gravitational energy = work done? (1 Viewer)

[dawso]

grrr....due to my computer blowing up in the holidays, i hav lost my summaries and therefore need 2 redo them, somethin that i never got and asked once on bos but the tread has now been deleted is the second dot point of this topic:

explain that a change in gravitational potential energy is related to work done.....

what do people hav for this dot point? i probly still wont get it but at least i will hav somethin 2 copy in2 my summary....

 

[velox]

Consider the work done in moving an object from the Earth's surface to a height, h metres.

W = F.d

Therefore, W = Fg.d (where Fg is the weight of the object)

Fg = mg (as weight of the object = mass x accelecration due to gravity)

Therefore, W = mg.d

Therefore, W = mgh (h is the distance the object has been moved)

Where:

W = Weight
F = Force
d = Displacement
g = acceleration due to gravity
h = distance object has been moved (in this case usually height-spacecraft)

taken from hsc.csu.edu.au

 

[zenger69]

Gravitational Energy is defined as the work done in moving an object from distance of infinite magnitude from the gravitational field to the gravitational field.

Work Done = Force x distance

Force = gm1m2/d^2

Work Done = gm1m2/d^2 x d (cancelling out one d)

Therefore gives the Gravitational Potential Energy formu GPE = gm1m2/d and is minus because gravity is pulling the object down.

Hope it helps.

 

[velox]

Can't we use this one aswell:

Gravitational potential energy Ep = work done to move to the point
= force required * distance moved
(Since work, W = fr)
= (mg) * h = mgh


I mean, which one is better?

 

[dawso]

Consider the work done in moving an object from the Earth's surface to a height, h metres.

W = F.d

Therefore, W = Fg.d (where Fg is the weight of the object)

Fg = mg (as weight of the object = mass x accelecration due to gravity)

Therefore, W = mg.d

Therefore, W = mgh (h is the distance the object has been moved)

Where:

W = Weight
F = Force
d = Displacement
g = acceleration due to gravity
h = distance object has been moved (in this case usually height-spacecraft)

taken from hsc.csu.edu.au

thought it looked familiar....

 

[velox]

So u dont understand that?

 

[dawso]

yeah i understand that much, but was just wonderin if there is any more to this dot point...,.but obviously not

 

[aim54x]

not much to add 3 this. i think everything has been covered.

don't u keep class notes???

 

[velox]

is the csu dot point answer i posted, adequate for this dot point?

 

[dawso]

yeah, of couse we hav notes in class, but u gotta hav responses for each dot point.....a specific answer is always better than the idea in general,

 

[zenger69]

trouble with using F=mgh is that it's not very accurate and not yr12 level which my teacher said.

And she penalised me on that question for using mgh. So everytime I get a dot point similiar to that, I use the gravitational force formula.

 

[velox]

Yeah i remember u stating something like that before. However i dont get it when you say this

Work Done = gm1m2/d^2 x d (cancelling out one d)

 

[JumboKHS]

Gravitational potential energy is a form of potential energy stored by a body caused by the virtue of its position (its location within a gravitational field).
A change in gravitational potential energy is caused by a change in the position of a body within a gravitational field, such a change requires a force to cause displacement. Since work done is a product of force and displacement it can be said that a change in gravitational potential energy is caused by work done.

And for those still abit brainfrozen here is a different way of looking at it, this will answer the next dot point as well.

Work done is equivilent to the change in energy = GPE1 - GPE2

*When an object is at a very large distance away (assume distance is infinite) then GPE=0. {Therefore GPE1 = 0}

*When an object is at a point within a magnetic field GPE = x {therefore GPE2=x}

Work done = | change in energy |
Work done = | GPE1 - GPE2 |
Work done = | 0 - x |
Work done = | -x |
Work done = gravitational potential energy

Thus the gravitational energy of an object is equal to the work done in moving it from a very large distance to a point within the gravitational field.
And
If Work done = gravitational potential energy,
than a change in one would equal to a change in the other.

Absolute value signs because in 'work done' questions it doesnt matter if your answer is +ve or -ve. Unless the chemistry junkies want to get technical about it.

 

[zenger69]

when you multiple by D is the distance the object has moved.

 

[m_isk]

Therefore gives the Gravitational Potential Energy formu GPE = gm1m2/d and is minus because gravity is pulling the object down.

Hope it helps.

Really, you should have a better explanation of why the expression for GPE is negative. Rightly, you derive that it is equal to GmM/d. As you can see from this expression, it will equal zero only when d is very large. You also know that the higher an object is, the greater its GPE. In light of this, the greatest value for GPE is at infinity, and everything else has a value less than that at infinity. But at infinity, GPE is zero, and hence all other values of GPE are negative. Hence the negtive sign in the expression (-GmM/d).