What happens to the energy when we accelerate and decelerate objects?

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In summary, Energy cannot disappear as we all know, but then I wonder: If we put force to an object and make it go at a high speed, and then put force to it to make it go back to a halt. Where did the energy go? We used force at acceleration and deceleration, but we only got a change in position for the object. Surely the energy is not lost. (of course we cannot retrieve the energy by forcing the object back in its initial position) but we only got a change in position for the object.
  • #1
disregardthat
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Energy cannot disappear as we all know, but then I wonder: If we put force to an object and make it go at a high speed, and then put force to it to make it go back to a halt. Where did the energy go? We used force at acceleration and deceleration, but we only got a change in position for the object. Surely the energy is not lost. (of course we cannot retrieve the energy by forcing the object back in its initial position)
 
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  • #2
Jarle said:
but we only got a change in position for the object.

What do you mean by this? Remember that acceleration is a vector quantity, and also that v=0 does not imply a=0, and so in your case you have a change in acceleration.
 
  • #3
When you make something to go at a high speed, you give it some kinetic energy. When you try to stop it, the energy normally turns into heat, another type of energy.
 
  • #4
cristo, what I meant was the we accelerate the object and then decelerate it to a complete halt, so it has no change in velocity at the end. A change in acceleration is a good explanation of transformation of energy to me, if we only accelerated it once, but then decelerating it (accelerating) til stop again, I don't see where the energy has gone.

haiha: So it must turn to some sort of energy, for example heat you say?
 
  • #5
Accelerating an object can add KE or remove KE, depending upon the direction of the acceleration compared to the velocity of the object. (It can even leave the KE unchanged, if the accelerating force remains perpendicular to the velocity--like something twirled in a circle.)

Of course, if you add or remove KE, that energy must come (or go) from somewhere.
 
  • #6
You did work on the object when accelerating it. The object did work on you when decelerating it.

Think of it this way: compress a spring, doing work to store potential energy in the spring. Now use that spring to accelerate your object.

Quickly run around in front of the object, place your spring in front of it and use the spring to slow it. Ignoring resistance or any kind of friction, the kinetic energy in the object will be exactly enough to compress the spring back to where it was before. If you don't like the idea of "running around in front of the object", imagine two different but identical springs. Same thing happens. Net result, you did work to compress a spring initially, and now you are back in that same situation. No energy change.
 
  • #7
Jarle said:
Energy cannot disappear as we all know, but then I wonder: If we put force to an object and make it go at a high speed, and then put force to it to make it go back to a halt. Where did the energy go? We used force at acceleration and deceleration, but we only got a change in position for the object. Surely the energy is not lost. (of course we cannot retrieve the energy by forcing the object back in its initial position)

The answer here is that the kinetic energy you put into the object was later transformed either into potential energy somewhere else, or into heat. For example, if you use a spring to stop the object, the kinetic energy from the formerly moving object will be transformed into spring kinetic energy. Or if you stop an object by friction (or some other non-conservative force), then the kinetic energy will be lost to friction. I could give slightly different answers based on the mechanism used to stop the object, but this is the basic idea.
 
  • #8
You did work on the object when accelerating it. The object did work on you when decelerating it.
So it's actually just a wrong perspective of seeing it from the beginning. I see it now. Does momentum count as energy, or more particulary: Is momentum kinetic energy?
 
  • #9
Yes and no. They are not the same thing. However, you may notice that momentum "p" is the derivative of kinetic energy with respect to velocity:

[tex]\[
\begin{array}{l}
E_k = {\textstyle{1 \over 2}}mv^2 \\
p = mv \\
\end{array}
\]
[/tex]
 
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  • #10
I see, thanks.
 
  • #11
Also, if you give something (like a car) a push, then run around to the other side of it and stop it, you'll expend roughly the same amount of energy stopping it as you did in starting it moving, since your muscles aren't springs.

In Hurkyl's example with the springs, the end result looks exactly like a mirror image of the initial setup and you're back to zero as if nothing had happened. In an example with a person, you've converted a lot of chemical energy to heat.

And to meet the two in the middle, if you use the car's engines to proved the push then the brakes to provide the stopping, it works exactly the same from an energy standpoint as if all the engine's initial energy input had been turned directly to heat.
 
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1. Where does the energy go when we use it?

When we use energy, it is converted from one form to another. For example, when we burn fuel, the chemical energy is converted into heat energy and light energy. When we use electricity, the energy is converted from potential energy to kinetic energy to power our devices. In both cases, some of the energy is also lost as heat.

2. Why does energy seem to disappear?

Energy cannot be created or destroyed, it can only be converted from one form to another. So, when we think energy is disappearing, it is actually being converted into a different form that we may not be able to see or measure. For example, when we turn on a light bulb, the electrical energy is converted into light energy.

3. Does energy ever run out?

The amount of energy in the universe is constant, but it can be converted into different forms. However, some sources of energy, such as fossil fuels, are finite and will eventually run out if we continue to use them at our current rate. This is why it is important to find renewable sources of energy.

4. What happens to the energy in food we eat?

The energy in food is stored in the form of chemical energy. When we eat food, our bodies break down the food molecules and release the energy, which is used to power our cells and keep our bodies functioning. Any excess energy is stored as fat for later use.

5. Can energy be recycled?

Energy can be converted from one form to another, but it cannot be recycled. For example, when we recycle paper, we are not recycling the energy used to make the paper, but we are conserving energy by using less energy to create new paper. However, some energy sources, such as solar and wind energy, are considered renewable because they can be replenished naturally, making them more sustainable options.

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