# Like what IS energy? relative to force

1. Dec 7, 2013

### alodia

I've been doing basic physics and i understand that to describe the

Kinematics or trajectory/movement of things, we use the relationship between position, displacement, velocity, acceleration, and time

while to describe the
Dynamics or cause of motion, we use the relationship between mass, acceleration, force, distance, energy, work, and time

both when combined will fully describe the mechanics or 'how stuff moves'.

but i'm lost at energy. what is it? i understand is a force is a push or pull, there's only 4 fundamental forces in the universe: gravity, electromagnetic, strong and weak nuclear force.

but what is this energy?
- is it like the means to generate a force?
- is it like the blood in your body to fuel a punch or a kick?
- is it like money that a company needs to operate?

now this work thing. are you saying that... since energy is like this internal thing, this much energy can only do so much work.

- this amount of energy can generate a force over some distance?
- this amount of blood can generate a punch or kick over some distance?
- this amount of money can sustain a company over some distance?

and therefore power is... the rate of work... so

- this amount of force over some distance every unit of time?
- this punch or kick over some distance every unit of time?
- this company operating some distance every unit of time?

....

thankS!

2. Dec 7, 2013

### Jimmy

You might want to look at this blog entry by ZapperZ:

https://www.physicsforums.com/blog.php?b=3203 [Broken]

Last edited by a moderator: May 6, 2017
3. Dec 7, 2013

### alodia

thanks but you've given me another rabbit hole rather than some satisfying treat so i can sleep well tonight

4. Dec 8, 2013

### Andrew Mason

Energy is defined as the ability to do work. Work is the application of a force (to a mass) through a distance. This might not be the most appropriate way of looking at energy for purposes in quantum physics, but it is still the best definition.

Changing the speed of a mass requires work. So energy is intimately connected to mass, time and distance. Our concepts of time and distance are tied to the concept of an inertial frame of reference, which requires mass. In a massless universe, energy would not have much meaning.

Energy is a mathematical device to keeping track of the capacity to perform work. Energy is viewed as "something" because it is conserved in all interactions of matter.

AM

5. Dec 8, 2013

### Staff: Mentor

It sounds like you already know what energy is. The usual mechanical definition is "energy is the ability to do work". That is what energy is.

The more important question is, given the fact that you already know what energy is, why do you feel uncomfortable with the concept?

6. Dec 8, 2013

### Malverin

When you apply a Force, you transfer Energy

If you push some object with a Force, it will move or deform.

The object will move fast when you apply Force, when it is not heavy and there is a little friction.

The object will deform fast, if it is soft.

The formula that describes connection between Force and Energy is

E= F * V*T = F*S

E => energy

F => force

V => movement (deformation) speed of the object
T => period of time in wich you apply the force to the object

S => the distance that object will travel (deform) due to the force

7. Dec 8, 2013

### Staff: Mentor

Only as long as the object is accelerated. A book pushes down on a table, yet no energy is transferred since neither the book nor the table are accelerated at all.

8. Dec 8, 2013

### Oldfart

But the table will be locally accelerated and deformed in way of the book, the table legs will be deformed. the floor in way of the table legs will be deformed, etc. This is easier to visualize if an anvil is placed on the table, but in any case energy is transferred to make these things happen, right?

9. Dec 8, 2013

### Malverin

It is deformed when it is not moved. I have written that.

10. Dec 8, 2013

### Staff: Mentor

But the force continues even though the object does not continue to deform or move.

11. Dec 8, 2013

### Andrew Mason

I don't mean to be overly-picky here, but the object can be accelerated without a transfer of energy. If the force/acceleration is perpendicular to the displacement, there is no work done so no energy is transferred.

AM

12. Dec 8, 2013

### Malverin

Resultant force will be zero at the end of deformation, because, elastic force will be with equal magnitude and opposite direction of acting force.
So there will be no more deformation and no more work will be done.

13. Dec 8, 2013

### Staff: Mentor

Absolutely. A planet in a perfectly circular orbit always maintains the same orbital energy.

14. Dec 8, 2013

### voko

Well, not exactly. The planet itself changes its shape somewhat (tides) even in perfect circular motion. And that shape-changing is not elastic, so energy is being lost.

15. Dec 8, 2013

### Staff: Mentor

The resultant force is zero, but the individual forces are still being applied. So it is incorrect to say that when you apply a force you transfer energy. And, as AM correctly pointed out it is possible to apply a non-zero resultant force without transfer of energy also.

16. Dec 8, 2013

### alodia

if the net force is non-zero, then there must be some change in acceleration, and therefore velocity and therefore transfer of energy. how's it possible to "apply a non-zero resultant force without transfer of energy."??

also, i get the textbook definitions.. but for me and many others who have to understand this from an intuitive perspective, energy just don't make sense. i get force, i can see it, pushing and pulling... but what i don't get is how pushing and pulling transfers some part of me.

like i get heat energy. its just there. no heat no movement. absolutely zero. is kinetic energy same as heat?... what about other forms of energy... there's only 4 forces. but how many energies?

17. Dec 8, 2013

### Staff: Mentor

Uniform circular motion. There is an unbalanced non-zero force (the centripetal force), a non-zero acceleration, a changing velocity, but no transfer of energy.

If you get the textbook definition then you understand energy. There isn't anything more to understand.

The problem isn't that you don't understand energy, it is that energy is such an important concept that you think that there must be some deeper hidden intuitive meaning that you are missing. You aren't. Energy is what the textbooks define it as, nothing more nothing less. It is indeed important, but not mysterious. If you understand the textbook definition of energy then you know what energy is.

Last edited: Dec 8, 2013
18. Dec 8, 2013

### alodia

i think a better question is why is energy necessary? or what was human trying to solve / understand that begets the idea and concept of 'energy'?

because necessity is the mother of all invention, so what was that turning point in history, what was that question or series of events that led to the idea and concept of 'energy'?

19. Dec 8, 2013

### Staff: Mentor

Oh, I do like that question. That is indeed a much better question.

I don't know the historical motivation for energy, but one thing that energy does is that it makes analyzing many systems much easier. For example, something sliding down a curved frictionless slide. You can solve such a problem using forces without energy, but you need to know the exact shape of the slide so that you can determine the normal force at each point. With energy, you can solve such a problem much easier. All you need to know is the height of the slide.

In more advanced problems it turns out that conserved quantities like energy are related to symmetries of the system. In the specific case of energy, it is related to time translation symmetry. So if a system is the same today and tomorrow (symmetry) then you know that there is a conserved quantity (energy). That conserved quantity, again, makes analyzing the system easier.

20. Dec 8, 2013

### alodia

yea i saw that khan academy video...i'm getting an inkling that energy is ... some internal stuff that can only be transferred, like in this slide example, and how much stuff you got depends on how much mass you have...

but then i'm confused about how something moving faster has more 'stuff' so something not moving has no stuff? i know when moving is kinetic energy and when stationary is potential... but if there's no height to fall from, then there's no stuff? what if there's no gravity and only masss. a floating mass object in complete vacuum of space with no forces... so no stuff?

21. Dec 8, 2013

### Staff: Mentor

I am not a big fan of using non-technical words like "stuff" in conversations like this. Since it is non-technical, you would have to give it a technical definition in order to apply it to physics. If that definition is the same as the definition of energy, then it is a duplicate word, and if not, then it is wrong.

Something which is moving can clearly collide with another object and do work. So something which is moving has energy. Something which is elevated can also fall and do work, so something elevated has energy. Something which can be burned can do work, ... Etc. If an object existed which could not do any work, then it would have no energy.

22. Dec 8, 2013

### alodia

what about an objecting in complete vacuum of space under no influence of any forces. does it have energy? does it have kinetic energy? does it have potential energy?

23. Dec 8, 2013

### Jimmy

Richard Feynman said this about energy:

The kinetic energy of a mass depends on its velocity and velocity is relative. If you and I are initially at rest relative to one another and I then accelerate to some velocity, relative to me, you will have a non-zero kinetic energy. However, I haven't given you any stuff. If I then match velocities with you, have I somehow taken something away from you?

Feynman's description of energy works just as well for momentum. However, people don't seem to get hung-up over momentum like they do with energy.

Last edited: Dec 9, 2013
24. Dec 8, 2013

### Staff: Mentor

Even an object at rest with no potential or kinetic energy still can do work due to its mass, by E=mc^2.

25. Dec 8, 2013

### Jimmy

The following book might shed some insight: