4A.3

[Morgan Carrington 2H]

Air in a bicycle pump is compressed by pushing in the handle. The inner diameter of the pump is 3.0 cm and the pump is depressed 20. cm with a pressure of 2.00 atm. (a) How much work is done in the compression? (b) Is the work positive or negative with respect to the air in the pump? (c) What is the change in internal energy of the system?

I'm not understanding how to calculate part a of the problem. I used w = P * A * D , but keep getting an answer far off from the correct one.

[Jeremy_Guiman2E]

You do use w = P * A * D!

w = 2 atm * (0.015 m)² * 0.2 m * pi
w = 2.8274 * 10^-4 m³ * atm

Then, you can multiply the answer by 101,325 Pa to convert from atm to Pa. Once you have your answer in m³ * Pa, it is equal to that amount in joules (J).

[kausalya_1k]

Should we always convert to J (using the 101325 Pa value) on exams? Correct me if I'm wrong, but I don't recall hearing that in lecture?

[rachelle1K]

I did this problem yet I keep getting my final answer as 28.6, which would round up to 29. The answer says it' supposed to be 28. Has anyone else been getting the same thing?

[rachelle1K]

Should we always convert to J (using the 101325 Pa value) on exams? Correct me if I'm wrong, but I don't recall hearing that in lecture?

I didn't hear him talk about it either in lecture, but it makes sense because work is equal to J and you have to make sure the units work out. Pa is equal to J.m^-3, and by multiplying by m^2 (area) and m (distance), we are left with the joules.

[DesireBrown1J]

You do use w = P * A * D!

w = 2 atm * (0.015 m)² * 0.2 m * pi
w = 2.8274 * 10^-4 m³ * atm

Then, you can multiply the answer by 101,325 Pa to convert from atm to Pa. Once you have your answer in m³ * Pa, it is equal to that amount in joules (J).

Where did you get 0.015m from and pi from?

[Reina Robles 2B]

You do use w = P * A * D!

w = 2 atm * (0.015 m)² * 0.2 m * pi
w = 2.8274 * 10^-4 m³ * atm

Then, you can multiply the answer by 101,325 Pa to convert from atm to Pa. Once you have your answer in m³ * Pa, it is equal to that amount in joules (J).

Where did you get 0.015m from and pi from?

I believe it's to convert the diameter (3.00cm) to area!

Area = pi*radius²

Diameter = 3.00 cm
Radius = 3.00 cm/ 2 = 1.5 cm = 0.015 m (Joules is m2.kg.s-2, so you need to convert cm to m)

Area = pi*0.015m²

[Reina Robles 2B]

I did this problem yet I keep getting my final answer as 28.6, which would round up to 29. The answer says it' supposed to be 28. Has anyone else been getting the same thing?

I got that too, but when I used 2.8*10^-4 instead of 2.8274*10^-4, I got 28.371, which rounds down to 28. Are we supposed to do it like this? I need to review significant figures.

[Zoe Gleason 4F]

I did this problem yet I keep getting my final answer as 28.6, which would round up to 29. The answer says it' supposed to be 28. Has anyone else been getting the same thing?

I also got 29. 28 is the product of using sig figs early, then rounding down. I think either answer would be correct, but my TA did tell us to not round until the very end (which would produce 29).