speedfan is an excellent program; however it can be inaccurate -- and the option to increase/decrease the speed of the computer fans is always handy :)
the conservation of momentum states that momentum is conserved in an isolated system - hence the vector sum of the momentum changes is = to 0 in an isolated system
yeh so the equation p = mv tells you a lot. basically that any increases in mass or velocity will increase the momentum, thus making it harder to stop. for instance it is more difficult to stop a car moving at 20ms-1 than at 10ms-1 if mass is kept constant