Arg, wtf... I cant figure this out.

[MikeMike]

Okay, its been 4 years since i took physics, i remember some, but not all. GF is however in physics while in animal sciences (yea, dont ask me why she will ever need physics as a vet, but you have to take it)

Anyways, its a basic problem, a bucket on a pulley is suspended over a body of water.

important stats

height from bucket to water 2.25m
radius of pulley .2m
mass of bucket is 1.5kg
the linear accel of bucket is 1.13m/s^2
time it falls is 3s

now, change in PE is m*a*change height
which means, it should be (atleast in my mind) m = 1.5kg, a = 1.13 OR 9.8 (we tried both) and deltaH should be 2.25m...
neither 33.08 nor 3.81

now, its something stupidly simple that i cant figure out... i have no idea what im doing wrong... help?!?

What is the change of the Potential Energy of the bucket from the top of the well to the water level?

 

[Jeff7]

Change in PE =
(Mass * height at the original location) - (Mass * height at the new location)

I think that should be it, check the units.

.....

Ah, PE = mgh. Right, there is gravity in there.

It's been only two years since I've had physics. How quickly it fades.

mgh(original) - mgh(new)

That "g" term is acceleration - gravitational acceleration.


So it started 2.25m up.
PE = 1.5kg * 2.25m * 9.8m/s²
PE = 33.075 kg-m²/s² <-- Relative to the surface of the water, that's all the potential energy the bucket has.

x = 0.5at²
x = 0.5 * 1.13m/s² * (3s)²
x = 5.085m <-- Distance the bucket would fall if it were to accelerate downward at 1.13m/s² for 3 seconds.
So the new height would be -2.835m?

Is there anything more to this problem? You mention a pulley, but it seems like that would be useless in finding a simple change in potential energy.



Do you know what the correct answer is? Is it a computerized system for entering answers, or out of a book?
Maybe it wants gravity entered as 9.81m/s² instead of just 9.8?

 

[Howard]

mg delta h is faster

 

[gorcorps]

Accel should definitely be gravity (9.81m/s), and everything else seems right. I'm not sure what else to change, other than maybe correcting the height with the pulley radius for some reason but that doesn't make any sense.

 

[Eeezee]

Potential energy is not mah, it's mgh. This specific type of potential energy is called "gravitational potential energy" in fact

You haven't actually stated the question though. What does the problem ask for? Does it just want potential energy? You should be getting about 33.1 Joules

 

[MikeMike]

What is the change of the Potential Energy of the bucket from the top of the well to the water level?

 

[Eeezee]

Okay, well, if it falls in 3 seconds at an acceleration of 1.13 m/s^2, the change in height is 5.09 meters, but that doesn't match the number you gave us. Would you mind copy-pasting the text verbatim?

 

[Eeezee]

Originally posted by: gorcorps
Accel should definitely be gravity (9.81m/s), and everything else seems right. I'm not sure what else to change, other than maybe correcting the height with the pulley radius for some reason but that doesn't make any sense.

m/s^2 you mean, right?

 

[MikeMike]

ugh, the height was 5.09 not 2.25 as she had written down... which well, seeing as how change in PE is the only equation that required the height, and she had the proper accel... i never thought about that.
Thanks eeezee for pointing that out.

 

[gorcorps]

Originally posted by: Eeezee

Originally posted by: gorcorps
Accel should definitely be gravity (9.81m/s), and everything else seems right. I'm not sure what else to change, other than maybe correcting the height with the pulley radius for some reason but that doesn't make any sense.

m/s^2 you mean, right?

yes