An object in space will move in a straight line at the same speed forever unless something hits it or pulls on it.

Sir Isaac Newton described this rule in 1687. He called the tendency to keep moving 'inertia.' Inertia is not a force but a property of all matter. The more mass an object has, the harder it is to change its motion.It is much harder to stop a heavy train than a light bicycle. Both might move at the same speed, but the train has more inertia. In the vacuum of space, there is no air to create friction. This allows objects to glide without losing energy.The Voyager 1 spacecraft is a real-world example. It was launched in 1977 and travels at over 38,000 miles per hour. It does not need engines to keep this speed because there is no air in space to slow it down.On Earth, seatbelts use this law to save lives. When a car stops suddenly, your body tries to keep moving forward at the same speed. The seatbelt provides the outside force needed to stop your body safely. Without that force, inertia would keep you moving until you hit the dashboard.