This is a pop quiz...have you studied?

[kevman]

MArked for later

 

[jagec]

Originally posted by: Jeff7

Originally posted by: DrPizza
And, (correct me if I'm mistaken), the two masses balanced on a lever resting on a fulcrum. The one on the left is 300kg. The "correct" answer is 100kg. I disagree. It's been 20 years since I've done such problems, but I thought center of mass was used, in which case the one on the right was 5 times as far away, not 3 times as far away. (Darn those profs who spent too much time on "point masses" and not enough on real world masses!"

For the lever question, I was also taught to model distributed masses as points for the sake of the FBD moment calculations. For that problem, it would be a 1:5 distance ratio, not 1:3.



- The naturally aspirated engine - don't know a thing about how engines work. I thought it'd be suction from the piston going down. Technically I guess you could say that suction doesn't really exist, and that it's always just outward pressure pushing in.

Another thing that bugged me about the balance was that when weights (especially tall weights) are put on a beam like that, the "balance point" will change depending on the initial tilt. I'd rather see one of these, which compensates for that problem. And I agree, it should be 1:5. I think I got that one wrong, because I picked the closest answer to 1:5.

The "suction" thing was a trick question. I got it right, but it's still a trick question.

 

[DrPizza]

Originally posted by: jagec

Originally posted by: Jeff7

Originally posted by: DrPizza
And, (correct me if I'm mistaken), the two masses balanced on a lever resting on a fulcrum. The one on the left is 300kg. The "correct" answer is 100kg. I disagree. It's been 20 years since I've done such problems, but I thought center of mass was used, in which case the one on the right was 5 times as far away, not 3 times as far away. (Darn those profs who spent too much time on "point masses" and not enough on real world masses!"

For the lever question, I was also taught to model distributed masses as points for the sake of the FBD moment calculations. For that problem, it would be a 1:5 distance ratio, not 1:3.



- The naturally aspirated engine - don't know a thing about how engines work. I thought it'd be suction from the piston going down. Technically I guess you could say that suction doesn't really exist, and that it's always just outward pressure pushing in.

Another thing that bugged me about the balance was that when weights (especially tall weights) are put on a beam like that, the "balance point" will change depending on the initial tilt. I'd rather see one of these, which compensates for that problem. And I agree, it should be 1:5. I think I got that one wrong, because I picked the closest answer to 1:5.

The "suction" thing was a trick question. I got it right, but it's still a trick question.

Somewhere along the way, I ran into a professor who constantly stressed that "suction" is not a force. Basically, I've seen 100 "trick" questions from a prof trying to make that point (and he did.) Oh, and I was tempted to choose the answer "closest" to the correct answer, but I finally decided, "well, the correct answer isn't here, so if I was doing it wrong, then the correct answer would be..."

 

[exdeath]

Originally posted by: DrPizza
Anyone have an answer yet regarding the incorrect question? (balancing masses)

You have to balance the torque so there is no net rotational force.

1 lb at 5 feet = 5 lbs at 1 foot = no net torque.

Which question was it again?

 

[KMc]

I got a 98%. I still don't see how the balance beam answer was 100kg?

 

[angminas]

80%, pretty good considering how long it's been, and that I never studied half of that stuff.

 

[PClark99]

:beer: 82%

I passed just barely.

 

[Lithium381]

i got 82