# What does it mean for energy to be lost? I thought that you could not lose energy!

## Main Question or Discussion Point

I got this question on a study sheet however, I am not sure if I understand, I thought that there was no such thing as a lost of energy because it can not be destroyed. However it can convert into a different type of enery. Please help me get a clue. Related Classical Physics News on Phys.org
berkeman
Mentor
Total energy should be conserved. What is the context of your question? What energies and events are we talking about?

Total energy should be conserved. What is the context of your question? What energies and events are we talking about?
There is nothing else in this question just that.

robphy
Homework Helper
Gold Member
energy can be transferred from one subsystem to another

Danger
Gold Member
It can also be bound into matter.

If you aren't accounting for thermodynamics, then energy can be "lost" due to friction. That's the usual context that I'm familiar with, unless you're talking about viscous losses, but that's more or less the same deal.

russ_watters
Mentor
There is nothing else in this question just that.
There has to be. The answer depends on the context. Two examples:

-Your house loses energy in the winter.
-A chemical reaction (such as buring fuel in your heater) converts chemical energy to heat energy and the total energy before and after is the same.

HallsofIvy
Homework Helper
Energy is conserved in a closed system. If the system is not closed energy can be lost outside the system. For example, if a mass is sliding down a slope with friction, the mass is losing energy to the slope. If you consider the mass and slope as a single system the energy is consered- the mass slows down but the slope's internal energy (temperature) increases.

rbj
i think that what they mean by "energy being lost" is that it was turned into heat and dissapated into the ambient enviroment.

in the winter, you can heat your house and then open the door and leave it open. you are losing heat but somewhere else, the temperature is gainig by a non-measureable amount.

There has to be. The answer depends on the context. Two examples:

-Your house loses energy in the winter.
-A chemical reaction (such as buring fuel in your heater) converts chemical energy to heat energy and the total energy before and after is the same.
:uhh: No there isn't... that was the question on the study sheet I assumed this mean that it was never really lost it was just converted into a different type of energy. In my text book it says that energy can not be destroyed but can be converted??

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Energy is conserved in a closed system. If the system is not closed energy can be lost outside the system. For example, if a mass is sliding down a slope with friction, the mass is losing energy to the slope. If you consider the mass and slope as a single system the energy is consered- the mass slows down but the slope's internal energy (temperature) increases.
okay this just sounds like energy being conserved to me ...no?

If the question is "what does it mean for energy to be lost?"

Then you basically have the right idea - it means that energy is transferred from the chief system of interest to something else. As others have pointed out, this is usually due to losses by heat (e.g. to the atmosphere).

HallsofIvy
Homework Helper
Energy is conserved in a closed system. If the system is not closed energy can be lost outside the system. For example, if a mass is sliding down a slope with friction, the mass is losing energy to the slope. If you consider the mass and slope as a single system the energy is consered- the mass slows down but the slope's internal energy (temperature) increases.
okay this just sounds like energy being conserved to me ...no?
Yes, that's what I said: "If you consider the mass and slope as a single system the energy is consered- the mass slows down but the slope's internal energy (temperature) increases."

My point was that if you do NOT include the slope itself in the "system", just the sliding object, then energy in the system is lost outside the system.

Yes, that's what I said: "If you consider the mass and slope as a single system the energy is consered- the mass slows down but the slope's internal energy (temperature) increases."

My point was that if you do NOT include the slope itself in the "system", just the sliding object, then energy in the system is lost outside the system.

Another way of seeing it is in terms of efficiency. Just about nothing in our practical world has 100% efficiency. Efficiency, in the case of thermodynamics and most other areas of phyics, refers to a discrepency in the input and the output of energy - the output may be less than the input, and hence it can be said that energy has been "lost" to the surroundings.

This happens to be in keeping with what I've been wondering about, so I won't start a new thread, but if it seems like I should, someone will pipe up about it, I expect.

If a brick is pushed gently in space, it will move uniformly until acted upon by an external force. If it then collides with another brick coming at it at the same speed, I know they don't suddenly stop (and now I'm thinking about it, I need to go out and find two billiard balls and roll them into each other to see what happens), and I'm imagining that they each impart kinetic energy to each other and they bounce away from each other... but I can't imagine if they're now going slower or not. Thinking back on billiards, it seems that the ball that is sent into another stationary one comes to a halt while the other one shoots off. It imparts all of its kinetic energy...

Anyway, I don't know how much sense that all makes.

Many thanks.

There are different types of collisions, and energy and momentum act in different ways in different situations.

Let us look at the perfectly elastic collision, in which (someone correct me if I'm wrong) both total energy and total momentum is conserved. If two bricks collide in space, for total energy to be conserved, they would both need to collide and travel back in the opposite direction at the same speed.

In a perfectly inelastic collision, where only total momentum is conserved, can be exemplified by two cars of the same mass, running at the same speed which crash head on into each other. They should both come to a halt. Where does the energy go you might ask? It is dissipated in sound, heat, and in deforming the bodies of the cars.

There are the in betweens as well, simply "elastic" and "inelastic" collisions, which you can go wikipedia or google if you feel so inclined :)

vanesch
Staff Emeritus
Gold Member
:uhh: No there isn't... that was the question on the study sheet I assumed this mean that it was never really lost it was just converted into a different type of energy. In my text book it says that energy can not be destroyed but can be converted??
I guess that the question is a "dissertation" question. A small statement is thrown at you, and you are supposed to write up a well-ordered reflection on the topic, with introduction, devellopment of argument, and conclusions.
The introduction analyses the different kinds of ways of looking at the problem, in the devellopment you treat these different things, and then you conclude.

Is it a question about efficiency? If so, it's just asking what percentage of the output energy is not "useful" output energy (the energy form you want the input energy to be converted to). For example, in a lightbulb, the percentage of energy converted to light would be recognized as "useful" output energy, and the percentage of energy converted to heat as energy that is "lost". If the purpose of the lightbulb was to heat your home, then the heat would be called "useful" output energy. If that's the case, I would also suggest you find a more efficient way to heat your home. So to find the amount of energy "lost", you would go:

useful output energy (J) = efficiency (%) * total input energy (J)
......................................................100

And then:

energy lost (J) = total input energy (J) - useful output energy (J)

Hopefully that was helpful to you. P.S. Ignore the dots. They're just there to put the 100 under the numerator.