If the net work done by external forces on a particle is zero, which of the following statements about the particle must be true?

The work-energy theorem states that Wnet = ∆K = Kf-Ki. Thus, if Wnet  = 0 then Kf - Ki  or 12mvf2-12mvi2, which leads to the conclusion that the speed is unchanged (vf -vi). The velocity of the particle involves both magnitude (speed) and direction. The work-energy theorem shows that the magnitude of speed is unchanged when Wnet  = 0 but makes no statement about the direction of the velocity.