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

We know work done by a force is given by which is zero in given question. And, can be written as .


So,


as ≠ 0 (it’s not possible).


This means the force vector and the velocity vector must be perpendicular to each other. So, force is depended on the velocity as change in direction of velocity will lead to change in direction of force. As magnetic field force does no work as it is always perpendicular to the velocity of particle on which it acts. Hence, force is velocity dependent when it can’t do work.


1