California Science Test (CAST) 8th Grade Practice Exam

Inertia is the tendency of objects to resist changes in their motion. This law says that an object at rest stays at rest, and an object moving in a straight line at a constant speed continues doing so unless a net external force acts on it to change that motion. In other words, if nothing pushes or pulls on an object, its velocity stays the same.

Think about a book sitting on a table: there’s no net force to move it, so it stays put. A puck sliding on smooth ice keeps moving in the same straight line because friction is minimal, until a force from friction, a collision, or a hit changes its speed or direction.

This idea differs from the other statements about motion. When there is a net force, motion changes in a way described by how force and mass produce acceleration. And the third law describes forces occurring in action–reaction pairs, not the tendency to keep moving or stay still. The conservation ideas deal with quantities staying the same in a system, not this specific behavior of motion without a net external force.