Why are tires filled with air instead of something less likely to go flat?
This is an interesting question with several answers. First, a solid rubber tire would have a huge mass and would require consider work to accelerate. Because it rotates as the car moves, a tire stores twice as much kinetic energy as the other parts of the cars. By reducing the mass of the tires, the car reduces the amount of energy it must put into the tires to get them moving and the amount of energy it must remove from the tires to stop them from turning.
Secondly, a solid rubber tire would be so hard that it would give the car a very rough ride. The air in the tires cushions the car against many of the rough spots it drives over. Without the air cushion, the wheels and axles would bound up and down with every pebble in the road.
Lastly, a solid rubber tire would be very expensive. The materials used in a tire are expensive and a tire’s cost should be roughly proportional to its weight. Since a solid tire would weigh much more than an air-filled one, it would also cost much more. Its tread would still wear out, so it wouldn’t last any longer than an air-filled tire.