Does a pencil have energy?

Username99

Sorry now for the stupid question, but I have been trying to teach myself some Physics and I have been struggling with the concept of work/energy.

My question is: Does a pencil sitting alone on a table have energy?

Energy is the ability to do work, anything that can do work has energy, i.e. anything that can exert a force through a distance has energy.

The pencil exerts a force on the table, its weight, (mass of pencil)(gravity) and likewise the table exerts a force on the pencil, but the pencil does not exert that force through a distance, neither dos the table.

Am I correct in saying therefore that the pencil has no energy?

Morbert

The scalar quantity we call "energy" is relative. One person might say the system of the pencil, the table, and the gravitational field has no energy. Another might say it does. Both are correct, provided they treat energy consistently.

FISMA

Learn and remember these two statements. I was taught them in secondary school and they worked well through University to this day.

1. Energy is a property of a system.
2. Work is a process by which energy is transferred or transformed.

Also, don't forget that introductory textbooks usually deal with point particles, at least until rotational chapters.

The bodies that intro Physics books deal with have no internal structures, like our bodies do or as a real pencil would be made up of.

locum-motion

Username99 wrote: »

...My question is: Does a pencil sitting alone on a table have energy?...

A pencil sitting on a table is acting as a store or repository for (at least) three different forms of energy:

1. Potential energy: The pencil is sitting at a height equal to the height of the table above a floor. If it falls, that potential energy is released in the form of kinetic energy.

2. Thermal energy: The pencil (presumably) is at room temperature. Put it in the fridge and its temperature will go down. This happens because some energy in the form of heat will be conducted out of the pencil and into the other items in the fridge, whose temperature will rise (although the chances of that rise being large enough to be detectable are slim).

3. Chemical energy: Burn the pencil. Although you have to put in heat energy first to set it on fire, once combustion begins to happen, your pencil will release its chemical energy in the form of heat.

There may be others, but those three come to mind immediately.

SOL

4. Nuclear energy

Username99

FISMA wrote: »

Learn and remember these two statements. I was taught them in secondary school and they worked well through University to this day.

1. Energy is a property of a system.
2. Work is a process by which energy is transferred or transformed.

Also, don't forget that introductory textbooks usually deal with point particles, at least until rotational chapters.

The bodies that intro Physics books deal with have no internal structures, like our bodies do or as a real pencil would be made up of.

So am I correct in saying energy is a property of system, just like charge is a property of an electron. You can not have an electron that does not have charge, so you can not have an object that does not have energy, be it gravitational, chemical etc...?

With regard to work, if I click my fingers and the table disappears then the potential energy that the pencil had, due to its position in the earths gravitational field, becomes kinetic energy.

If I am correct so far, and work is the process by which energy is transferred from one form to another.

And work is (force)(distance), Force is (mass)(acceleration), so lets give the pencil a mass of 10 grams, therefore the force is (.010)(9.8) = .098N

And if the table is 2 metres off the ground, the work done is:

(.098)(2) = .196J

Is it correct then to say that, .196J of energy are used transferring the pencils potential energy into Kinetic energy?

Delphi91

Username99 wrote: »

...With regard to work, if I click my fingers and the table disappears then the potential energy that the pencil had, due to its position in the earths gravitational field, becomes kinetic energy.

If I am correct so far, and work is the process by which energy is transferred from one form to another.

And work is (force)(distance), Force is (mass)(acceleration), so lets give the pencil a mass of 10 grams, therefore the force is (.010)(9.8) = .098N

And if the table is 2 metres off the ground, the work done is:

(.098)(2) = .196J

Is it correct then to say that, .196J of energy are used transferring the pencils potential energy into Kinetic energy?

Your calculation is correct. I would, however, change your last statement.

When the pencil is on the table, it has 0.196J of potential energy (due to the work done against the gravitational field in lifting the pencil onto the table). If the table disappears, then the potential energy the pencil has is converted into kinetic energy as the pencil falls (Principle of Conservation of Energy).

As the pencil falls, its height above the ground reduces, so its potential energy (given by mgh) reduces also. Its kinetic energy (given by 1/2 mv^2) increases because the pencil is gaining speed as it falls. However, at any given point in the drop, if you calculate the potential energy and the kinetic energy of the pencil and add them together, you will still get 0.196J

Therefore it would be more correct to say that 0.196J of potential energy is converted into kinetic energy during the fall.

FISMA

Username,
I think you've got it.

Most Physicists will speak about potential energies of a single object, like satellites or a pencil, as Delphi did above, and I regularly do. That's not what we really mean.

However, more strictly speaking, the Satellite alone does not have GPE, rather the Satellite-Earth system has stored GPE. Likewise, the pencil-earth system has an amount of stored gravitational potential energy. The pencil did not get on the table by magic, it was placed there. There was work done ON the pencil BY a person. The pencil gained energy at the expense of the person who lost energy.

When work is done between two bodies energy is transferred. However, inside of a body, like your stomach, energy is transformed.

For example, chemical potential energy from food is transformed into kinetic energy of the center of mass of muscles.

The difference between process and property is subtle, but important.

Open a Physics text book and you will see numerous instances of changing energies ΔKE, ΔU, and so forth. However, and someone correct me if I am wrong, you will never see a ΔW, where W is work.

Also, although we have a Law of Conservation of Energy, we are free to discover new energies at any time. Nuclear Energy is still quite "new," relatively speaking. So to is electrical potential energy.

So feel free to discover a new energy at any time. Just make sure you conserve it!:pac:

Username99

Thanks for the help guys, I think I have it now.

Lbeard

FISMA wrote: »

1. Energy is a property of a system.

No, it is temporary displacement from null in any field.

*see how I fitted in space and time in there

My definition is a little too limited but, energy = f(t,x) is a little more meaningful than, energy = f(p) Where p = a property. Which is as meaningful as, Jimi Hendrix = f(guitarstrings). Even MileyCyrus = f(BillyCyrus) could have more meaning.

Energy = f(d), where for the domain d < 0, d > 0, d != 0. Where d is any difference, or non-zero value.