is falling but has not achieved terminal velocity, the force of gravity is greater than the force of air resistance (otherwise, the object wouldn't be accelerating downwards). Therefore, the magnitude of work done by gravity is greater since displacement is same for both of them.

As for the second half, when force and displacement are in the same direction, W is positive. This is why work done by gravity is positive. On the other hand, when you look at your force diagram, force of air resistance is pointing upward, meaning force and displacement are in the opposite direction such that work is negative.

 

[AnastomoSteve]

Not sure if you still need this, but maybe others can benefit from it. When the object is falling but has not achieved terminal velocity, the force of gravity is greater than the force of air resistance (otherwise, the object wouldn't be accelerating downwards). Therefore, the magnitude of work done by gravity is greater since displacement is same for both of them.

As for the second half, when force and displacement are in the same direction, W is positive. This is why work done by gravity is positive. On the other hand, when you look at your force diagram, force of air resistance is pointing upward, meaning force and displacement are in the opposite direction such that work is negative.

The key here is that the question states "during the first 600 m of the fall." The passage clearly indicates that terminal velocity is not reached until 600 m so during this time the object is accelerating downward (the forces are NOT equal).

 

[drechie]

can anyone help me on #8 it seems like a super easy problem that i got totally wrong. i know one mole = 22.4 liters at STP. So what I thought to do was use a proportion. One mole = 22.4 Liters. so 22.4 * 6/29 = # moles of N2.

4 moles of reactant produces 29 moles of product, 6 of which come from N2. where did i go wrong?


8. At STP, the volume of N2(g) produced by the complete decomposition of 1 mole of nitroglycerin would be closest to which of the following?

A) 5 L B) 10 L C) 20 L D) 30 L

 

[Cawolf]

@drechie

You have the idea right, but you only need to evaluate the ratio of product (N2) to reactant (NTG).

This is 6 mol N2/4 mol NTG, not the ratio you listed above.

When using dimensional analysis (which would have helped you avoid this error), only use a single ratio at a time.

Therefore the answer should be closer to 30, choice D.