# Force and Energy

1. Mar 6, 2013

### g.lemaitre

Energy causes objects to move. Force causes object to change direction, accelerate or decelerate, correct?

2. Mar 7, 2013

### Menaus

Energy doesn't really make objects move, it is our abstraction of the idea of motion, usually in joules. What makes an object move is the force of the field or object moving it, of which manifests our idea of 'energy'.

3. Mar 7, 2013

### g.lemaitre

Don't really understand what you mean. If anyone else wants to chime in I would really appreciate it.

4. Mar 7, 2013

### Crazymechanic

Well maybe it would be good for you to explain what you mean by energy yourself.Energy can be classified in different forms.
One way for example to measure the potential energy of a piece of matter is measuring it's temperature as temperature arises from the movement of elementary particles and atoms in that material.So the lower the temperature the lower the movement the lover the kinetic energy or those particles etc.you see different ways of describing what energy is come into play.

Mechanical energy , when a engine is running measured from the rotor for example.but the rotor is pushed by the electromagnetic field in the electromotor so that's a different kind of energy.One form of energy can be converted to another with some energy loss in the conversion process due to non ideal conditions in real life apparatus.

Well in short force is something with which you measure a acceleration of a car or a rocket but the energy is with which you describe the rocket fuel or gasoline like how much usable energy can be extracted from it in a certain burning process to achieve or convert that do a certain force or thrust or so via different mechanisms.Like the rotating crankshaft /gearbox/differential and wheels.

i suggest read up on energy and force on WIKI for starters.

http://en.wikipedia.org/wiki/Force

http://en.wikipedia.org/wiki/Energy

5. Mar 7, 2013

### MikeGomez

Momentum causes objects to move.

If you're clever you can equate momentum to energy, so in that sense you are correct.

This is correct. Forces change the momentum of an object.

6. Mar 7, 2013

### e.chaniotakis

Motion can be equally described by means of force and by means of energy.
By definition:

-Momentum is the product of mass and speed. It defines the kinetic state of a body

-Force is the cause that makes things change their kinetic state (a.k.a momentum) and create stress. By saying kinetic state I mean both the value of momentum and its direction.

- Energy cannot be defined. It is something like matter. We cannot define it, yet we use it whatsoever. Every body has energy (mass). A moving body has kinetic energy.
When force is applied to a body, its speed changes and thus its kinetic energy changes.
When a body is in a field it has some potential energy (taken that the field is conservative).
Energy is transfered from body to body, and changes forms through physical processes.E.g kinetic energy can become heat after a collision.

7. Mar 7, 2013

### sophiecentaur

I think you are looking for a distinction between two quantities that can actually be looked at as just part of a bigger picture. You are raising and trying to answer a "what is it really" question and there is no answer to that sort of question. We use appropriate quantities to solve appropriate problems - there doesn't need to be any conflict as long as you don't insist on classifying things too rigidly.
You often find that there are alternative ways of approaching problems - you have a choice between a 'forces' approach and an 'energy' approach. They can produce the same answer. When you have done this successfully a few times, you get to realise it's only different facets of the same thing.

8. Mar 7, 2013

### g.lemaitre

No, I'm not. I'm aware that we cannot know what things are because to say what something is you have to say it is like something else which doesn't answer the question. I want to know what energy and force are related to. Is energy related to motion and is force related to change of direction? I've already gotten pretty good answers however.

9. Mar 7, 2013

### ZapperZ

Staff Emeritus
$$\mathbf F = - \mathbf \nabla U$$

There ya go!

Zz.

10. Mar 8, 2013

### sophiecentaur

That's how they are related - and very succinctly put. I wasn't sure that's what he meant by "related to", though.

11. Mar 8, 2013

### g.lemaitre

I guess what I'm trying to answer is why do sub atomic particles move? I thought it was because they have energy. I'm not going to ask why they have energy because I know that can not be answered. I just want to make sure that that idea of mine was correct.

12. Mar 8, 2013

### ZapperZ

Staff Emeritus
This question cannot be answered because it is vague and loaded with undefined situations.

What "sub-atomic particles"? Are you referring to electrons in atoms? Are you referring to conduction electrons in metals? Are you referring to quarks in nucleus? These are all very different situations and have different physics associated with them!

Secondly, "force" is not well defined concept, especially in QM and QFT. Yet, you are still playing with that word and demanding how they all fit in.

Finally, look back at your original question:

Rather than trying to figure out how "sub-atomic particles" move, I think you should make many steps back, way back, and start with basic classical mechanics first. You are not yet equipped to understand how the concept of energy and force are applicable (or not) for those sub-atomic particles.

Zz.

13. Mar 8, 2013

### g.lemaitre

Well, is there any subatomic particle that is ever not moving? Isn't the property that we use to account for particles perpetual motion that they have energy? I realize of course that I could go back and study classical mechanics. You can always do that. But every day a student of physics should ask 2 or 3 questions in physicsforums in order to really progress and understand.

14. Mar 8, 2013

### ZapperZ

Staff Emeritus
Says who? Who made up such a rule? You?

One can also sit back and learn from existing discussions rather than ask something way beyond one's head!

Zz.

15. Mar 8, 2013

### g.lemaitre

Remember the phrase I think by Twain better to be a fool and keep silent than open your mouth and prove it. If you just read physics and never open your mouth you'll never find out if you're a fool. You can read something and understand it incorrectly. The only way to find out if you're wrong is to, one, ask someone more knowledgeable than you, or, two, just hope you have an epiphany which rarely happens.

In regards to your relation of E to U, that's fine, you can relate energy to U, but I want to understand energy in terms of how it is related to motion.

16. Mar 8, 2013

### ZapperZ

Staff Emeritus
You quoted Twain, and then proceeded to contradict it.

It still doesn't change the fact that you simply can't jump into physics right in the middle. You can't talk about energy and "sub atomic particle" when you have poor understanding of basic mechanics. Knowledge is not a series of disjointed pieces of information.

Zz.

17. Mar 8, 2013

### sophiecentaur

So, in the past, no one could have understood anything as a result of personal effort and reading? PF just wasn't around in the formative years of the really bright Scientists around today. The instant gratification in the form of answers from someone who has identified your personal mis-conceptions and then answering in just the right way would be an immense luxury, I think.

If you were prepared to PAY a personal tutor to be taught by the one-to-one question and answer method then you might find someone to indulge you. But it would cost you. If you lived locally, I would even offer, myself but I doubt whether you live in the South of England (statistically). Have you thought of reading all the faqs and searching the threads on this topic before leaping in half way through, trying to make spurious connections between random ideas?

If you are interested in making connections between Energy and Motion of particles, look up kinetic theory in any good text book or on the hyperphysics site (very well put together). There is a lot of info available for those who are prepared to read it.

18. Mar 8, 2013

### Popper

This is an incorrect way of thinking about motion. A body in motion has energy. It is not the cause of the motion. A particle can be moving at constant body with nothing causing it to move.

To refer to force as causing acceleration used to be the way physicists looked at force. However in modern times we no longer think that way. Instead we define force a the time rate of change of momentum. When we look at particular sitations such as an electron in an electric field we can say that the electric field causes the electrons momentum to change. The rate of change is then quantified as F = dp/dt.

Last edited: Mar 8, 2013
19. Mar 9, 2013

### g.lemaitre

It is easier to rid yourself of false beliefs by posing questions to experts than it is to read books written by experts. Let me prove this logically.

1. Expert says A and B are true, therefore I believe A and B mean that C is true.
2. If C is false and an expert is present, the expert will tell me
3. If C is false and an expert is not present, then the expert will not tell me.
4. Therefore, uttering beliefs in the presence of experts is the more likely to remove falsity of belief than uttering beliefs in the absence of experts.

20. Mar 9, 2013