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Ball attached to String (Potential Energy) 
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#1
May1411, 02:18 PM

P: 20

1. The problem statement, all variables and given/known data
A 2.40 kg ball is attached to a ceiling by a 2.00 m long string. The height of the room is 3 m. What is the gravitational potential energy of the ball relative to: a) the ceiling? b) the floor? c) a point at the same elevation as the ball? Variables P for potential energy m for mass in kg g for gravity h for height 2. Relevant equations P=mgh 3. The attempt at a solution For b), I found the answer which was 23.52. For a), though, I got 70.56 because P=(2.4)(9.8)(3)=70.56. The 3 is from the fact that the ceiling is 3m off the ground and a) is asking about the ceiling. It says that I'm wrong For c), I don't have any idea of what they're talking about. Is it the same thing as b)? 


#2
May1411, 02:53 PM

P: 1,261

Energy is always relative. Potential energy is defined as the potential energy of one point, vs. the potential energy at another point. The equation, more precisely, should be written [tex]U = mg \Delta h[/tex] for some different in height [tex]\Delta h \equiv h  h_0[/tex].
Usually the 'reference' point ([tex]h_0[/tex]) is taken to be "zero height" ([tex]h_0 = 0[/tex]), and that is often either sealevel, or groundlevel, or floorlevel, etc. 


#3
May1411, 02:59 PM

P: 20

ok, so I did 2.4(9.8)(1)=23.52 for a). i still get it wrong....
and for c), do i just do 2.4(9.8)(2) because it's the inverse? 


#4
May1411, 03:04 PM

P: 1,261

Ball attached to String (Potential Energy)



#5
May1411, 03:16 PM

P: 45




#6
May1411, 03:18 PM

P: 20

so will a) be negative because 23=1? therefore, will the answer be 23.52?
and will c) be zero because 22=0 and 2.4(9.8)(0)=0? 


#7
May1411, 03:23 PM

P: 20

@vespa71
the problem said the ceiling to the floor was 3m 


#8
May1411, 03:24 PM

P: 45

a) will be negative becaus there's a 2!! meter negative drop from the ball to the ceiling. c) is zero as there's no drop. Well done.



#9
May1411, 03:33 PM

P: 45

I recommend to make a simple drawing to visualize the problem. Best of luck



#10
May1411, 03:35 PM

P: 20

well ok, i did 2.4(9.8)(1). that gives me 23.52. it still tells me i'm wrong



#11
May1411, 03:37 PM

P: 20

oh and thanks for c). i got it right.



#12
May1411, 03:51 PM

P: 45

If you have a 2m drop from ball to ceiling, and a 1m drop from ball to floor, and a 0m drop from ball to somthing on the same level, I think it will solve.



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