Inertia

Inertia is the tendency of an object to keep doing what it is already doing. An object at rest wants to stay at rest. An object that is moving wants to keep moving in a straight line at the same speed. To change what an object is doing, something has to push or pull on it. That push or pull is called a force.

The idea of inertia was worked out by the English scientist Isaac Newton in the 1680s. He wrote it down as his first law of motion. It is sometimes called the law of inertia. Before Newton, most people thought moving objects naturally slowed down on their own. Newton showed that something else, usually friction, was slowing them down.

You feel inertia all the time. When a car brakes hard, your body keeps moving forward for a moment. Your body was going the car's speed, and it wants to stay at that speed. The seatbelt pushes back on you and changes your motion. That push is a force. Without the seatbelt, inertia would send you right into the dashboard.

You can feel it the other way too. When a car speeds up from a stop, you feel pressed back into the seat. Your body was at rest and wants to stay at rest. The seat has to push you forward to match the car's new speed.

How much inertia an object has depends on its mass. Mass is how much matter something is made of. A bowling ball has much more mass than a tennis ball, so it has much more inertia. It takes a stronger push to get the bowling ball rolling, and a stronger push to stop it once it is moving. This is one reason a loaded truck needs a much longer distance to stop than a small car.

Inertia explains something strange about space. On Earth, a rolling ball slows down because friction from the ground and air steal its energy. In space, there is almost nothing to cause friction. A spacecraft can turn off its engines and coast for years. The Voyager 1 probe, launched in 1977, is still flying outward through space today, more than 15 billion miles from Earth. Inertia is carrying it the whole way.

The next time you slide across a wood floor in your socks, notice how hard it is to stop. That is inertia, and every object in the universe has it.