EMF,

Impact isn't just impact. In physics you'll have to do the bouncing ball test. This shows that energy doesn't just go one way.

Here's a better example. You've played with the Newton's Collision Balls which shows the transfer of energy.

You also know that the swinging back and forth will eventually stop. Why? The energy doesn't just go back from one end of the ball to the other end of the ball. If it did then you have just solved the world energy crisis by making a perpetual machine. However, it does stop.

Why?

First, because energy the energy is dispersed into the air molecules. You have resistance when the ball on the end swings up in the air (air resistance). The energy from the swinging metal ball needs must repel the every constance moving air molecules. This takes energy. The energy is transfered to the air molecules, both ways, too, coming up and down. Energy lost.

Second, each time energy travels through the metal balls in the middle, energy is lost. Because every single object is attached to the frame with strings, the energy will be transfered throughout the the strings to the frame and then to the desk it's sitting on and further on and then back. Of course very little energy, very negligible amount, comes back because the energy is diverted through the whole ground and thus the world (of course it doesn't end there. The entropy of the world is always increasing). Thus, further lost of energy.

The energy lost is fairly minimal (but still high) as you can see because the process of the collision can go for several minutes.

Now picture Newton's Collision Spheres, except replace the spheres with squares.

Doesn't work as well doesn't doesn't? Right exactly why? Exactly what I said about flat spines. Energy dispersion is quicker. First the face of the square thus the energy hitting the face isn't concentrated, but spread. This means that more the energy will spend more time within the square (the sphere works better because the impact is focused on one place). Because of that more energy will be lost the the frame.

Trust me this makes sense. Why do pulley's work? Dispersing of energy through the pulley system. Why do gears work? Dispersing of energy?

It's also the reason why, so far, in this world there are no such thing as perpetual machines.

You can even calculate all the energy lost, transfered, etc.

It's much better if I can demonstrate this instead of explaining.