Newton's first law of motion, often called the law of inertia, states that an object at rest stays at rest, and an object in motion stays in motion with the same speed and in the same direction (constant velocity) unless acted upon by an unbalanced external force. It defines that objects resist changes to their state of motion.

Newton's first law of motion, often called the law of inertia, states that an object at rest stays at rest, and an object in motion stays in motion with the same speed and in the same direction (constant velocity) unless acted upon by an unbalanced external force. It defines that objects resist changes to their state of motion.

Newton's first law of motion, often called the law of inertia, states that an object at rest stays at rest, and an object in motion stays in motion with the same speed and in the same direction (constant velocity) unless acted upon by an unbalanced external force. It defines that objects resist changes to their state of motion.

In a vacuum, a feather and a hammer hit the ground simultaneously because the absence of air removes air resistance (drag), which is the only force that normally causes lighter or less dense objects to fall slower. Gravity accelerates all objects at the same rate, regardless of their mass.