Inertia
When something’s stopped, it’s not going to move unless something moves it! That’s inertia… and once something gets going, it’s going to keep on going until something stops it. It’s all physics! Learn more about inertia…
http://library.thinkquest.org/10796/ch3/ch3.htm
http://www.mansfieldct.org/Schools/MMS/staff/hand/lawsinertia.htm
http://id.mind.net/~zona/mstm/physics/mechanics/forces/galileo/galileoInertia.html