A particle of mass m is driven by a machine that delivers a constant power k watts. If the particle starts from rest the force on the particle at time t is
$\sqrt{\dfrac{mk}{2}} t^{–\dfrac{1}{2}}$
$\sqrt{mk} t^{–\dfrac{1}{2}}$
$\sqrt{2mk} t^{–\dfrac{1}{2}}$
$\dfrac{1}{2}\sqrt{mk} t^{–\dfrac{1}{2}}$