I've been breaking my head a couple of hours to figure out the concept of momentum, and it's difference between kinetic energy. Here's what i came up with, please correct the things which are wrong. The blue object has a starting velocity [tex]v_1[/tex] so it has a momentum [tex]v_1 \cdot m_1[\tex]. The green rectangle represents a closed system. As the blue object moves, it looses its kinetic energy do to friction, and at the same time it transfers the momentum to the system. Since there is a force on both the blue and the green rectangle, each of them is accelerating in the direction of the force. When all the kinetic energy is converted to heat, the blue object stops, and at that same time, all of the momentum has been transfered to the green rectangle. Just one thing, to avoid confusion. I am aware that I cant say that because it would be normal that i observe the relative movement of the green object compared to the STATIC blue rectangle (because i defined it as the closed system). I did so on purpose. While trying to figure out the relationship between friction and momentum i tried observing a calculator on my desk. I push my calculator, it moves a bit and then stops do to friction. I couldn't see where the movement went because i considered my table the closed system and it didn't move when i pushed my calculator (at least not enough for me to notice :) ). If I could notice, i would see that the table transfer its momentum to the house floor, which transfers its momentum to the planet earth. I hope you understand why i called the green rectangle a closed system, and then observed he whole drawing from a even larger perspective. To me, limiting myself to a closed system (in the real meaning of that word) was the main problem of failing to understand momentum. This leads me to another conclusion. The kinetic energy of a moving object can't all be converted to heat (or sound) but must also partially be transfered to a different object in kinetic energy form. I think there must be a formula which defines how much of the energy must remain kinetic.