Which principle explains that an object at rest stays at rest unless acted upon by an external force?

• A. Gravity
• B. Law of inertia
• C. Conservation of energy
• D. Friction

Answer: (B)

Inertia explains this idea—the tendency of objects to resist changes in their motion. Newton's first law says an object at rest stays at rest unless a net external force causes it to move. When nothing unbalances the forces on an object, there’s no acceleration, so its velocity remains zero. For a book on a table, gravity pulls down and the table pushes up; those forces balance, giving zero net force and leaving the object at rest. If you apply a push, a net force appears and the object accelerates in that direction, following F = ma. The other choices don’t capture this general rule: gravity is just a real force, not the principle that keeps things from starting to move; conservation of energy is about energy accounting, not whether something remains still; friction is a force that can oppose motion (and static friction can hold something in place) but it’s the inertia that tells us why motion doesn’t begin unless a net external force acts.