# Conservation of energy?

1. Mar 18, 2004

### expscv

it says that "energy can not be create or destroyed but from one form to another"

is this work for human? since we eat and use the chemical energy in the muscle to do work = running = kinetic energy but where is kenetic energy transfer to?

can anyone anlyse the energy in a car? oil ===> kinetic energy ===> heat and sound?
but the input and output of energy does not seems to balance.

energy seems to be destroyed

thanks

2. Mar 18, 2004

### Bystander

Sounds like homework --- but, it's the concepts that are giving you trouble, so post a little more on what makes you think energy is "being destroyed."

3. Mar 19, 2004

### expscv

asd

the chemical energy of oil is transform to kinetic energy

but kinetic energe is not trasform to any other form.

it is been destroyed?

4. Mar 19, 2004

Most of it is lost to friction, sound, and heat.

5. Mar 19, 2004

### expscv

umm sounds right

but is fiction a type of energy?

6. Mar 19, 2004

### Staff: Mentor

No, friction is a force which combined with motion and time gives energy. And energy = heat.

Friction in your car, working muscles, whatever: one way or another, it all ends up as heat.

7. Mar 19, 2004

### expscv

yes but the problem ---> can the amount of heat been produced by kinetic energy

be transform into kinetic energy again? exactly as before?

it seems not possible.

8. Mar 19, 2004

It's very difficult to collect energy lost due to friction. In fact, if you managed to pull it off, you'd be an extremely rich man.

9. Mar 19, 2004

### Chen

Collecting the "wasted" energy back would probably require an investment of additional energy, anyway.

10. Mar 19, 2004

### expscv

haha thanks never mind about rich

so: the conservation of energy must be true and that the amout of energy you used or applied somehow trasfer to other

so that kietic energy can trans to many other form like

light heat sound chemical, but how do i calculate the
magnitude?

i only know how to calculate potential to kinetic but not kinetic energy to any other.

11. Mar 19, 2004

It's most easily done by calculating the loss of energy.

For example, if you have a 2kg car that's rolling around at 10m/s at one point in time and then you measure it to be rolling around at 5 m/s a few seconds later, the difference in energy is

1(5^2 - 10^2) = -75J,

so 75J of energy went somewhere. The only real place it could have gone is heat/sound, so there you have it.

12. Mar 19, 2004

### expscv

oh i got it

now i m alive again haha thx a lot

13. Mar 19, 2004

### expscv

thx all!

14. Mar 19, 2004

### revesz

i might be wrong but i think i have thought of another place the energy might go. back to the car slowing down 5km, its tires exert a force on the ground, pushing whatever is under it in the same direction of its movement, so the slight momentum of the car could be transfered to the momentum of the earth itself. The earth will never slow or speed up due to this though, because when the car sped up it also put energy going the other way, so it neutralizes the change. if you throw yourself into a wall, the earth will spin faster, but you had to start running first and that already slowed the earth down. of course the effects would be so minute that they could never be observed.

just a question. if one car going one way crashes into a car going the other way, what happens to the energy, they both stop, and the energy seems to be gone. mabye thats because the energy was opposite in the first place...

15. Mar 19, 2004

### Chen

So if all people around the world starting running in the right direction together, we could stop Earth?

16. Mar 19, 2004

### Chen

For starters, energy is a scalar and not a vector. Therefore the energy of one car can't be the "opposite" of the other car. There is nothing puzzling about this situation - when the cars crash you hear a big bang, don't you? And some heat develops, doesn't it? And the cars are suddenly reshaped, aren't they? That's just some some the processes that use up the original energy.

17. Mar 19, 2004

### revesz

actually the energy is used in stopping the cars, one car exerts a force on the other car with enough force to stop the car, and vise versa, so each take the other cars energy to change their momentum. That makes perfect sence to me, but then what happened to the momentum?

momentum is relative, so it must be a vector, it is directional. If every particle of mass in the universe were moving the same way it would have no kinetic energy, it is only when something is moving the other way that there is momentum. Actually all things have momentum even when there still, they stay on their own place in space-time, which may or may not be moving relative to some other point in space-time, and they stay there untill something else moves them, off that point in space-time. it is imposible to go from a stand still to moving north, without exerting a force south, it is impossible to be heading north and turn and go west without exerting a force east. That is because there is enough momentum going north to go west, but first you must accellerate in that direction. I dont remember what i was trying to get at now... o ya, momentum is scaler but it has three dimentions, the energy can be converted from one dimention to another. What i mean by dimention is left right, momentum, is different than front back momentum, and up down momentum. all are compatable with each other, exept for the opposite. which means that left doesnt cancel out up or down or front or back, but it does cancel out right. Left cancels right, up cancels down, front cancels back. It is true, think about two object heading toward each other. They dont give all their energy to heat and sound when the collide, they cancell out the others momentum, like tug-o-war

Last edited: Mar 19, 2004
18. Mar 19, 2004

### Chen

You seem to be confusing momentum with energy and vice versa. I will just point out that momentum is defined as $$m\vec v$$ and therefore it is a vector, not a scalar. A scalar cannot have three dimensions, and it cannot be exchanged between dimensions. If the two cars have the same mass and the same velocity but in different directions ($$\vec v_1 = - \vec v_2$$), their momentums are also equal but in different directions ($$\vec P_1 = - \vec P_2$$). However, their kinetic energy is exactly identical, since it defined as $$\frac{1}{2}mv^2$$, so the direction of the velocity doesn't matter at all. The two cars together have $$mv^2$$ kinetic energy, not zero.

But you are right that energy is "used" by each car to exert a force on the other to stop it. But force is not a form of energy, and the energy goes somewhere else. For example, when you push a crate across the floor, you exert a force on it, which gives the crate kinetic energy. The force itself is not the energy. Now if there is also friction between the crate and the floor, the crate's velocity will be smaller which means its kinetic energy is also smaller. The "missing" energy does not become a force, the friction, it becomes heat which is caused by the friction. See what I mean?

Last edited: Mar 19, 2004
19. Mar 19, 2004

### revesz

yes i think i am confused with what force, kinetic energy, momentum, and energy are. i sometimes use them interchangably. but there is still a problem that i will try and outline. a force pushing left will cancel out a force pushing right, so there will be no energy used. I was using the cars momentum as the pushing force. The kinetic energy stores the force. But i can put it into another analogy. two teams of very strong body builders are having a game of tog-o-war. They are exerting a force for 3 minutes, so they must be using enegy, but if the rope stays still, how can that be true? is it all converted into heat? how? there is no movement, so there can be no friction. but there is still a force, so there must be energy used. is this right, or is energy only used when work is accomplished?

now i will give another example of the car collision, but now it is two rockets in space. two rockets in space are facing opposite direction, and are accelerating away from each other. Attached to both the rockets is a rope, capabale of stopping both ships if they come to far apart. Now both ships hit the throttle and are soon going extremely fast in opposite directions, away from each other. When the rope stags them, they come to an instant halt, loosing all their momentum, not very much sound or heat would be made in this senario. How is the kinetic energy conserved?

20. Mar 19, 2004

### Chen

Yes, the forces cancel each other so that the object experiences no acceleration and its velocity doesn't change (whether it's resting or not). Energy is still being wasted, of course. Could you stand a push a wall real hard, for hours and hours? Probably not, you would get tired. Why? Because you are spending energy... where that energy goes, you really should ask a biologist. But when you push the wall, your muscles do work even if the wall doesn't move one bit.