work question

[not respect]

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this was on kaplan FL7

50. crane lifts a 1000 kg beam, sets it on a scaffold 100 m off the ground. All the following are true except:

a) net work done is 9.8e5 J
b) net work done is 0 J
c) work done by gravity is 9.8e5 J
d) work done by crane is 9.8e5 J

Now from good test taking logic i easily figured this out but i don't get the physics behind it. Can someone explain to me a little bit? The bar is lifted off the ground, doesn't that mean there's net work on it? thanks.

 

[murdoc9]

the k-work theorem says that the net work done on an object is equal to its change in kinetic energy. since the object starts and ends at rest, there is no net work.

 

[not respect]

Yeah but it has increased potential energy, surely that must come from net work?

 

[not respect]

Oh wait, the work done to move it against gravity is so much but the net work done on it is 0? is that right?

 

[murdoc9]

yeah i think it's ok to look at it like that- even though work is scalar, to say that so much work done by the crane was matched by gravity. whereas with kinetic energy, if you have any kinetic energy, you've matched the work done by gravity (or any other type of force for that matter) and then some. somebody can correct me if that's not a valid interpretation of the k-work theorem, but that's all i got.

 

[not respect]

your kaplan PS scores are f-in amazing so obviously you are right 🙂 did you start out that high or did you study a lot? i understand everything but theres SO MANY trick questions so i usually get ~10 wrong (on the FL 🙁 )

 

[murdoc9]

haha thanks. i didnt start that high, i think i started around 9 or 10 before any studying. ive been studying pretty steadily for a month. contrary to what many think, i think EK 1001 is a great way to improve the physical sci score. basically all that i do is take a practice test, pick out my weaknesses, and then hammer away at them with EK questions.

 

[ecoli]

wait... so what's the answer?

 

[not respect]

the answer is B, the net work done is 0.

 

[ecoli]

well that makes sense. Since you're lifting an oject up, there will be a net work exerted, so since its an 'except' question, that's exactly what I expected.

 

[not respect]

err i meant A* sorry typing too fast 😛 B is true actually, it seems weird but it is.

 

[bluemonkey]

Why is that weird? There is no net work done of the object since the forces are in opposing directions and you are dealing with conservative forces...

 

[ecoli]

Why is that weird? There is no net work done of the object since the forces are in opposing directions and you are dealing with conservative forces...

oh good point. I just realized that if b is wrong, c and d couldn't possibly be right either, since they act opposingly.

 

[TawMus]

Why is that weird? There is no net work done of the object since the forces are in opposing directions and you are dealing with conservative forces...

work is not a conservative force.