Objects that are moving tend to keep moving unless acted upon by an outside force. This is called inertia. It is part of Newton's First Law of Motion. In this case, both the chair and the books were in motion. Because the books were not attached to the chair, they continued to move forward when the chair was stopped. Weight finally made the books fall to the floor.

The student is able to communicate that both the chair and the books were in motion. When the chair was stopped, the books continued to move forward because objects in motion tend to continue moving. They can identify this property as inertia. The student is able to communicate that the books continued to move forward because objects in motion tend to continue moving. They can identify this property as inertia. The student is able to communicate that stopping the chair made the books fall. The student is able to communicate that the books fell off the chair.