I have some problems understanding when a system is isolated or not. I know the definition: Well, my confiusion is better shown by an example. Consider an Merry-go-round (MGR): A person runs towards the MGR while the MGR stand still. Call this situation 1). He the jumps on it. This obviously makes the MGR rotate. Call this situation 2). Define that for the spin, the center of rotation is in the middle of the MGR. Im confused regarding what properties is conserved. Angular momentum, energy and (linear momentum). If you dont care about my ramblings, you can stop read now ;p I would think that momentum (linear, p=mv) is not conserved. In 1) the person running makes the system have a p>0, but in 2) the system will only have angular momentum, i.e p=0. Is this correct? I would say that energy is not conserved due to the fact that it is friction that makes the person stick to the MGR. But im not sure about this. I can think of situations where one would not need friction to stick to the MGR, for example if one run into a wall placed on the MGR perpedicular to it. This makes me confused. And then you have angular momentum (spin). The person has angular momentum around the axis of rotation just before he hits the MGR in 1). in 2) the MGR and person both have Angular momentum. I would say that the spin is conserved, because there is no external net torque acting on the person-MGR system. However, you have that friction again.. One can perhaps look at the friction as a torque.