• The motion of an object is determined by the sum of the forces acting on it; if the total force on the object is not zero, its motion will change. The greater the mass of the object, the greater the force needed to achieve the same change in motion. For any given object, a larger force causes a larger change in motion. (6-8)
• Newton's second law accurately predicts changes in the motion of macroscopic objects. (9-12)

• 6-8: 4F/M3a. An unbalanced force acting on an object changes its speed or direction of motion, or both.
• 9-12: 4F/H7. In most familiar situations, frictional forces complicate the description of motion, although the basic principles still apply.

• 6-8: 4G/M1. Every object exerts gravitational force on every other object. The force depends on how much mass the objects have and on how far apart they are. The force is hard to detect unless at least one of the objects has a lot of mass.

It can be difficult for beginners to recognize different force interactions, especially since these concepts sometimes run counter to the student's intuition. This interactive assessment lets them practice in a self-directed environment. They view 11 common physical situations, then decide which forces are present. Afterward, they use a pull-down menu to view correct answers -- all accompanied by explanations.

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