# What is consumed when an object falls down ?

1. Jul 31, 2013

### bksree

When work is done using an energy source, say a battery, energy is consumed and the battery is able to do work till its chemical energy is consumed.
Everytime an object falls down work is done by gravity ? Suppose everybody in the universe starts dropping objects into canyons, valleys and the oceans on earth, then why isn't it possible to exhaust gravitational energy.

2. Jul 31, 2013

At the first place,Energy is required to move objects up the canyons valleys etc.

3. Jul 31, 2013

### ProfDawgstein

If you drop something gravity does work on the object, but it also decreases its potential energy.
The opposite would be lifting something up, which does increase its potential energy.
That means that gravitational energy always reduces potential energy, so it does not get exhausted.

4. Jul 31, 2013

### bksree

Let's consider the case where people are pushing the earth around them down (no lifting objects to heights), say like standing on top of the grand canyon and all other canyons and blasting the earth down or people are standing around lakes, wells & oceans and pushing the earth down.

5. Jul 31, 2013

What do you mean by pushing the earth down?

6. Jul 31, 2013

### bksree

What was meant is nobody is lifting objects to heights and then pushing it down. People are just standing near large canyons, valleys etc, digging out large boulders and pushing it down.

7. Jul 31, 2013

### ProfDawgstein

But the objects already have potential energy, if not they would not be up there (waterlevel = 0 potential energy).
The objects would just fall and lose potential energy (and gain kinetical energy).

8. Jul 31, 2013

### A.T.

This will fill the canyons, and at some point you have equal potential around the globe.

9. Jul 31, 2013

### ProfDawgstein

thus creating a "perfect" sphere?

10. Jul 31, 2013

### bksree

means value of g will change ?

11. Jul 31, 2013

### ProfDawgstein

g will be constant everywheres, because everything will be reduced to the same height.

12. Jul 31, 2013

### phyzguy

I would argue that it is possible to exhaust gravitational potential energy. The universe started in a very uniform state with a very large amount of gravitational potential energy. As the matter "clumps up" into ever larger assemblages of mass, the usable gravitational potential energy is being used up. As the universe ages, more and more of the mass will be gathered into black holes, from which it is not possible to extract energy (at least once they spin down). This is more properly analyzed in terms of gravitational entropy. Penrose' book "The Road to Reality" discusses in detail how the universe was formed in a very low state of gravitational entropy and how the entropy is increasing enormously as matter clumps up to form black holes.

13. Jul 31, 2013

### B4ssHunter

because gravitational energy does not depend on a source, just like magnets you can keep coupling two magnets forever , you will have to spend some energy though to let them go away from each other , actually , when you drop something into earth , energy is somehow relieved , dropping something into earth or letting two magnets come into each other does not spend energy, but it actually gives energy

Last edited: Jul 31, 2013
14. Jul 31, 2013

### Staff: Mentor

Energy isn't really 'consumed'. Potential energy from the battery is simply transformed into other types of energy and moved elsewhere. Likewise, gravitational potential energy is transformed into kinetic energy as the object falls. After impact, ALL of the kinetic energy is transformed into heat and other types of energy. So what used to be gravitational potential energy is eventually transformed into other types and it takes work to give that object it's gravitational potential energy back.

You could say that the objects gravitational energy, with respect to the earths surface, has been exhausted once it impacts the ground.

15. Jul 31, 2013

### bksree

I agree that gravitational potential energy is finally converted into heat. The question is shouldn't there be a corresponding decrease in the source of grav. pot. energy ? How is this manifested physically ?

16. Jul 31, 2013

### Staff: Mentor

Let's say I drop a ball. The ball hits the ground and the kinetic energy is converted into heat and such. Until that energy escapes the system (the Earth and the ball) there is no decrease in the source. (If by source you mean gravity)

But let's say some of that heat is radiated away from the Earth is has escaped our system. Now we have less energy, less mass, and less gravity than we had before.

17. Aug 1, 2013

### sarose

Let us consider a body of mass m at height h from ground at point A (say) initially at rest is falling under the action of gravity. If g be the acceleration due to gravity, then at point A,
kinetic energy= 1/2(mv^2)=0 (because object is initially at rest)
potential energy=mgh
Therefore at point A total mechanical energy=mgh

after covering distance x particle reaches at point B,which is h-x above the ground level.
at this point velocity v* of object can be obtained by using equation,

v*^2=u^2+ 2gx
since u=0
v*^2= 2gx
and kinetic energy at point B is 1/2( m 2gx)=mgx
potential energy = mg(h-x)=mgh- mgx
therefore total energy=mgh

now at ground velocity of object is given by
v**^2=u^2+2gh (since object traversed the distance h)
=2gh ( since u=0)
kinetic energy at ground=1/2(m 2gh)
=mgh
potential energy=0 (since P E is measured from ground and h=0)
therefore total energy= mgh

Hence, from above we can see that total mechanical energy is constant throughout the motion of object.
The force is to be done when object moves against the gravity, but your case is opposite of that. Means falling object is not against the gravity.

18. Aug 1, 2013

Is potential energy at sea level zero?

19. Aug 1, 2013

### Staff: Mentor

The answer is simply no as can be obviously seen by the fact that you can lift the object back up and reset and repeat the scenario over and over again, forever.

Consider rain.

20. Aug 1, 2013

### Staff: Mentor

It is typically defined to be zero, but it doesn't have to be.

21. Aug 1, 2013

### bksree

Thanks to all for the inputs.
Many of the replies here have assumed that the object is lifted up to a height and then dropped down. This is not the scenario I am imagining.
The situation is : People just drag the boulders, earth, whatever and drop it into the nearest well, lake, valley, cavern , whatever.
When each of these bodies falsl into the nearest depth, work is done by gravity which is converted into heat when the body strikes the floor.

Since work is done by gravity each time, shouldn't its source (whatever that is) start running out of energy ? How is this physically manifest finally ?

22. Aug 1, 2013

### phyzguy

As others have pointed out, if you keep doing this, eventually the Earth will be perfectly spherical, at which point you can no longer extract any gravitational potential energy unless you supply energy by lifting something up.

23. Aug 1, 2013

### D H

Staff Emeritus
No, it won't. The Earth is spinning.

24. Aug 1, 2013

### Staff: Mentor

Most of the potential energy has been there from the beginning. Every particle that makes up the earth and everything around is some distance from the center of the earth, and energy would be released (work would be done) if that particle were to fall all the way to the center.

When something moves downwards (a boulder rolls into a valley, water flows downhill, we fill in a low spot in front yard) the gravitational energy that's released does some amount of work and eventually ends up as heat. However, that can only happen if there's a height difference somewhere. We need a high spot and a low spot so that something can move downwards from the higher spot to the lower spot - and every time that happens the high spot gets a bit lower, the low spot gets a bit higher, and we have less height difference to play with.

1) The earth never "runs out" of gravitational potential energy, because every particle on the surface of the earth is about 4000 miles from the center of the earth, and that's a long ways to fall.
2) We could, however, eventually run out of usable gravitational potential energy (where "usable" means that it can be turned into work and kinetic energy). Once every low spot has been filled in and every high spot has been knocked down there are no longer any height differences, and without these height differences there's no possibility of moving downhill. The world would have to be a smooth and nearly perfect sphere.
3) However, the supply of usable gravitational potential energy is being continuously replenished: The movement of continental plates pushes up new mountains (imagine the amount of work done if we could drop the Himalaya mountains into the Marianas trench); the sun heats water so that it evaporates and rises; we can use the chemical energy of our muscles to throw things into the air; and so forth.

Last edited: Aug 1, 2013
25. Aug 1, 2013

### Staff: Mentor

No. Gravity does not require energy to function. And by that I mean that a book resting on a table is pulled down by gravity, but no work is done by the book and no energy is transferred anywhere. None of the four fundamental forces of nature require energy to function.