Have already received excellent help in understanding this but might need a bit more. Suppose we have a gas inside a cylinder with a piston in it. Now my teacher said that to compress the gas quasistatically we would need to press in the piston with a speed, that is slow compared to the speed of sound, which makes sense as this is the speed at which pressure differences travel. Now my question is: Don't we also need to press in the piston with a force only infinitely larger than the force from the internal pressure? I mean if we have a gas in a cylinder with a piston of area 0.01m^3 and pressure of 100Pa such that the internal force on the piston initially is 1N. Then we couldn't really achieve a quasistatic compression by exerting 100N on the piston could we? But is this maybe accounted for in saying that the speed should be sufficiently slow? Overall I think this way of thinking of a proces is quite weird? Do they occur often in nature? Because it would seem that to compress something quasistatically you would the whole time have to keep an eye on not exerting a too large force..