-- Since our athlete is running at constant velocity, AND the javelin is
staying with him, the speed of the javelin itself must also be constant.
-- Therefore, the force of 5N that he's applying to the javelin can't possibly
be in the direction that he and the javelin are moving, otherwise the javelin
would be accelerating.
-- So we know that the force is perpendicular to the javelin's motion ... like
probably straight up to prevent it from falling to the ground.
-- Since none of the force is in the direction the javelin is moving, the force
does no work on the javelin. Zero.
