If the velocity of {displaystyle A}A be increased at the rate {displaystyle {frac {du}{dt}}}{displaystyle {frac {du}{dt}}}, then in order to prevent {displaystyle B}B from moving a force, {displaystyle eta ={frac {d}{dt}}(Mu)}{displaystyle eta ={frac {d}{dt}}(Mu)} must be applied to it.