Ball attached to String (Potential Energy)


by goluigi2196
Tags: attached, ball, energy, potential, string
goluigi2196
goluigi2196 is offline
#1
May14-11, 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)?
Phys.Org News Partner Science news on Phys.org
Better thermal-imaging lens from waste sulfur
Hackathon team's GoogolPlex gives Siri extra powers
Bright points in Sun's atmosphere mark patterns deep in its interior
zhermes
zhermes is offline
#2
May14-11, 02:53 PM
P: 1,262
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 sea-level, or ground-level, or floor-level, etc.

Quote Quote by goluigi2196 View Post
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
While the ceiling is 3m off the ground ([tex]h_0 = 3[/tex]), it is only 1m away from the ball (the ball is what you're finding the potential energy of). h = 3m - 2m = 1m

Quote Quote by goluigi2196 View Post
For c), I don't have any idea of what they're talking about. Is it the same thing as b)?
If they're asking for the potential of the ball with respect to something at the same height, what is the difference in height [tex] \Delta h[/tex]?
goluigi2196
goluigi2196 is offline
#3
May14-11, 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?

zhermes
zhermes is offline
#4
May14-11, 03:04 PM
P: 1,262

Ball attached to String (Potential Energy)


Quote Quote by goluigi2196 View Post
ok, so I did 2.4(9.8)(1)=23.52 for a). i still get it wrong....
Sorry, forgot to highlight a key point. In this case, the ball is lower than the reference point. I.e. [tex]\Delta h = h - h_0 = 2m - 3m[/tex]

Quote Quote by goluigi2196 View Post
and for c), do i just do 2.4(9.8)(2) because it's the inverse?
Its not the inverse problem. Its asking what is the potential difference between something at h = 2m, and a reference point at [tex]h_0[/tex] = 2m
Vespa71
Vespa71 is offline
#5
May14-11, 03:16 PM
P: 43
Quote Quote by goluigi2196 View Post
..attached to a ceiling by a 2.00 m long string
How far is it between ball and ceiling? Ball and floor?
goluigi2196
goluigi2196 is offline
#6
May14-11, 03:18 PM
P: 20
so will a) be negative because 2-3=-1? therefore, will the answer be -23.52?

and will c) be zero because 2-2=0 and 2.4(9.8)(0)=0?
goluigi2196
goluigi2196 is offline
#7
May14-11, 03:23 PM
P: 20
@vespa71

the problem said the ceiling to the floor was 3m
Vespa71
Vespa71 is offline
#8
May14-11, 03:24 PM
P: 43
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.
Vespa71
Vespa71 is offline
#9
May14-11, 03:33 PM
P: 43
I recommend to make a simple drawing to visualize the problem. Best of luck
goluigi2196
goluigi2196 is offline
#10
May14-11, 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
goluigi2196
goluigi2196 is offline
#11
May14-11, 03:37 PM
P: 20
oh and thanks for c). i got it right.
Vespa71
Vespa71 is offline
#12
May14-11, 03:51 PM
P: 43
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.


Register to reply

Related Discussions
Ball twirling on string attached to stick Introductory Physics Homework 11
Potential Energy! (A box attached to a spring which is attached to the ceiling.) Introductory Physics Homework 13
Calculating the tension attached to a ball attached to coaster rolling down incline Advanced Physics Homework 0
Roller coaster, ball attached to coaster by a string... calculate various tension Advanced Physics Homework 0
a ping-pong ball attached to a string in a jar of water Introductory Physics Homework 2