1. The problem is to explain the meaning of inertia in physics in simple words. 2. Inertia is a property of matter that describes an object's resistance to changes in its state of motion. 3. This means that an object will stay at rest if it is not moving, or keep moving at the same speed and direction if it is already moving, unless a force acts on it. 4. The more mass an object has, the greater its inertia, so heavier objects are harder to start moving or stop. 5. Inertia is the reason why you feel pushed back in a car when it suddenly accelerates or why it is hard to stop a rolling ball. 6. The concept of inertia is fundamental in Newton's First Law of Motion, which states that an object will maintain its motion unless acted upon by an external force. Final answer: Inertia is the tendency of objects to keep doing what they are already doing, whether staying still or moving, unless something forces them to change.