Inertia is the tendency of objects to keep doing what they're already doing. A ball sitting still on the ground tends to stay still. A hockey puck sliding across ice tends to keep sliding. This resistance to change is inertia.

Isaac Newton described inertia as a fundamental idea in physics. Every object has it, from tiny pebbles to massive planets. The more mass something has, the more inertia it possesses. That's why it's easy to push a shopping cart but almost impossible to push a car: the car has much more inertia.

You feel inertia when a bus suddenly stops and your body lurches forward. Your body was moving with the bus, and inertia made it want to keep moving even after the bus stopped. Seat belts exist because of inertia.

The word also describes a kind of mental or emotional resistance to change. If your family has always celebrated holidays the same way, there might be inertia against trying something new, even if the new idea is good. Inertia isn't always bad, but it means you need extra energy or effort to start something new or change direction. Scientists and engineers have to account for inertia constantly, from designing spacecraft to planning how quickly a train can stop safely.