Force must be positive, plotted above the time axis.

A farmer is pumping water from a well using an old-fashioned hand pump. Students are asked to sketch a plot of the force exerted by the farmer's hand on the pump handle versus time.

Correct Answer 

Students who suffer from this misconception will plot the force entirely above the time axis, failing to properly take the change in direction into account in their sketch. Shown below is a common misrepresentation of a force versus time sketch for the farmer's hand.

 

Related misconceptions/student difficulties that may arise in the question:

Students have difficulty relating real world motions to a graph and vice-versa.

Velocity must be positive.