Show that a force that does no work must be a velocity dependent force.

Explanation

Ket bi wirj us done by a force, so we have
`dW=F.dl=0`
`impliesF.vd=0`
`impliesF.v=0` (since dl=v dt and dt `ne0`)
thus, F must be velocity dependent which implies that angle between F and v is `90^(@)`. If v changes (direction), then (directions) F should also change so that above condition is satisfied.