# The Naked Scientists Forum

### Author Topic: Force vs energy?  (Read 28000 times)

#### RobotGymnast

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##### Force vs energy?
« on: 21/02/2009 13:14:21 »
Simple question. I understand that force is (according to wikipedia) "anything that can cause mass to change its velocity", and that energy is "the amount of work that can be performed by a force". What's the difference? When something begins dropping down to the planet, is that force or energy? Is there gravitational energy up above the planet, or is it gravitational energy?

#### lyner

• Guest
##### Force vs energy?
« Reply #1 on: 21/02/2009 13:44:55 »
Force can be described as you say. But a force, steadily pushing on something stationary, involves no work (or transfer of energy) at all. It's only when the force actually pulls or pushes something by a distance that energy is involved.
We say that the Work done (energy transferred) is the force times the distance moved in the direction of the force.
There is neither gravitational force or energy "up there". What there is, is a field. When you put a mass up there, there will be a force on the mass and work was done in getting it there - that's the gravitational energy.

#### lightarrow

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##### Force vs energy?
« Reply #2 on: 21/02/2009 14:13:51 »
Simple question. I understand that force is (according to wikipedia) "anything that can cause mass to change its velocity",
Simple question: when a mirror reflects off a light beam (which is massless) does it make a force on the beam? According to what you write, it shouldn't; however the mirror recoils after reflection, so a force has been applied to it. How would you explain this fact?
P.S. It's to make you reason in a physical way about it.
« Last Edit: 21/02/2009 14:16:25 by lightarrow »

• Neilep Level Member
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##### Force vs energy?
« Reply #3 on: 21/02/2009 14:17:03 »
I remember explaining this to yor_on in a different thread which may interest you.

http://www.thenakedscientists.com/forum/index.php?topic=19107.0

#### RobotGymnast

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##### Force vs energy?
« Reply #4 on: 21/02/2009 19:51:58 »
I'm understanding a bit more.. what I'm getting is that it's a force when it's not acting upon anything? and you would call it energy when it starts acting on something and moving it?

About the light question, the wording confused me a bit... the main thing that stood out was the correlation between your question and your username =P. What do you mean "does it make a force on the beam"?

#### lyner

• Guest
##### Force vs energy?
« Reply #5 on: 21/02/2009 20:30:19 »
There's no force if there's nothing to act on. Newton added his Third Law of Motion  to take account of this. If you can make a force act on something, there must be an equal and opposite force (a reaction). You can't push against the air with your hand - it just slips through and, however 'strong' you are, you still can't impress a force on it (that's an over simplification; there will be a tiny force of a few milliNewtons involved in shifting a bit of air, but you take my point?
The notion of a Field more or less takes care of your original queston (but there is still some argument about it). Lightarrow's post gives a bit of a problem if you just consider Forces; he is being a bit mean here, actually and is pointing out that things are not so simple - ever! There are two other quantities called Impulse and Momentum which may help you.

The Force is a way of finding how an object will behave under a set of circumstances. A force isn't 'just there' - it just describes the effect of one thing on another.

#### RobotGymnast

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##### Force vs energy?
« Reply #6 on: 21/02/2009 20:32:17 »
So gravitational force exists, but when do you call it energy and not force? This is what I'm confused on (or rather, that's my focus.. I'm confused on most of this)

#### Vern

• Neilep Level Member
• Posts: 2072
##### Force vs energy?
« Reply #7 on: 22/02/2009 03:51:19 »
So gravitational force exists, but when do you call it energy and not force? This is what I'm confused on (or rather, that's my focus.. I'm confused on most of this)
I view it as force is that which operates upon an entity to urge it to change, energy is expended when that entity responds to the force and experiences change as a result of that force.

#### Hei-Tai

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• Posts: 59
##### Force vs energy?
« Reply #8 on: 22/02/2009 06:13:56 »
So gravitational force exists, but when do you call it energy and not force? This is what I'm confused on (or rather, that's my focus.. I'm confused on most of this)

Basically force and energy is same thing=== matter motion/function result,,but i think that different is that force we can hmm.see directly,,like mechanical motion,,it is quite clear,,but motion which is non-clear like electricity motion or heat-energy,,and  therefore we use word energy and so on.

Gravitational force is different,,,what is that? Some-kind of wire between planets?

We can use gravitational-mathematics when there is mechanical link between objects like hammer-ball.

We use that same gravit-math when calculate planets rotating-things and it give us enough accurace todays life.

But is it the same thing, situation?

There is no wire between planets,,,i dont see that that kind of wire

So i think that planets born-lay-swim-fly in some kind of space-matter and when we calculate planets we must take that also in the math,,i mean that constant G-force is not constant at longer time.

Todays gravit-math is enough to our life time,,,but science must be accurace,,,wire dont exist between moon-earth like between hammerball-human,,,but still there is some kind of connection between planets,,,otherwice moon dont round earth year by year.

#### lyner

• Guest
##### Force vs energy?
« Reply #9 on: 22/02/2009 11:23:03 »
So gravitational force exists, but when do you call it energy and not force? This is what I'm confused on (or rather, that's my focus.. I'm confused on most of this)

I was trying to draw a distinction between the two. Energy is needed to move a force through a distance- force TIMES distance. So they can't be the same thing. One involves movement and the other doesn't.
Of course gravitational force exists - it keeps you on the ground. You can change your gravitational potential energy by going up or down and then Work is done.

#### lightarrow

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##### Force vs energy?
« Reply #10 on: 22/02/2009 12:34:40 »
About the light question, the wording confused me a bit... the main thing that stood out was the correlation between your question and your username =P. What do you mean "does it make a force on the beam"?
You wrote: <<I understand that force is (according to wikipedia) "anything that can cause mass to change its velocity">>
If that were true, then a mirror should make NO force on a light beam reflecting off it, because a light beam has no mass, but its velocity changes (from +c to -c) after reflection. Then, how it's possible that the mirror recoils after reflection (as experimentally oseved)? According to the third law of dynamics, if a body A makes a force on a body B, then the opposite hold true as well: the body B makes a force on the body A. Since the mirror has recoiled, light must have made a force on it; then, in the same way, the mirror should have made a force on the light beam!

Infact the mirror DOES make a force on the light bem. A force is defined as a time variation of momentum, and light has momentum (even if it has no mass). So, that definition of force that you wrote is not correct, in the sense that is not general.
« Last Edit: 22/02/2009 12:43:15 by lightarrow »

#### swansont

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• Posts: 62
##### Force vs energy?
« Reply #11 on: 22/02/2009 13:33:38 »

Basically force and energy is same thing=== matter motion/function result,,but i think that different is that force we can hmm.see directly,,like mechanical motion,,it is quite clear,,but motion which is non-clear like electricity motion or heat-energy,,and  therefore we use word energy and so on.

One cannot simply equate force and energy; if the force is perpendicular to the motion there will be no work, and thus no change in energy.

#### lyner

• Guest
##### Force vs energy?
« Reply #12 on: 22/02/2009 14:10:46 »
Hai Tai.
Saying force and energy are the same thing is total nonsense. If you know so little Physics, you should not make authoritative statements. Your model is total fantasy; it doesn't match reality and there is no experimental evidence for it.
When you have made something up in your head, you should treat it as such. Do you think all textbooks are rubbish?Have you read any?

#### Vern

• Neilep Level Member
• Posts: 2072
##### Force vs energy?
« Reply #13 on: 22/02/2009 14:39:11 »
Quote from: lightarrow
Since the mirror has recoiled, light must have made a force on it; then, in the same way, the mirror should have made a force on the light beam!
The light beam then must be changed in some way after the impact with the mirror. I suspect it would be wave-length shifted slightly toward the red.

#### RobotGymnast

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##### Force vs energy?
« Reply #14 on: 22/02/2009 15:23:39 »
Alright I think I get it, all in all. Thanks for clarifying the definition of force for me.
Although the velocity only changes from +c to -c if it comes to the mirror head-on, doesn't it? If it came at an angle, the right-left component of the direction would be the same, wouldn't it? Ah well, I realize it was only meant for clarification, and it worked; I guess that's what's important.

#### lyner

• Guest
##### Force vs energy?
« Reply #15 on: 22/02/2009 16:42:20 »
RobotGymnast
Glad to have helped to sort of something for you.
Btw, excellent to give your info in your sig. People should do that (or fill in their profile) as a matter of course.

#### lightarrow

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##### Force vs energy?
« Reply #16 on: 22/02/2009 19:10:02 »
Quote from: lightarrow
Since the mirror has recoiled, light must have made a force on it; then, in the same way, the mirror should have made a force on the light beam!
The light beam then must be changed in some way after the impact with the mirror. I suspect it would be wave-length shifted slightly toward the red.
Certainly.

#### lightarrow

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##### Force vs energy?
« Reply #17 on: 22/02/2009 19:13:12 »
Alright I think I get it, all in all. Thanks for clarifying the definition of force for me.
Although the velocity only changes from +c to -c if it comes to the mirror head-on, doesn't it?
Of course.

#### RobotGymnast

• Jr. Member
• Posts: 47
• Dum de dumm...
##### Force vs energy?
« Reply #18 on: 22/02/2009 19:55:11 »
Okay, I think I get it for the most part. Thanks everybody

#### The Naked Scientists Forum

##### Force vs energy?
« Reply #18 on: 22/02/2009 19:55:11 »