 The requirement for power input to an ideal mechanism to equal power output provides a simple way to compute mechanical advantage from the input-output speed ratio of the system.The power input to a gear train with a torque "TA" applied to the drive pulley which rotates at an angular velocity of "ωA" is "P=TAωA".Because the power flow is constant, the torque "TB" and angular velocity "ωB" of the output gear must satisfy the relation
which yields
This shows that for an ideal mechanism the input-output speed ratio equals the mechanical advantage of the system.