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Hello,
I have a very bright 8th grade student who asked me something I can't answer.

If one were to put some sort of mechanism underneath a big rock that converted the pressure of the rock against the earth into electricity, then how is that not some sort of perpetual energy source? The gravitational potential energy is not changing if the rock is not moving; yet the pressure it exerts must be a kind of energy that can be theoretically converted into electricity, no? The gravitational energy does not "wear out" in this situation... so isn't that a perpetual energy source?

I'm just not sure how to answer his question. He and I both think we understand the basic concepts of gravitational potential energy and kinetic energy. Any guidance you can give us on answering his question would be most appreciated. thank you!

Answer 1:

From the way you ask your question I am guessing that you realize the answer is no - this is not a perpetual energy source. To understand why, think about the forces acting on the rock. The obvious one is the gravitational force on the rock, pulling it down. But if that were the only force, then the rock would be accelerating to the center of the earth. The rock is stationary, so there is also a force pushing up on the rock. If your huge rock is sitting on the ground, then the ground is pushing up on the rock just as hard as gravity is pulling it down. If you stick some sort of device under the rock, then that device will be exerting just as much force upwards on the rock as the rock exerts downwards on the device, so there can't really be any net gain there. Plus you would have to lift up the rock to put the device underneath it, so you would actually have a net loss of energy.

In general you won't be able to get energy out of the forces acting on a stationary object for this very reason. To get energy from a force, the force has to be doing some work ("work" in the technical sense of force*distance, measured in Joules...and note that 3.6 million Joules = 1 kilowatt-hour, which is the unit we use to measure electricity). A good example is moving water (which is moving because of gravity) turning a turbine in a hydroelectric dam. Without the turbine, you can't get electricity out of the water being held behind a dam, because the dam is pushing back on the water just as hard as the water is being pulled against the dam. Put the turbine in, and the forces no longer balance, so the gravitational force can do some work by turning the turbine, which generates electricity. You mentioned potential and kinetic energy, which is at the crux of this - you can have a huge amount of potential energy, but you can't use any of it unless you turn it into kinetic energy. A stationary rock is all potential energy and no kinetic energy, so it's not doing any work.So you can get some energy out of your huge rock, but you would have to roll it up a hill and find a way to capture the work that gravity does as it rolls down the hill. Of course it will still take more energy to roll the rock up the hill than you can get on the way down. Getting energy from gravity only really works in the hydroelectric example because the sun does the 'rolling up the hill' step - it heats the ocean and gets the water to evaporate into the atmosphere, where it can rain down upstream of the dam.

Cheers,

Answer 2:

If one were to put some sort of mechanism underneath a big rock that converted the pressure of the rock against the earth into electricity,then how is that not some sort of perpetual energy source? The gravitational potential energy is not changing if the rock is not moving; yet the pressure it exerts must be a kind of energy that can be theoretically converted into electricity, no?NO!! The rock is not moving, so there is no change in its potential energy, hence no kinetic energy that can be harnessed and used for electricity! Pressure is NOT the same thing as energy! The pressure of the rock is equal to its weight divided by the contact area of the bottom surface of the rock and the earth. BUT - this is BALANCED by the upward pressure of the earth on the rock. The rock is in equilibrium - the weight of the rock is really the gravitational pull of the earth on the rock,measured FROM THE CENTER OF THE EARTH. This pulls the rock DOWN towards the center of the earth. The gravitational pull of the earth on the rock(and the rock on the earth!) causes the rock to push on the surface of the earth. The surface of the earth pushes back on the rock! This is described by NEWTON'S THIRD LAW: action and reaction. The push of the rock on the surface of the earth is balanced by the push of the surface of the earth on the rock. PRESSURE IS NOT THE SAME THING AS ENERGY.

The gravitational energy does not "wear out" in this situation... so isn't that a perpetual energy source? I'm just not sure how to answer his question. He and I both think we understand the basic concepts of gravitational potential energy and kinetic energy.

These are actually very tricky concepts. Both gravitational potential energy and kinetic energy are always measured WITH RESPECT TO a certain reference frame. For example: you think you are at rest now, and have zero kinetic energy; this is true from the perspective of the room in which you are sitting and looking at your computer. But, from the point of view of an observer outside the earth, you are moving with the velocity of the rotation of the surface of the earth plus the velocity of the earth around the sun plus the velocity of the sun around the galaxy!

Gravitational potential energy is always measured WITH RESPECT TO A CERTAIN DISTANCE from the center of the body that you are investigating,such as the Sun, earth, or another planet, etc. The surface of the earth is where we take the gravitational potential energy of falling bodies to be ZERO in our everyday lives.

When you do work to raise an object above the ground, you increase its gravitational potential energy relative to the earth, and when you let it go, the gravitational potential energy is converted into kinetic energy.Some of this energy goes into heating the contact surface between the object and the air, due to friction as it falls, and some fraction goes into heating the contact surface between the rock and the ground when it hits the ground, due to friction at the contact surface. Some small amount goes into the sound that is produced when it hits the ground also. When the object comes to rest on the earth, that's IT. Finished. No more kinetic energy when the rock is resting on the earth, and no more gravitational potential energy until some force comes along and lifts it up again.

Think about it: if there were energy generated just by rocks sitting on the earth, we would not be here, because the earth would have melted long ago!

A word of caution: ANYTIME you think you have invented a perpetual motion machine, you are thinking of something incorrectly.

For a great source of information, please read this awesome little book:FEAR OF PHYSICS by Lawrence Krauss. It explains lots of the semisconceptions and is like a bible for science teachers. You can order it on line or get it from Borders, Barnes & Noble, etc.


Answer 3:

Great question. The answer is that force alone is not enough to generate power. That force must move through some distance. You can push on a rock from below or above until your legs give out, but if it doesn't *move*, you have technically done zero work (transfer of energy). Energy is measured as force times distance moved. So you are actually correct in recognizing that gravitational potential energy is not being changed. In a way, the "perpetual energy source" is actually just "perpetual energy storage." The rock has a certain amount of gravitational potential energy (the Earth is tugging at it), so if you had a deep hole under the rock, the weight (force) of the rock could be converted into useful energy. Otherwise it's just being stored as long as it sits motionless.


Answer 4:

The precise point is that gravitational energy is potential energy unless it makes something move, and then the energy is converted to kinetic energy, no work can be done. So a big rock at the top of a hill has no kinetic energy. Its only when it rolls down that work is done, and this can be converted to useful energy. When the rock reaches the bottom of the hill, it can't role any more, and to make it do useful work, it has to be rolled uphill again. So its not perpetual.

This is the principle of a water wheel, and its not perpetual.


Answer 5:

Energy can only be harvested if a change in energy states is occurring. You cannot harvest energy from a static system, even if the forces in that static system are extremely powerful. So simply having a huge rock sitting on a pressure plate would not generate any energy. You would only get energy by changing the system, e.g. pushing the rock off of a cliff and having it land on your pressure plate (and it would only give you energy when the rock hits, not afterwards).

This principle, incidentally, is known as the Second Law of Thermodynamics: to be able to do work, you have to do an equal or greater amount of work elsewhere.



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