Book: #2

     Using computer generated animation these sequences show how a body responds to a force according to Newton's Law: with examples of no force, constant force and changing force. 

The conventions used in the film are all shown in the figure: the body is the circle, the direction of the force is along the arrow and the magnitude of the force is proportional to the length of the arrow. The relative velocity is visualized by marking the position of the body periodically like a stroboscopic photo of the moving body: so the marks do not distract the viewer, each mark disappears after 8 time intervals. 

Since the mass of the body is constant the change of velocity dv/dt is always proportional to the force F. When a force is applied (arrow) the spaces between successive sparks show a constant change; a change of velocity, in the direction of the force; if the force increases (larger arrow) the spaces between successive marks show a larger change. In the last case of the steadily varying force, it is the direction of the force and not the magnitude which changes.