# The energy is a system a property

## Work, energy and performance

The conservation of energy contradicts our everyday experience: A pounding rubber ball never jumps as high as it does when you let it go for the first time and at some point remains on the ground - where has the positional energy gone? A rolling skateboard stops at some point - where has the kinetic energy gone?

Even if it's hard to believe: with a thermal imaging camera you can see that the floor, the rubber ball, the rollers of the skateboard and the street have become a little warmer: Positional and kinetic energy have been converted into internal energy / thermal energy and are partially transferred to the ground, rollers and road. Since we can do little with this inner energy, we say that the initial energy has been "devalued".

Start the animation in Fig. 4. You will again see a multitude of jumps while jumping on the trampoline. Now, however, at the end of a jump, the purple ball no longer comes to its original height, but comes to rest a little lower. At the same time, after a few jumps, you can see that the jumping mat is changing its color to red; this should represent a slow warming of the jumping mat.

What does this tell us about the energy?

• First the ball has positional energy.
• This positional energy becomes less and less over time, but the internal energy of the trampoline increases.
• In the end, only the trampoline has internal energy.

The position energy of the ball at the beginning is thus converted into the internal energy of the trampoline over time and is "devalued" in the process. We therefore state: