Inertia is the resistance of any object to any change in its velocity. An object at rest will remain at rest unless a force acts upon it; likewise, an object in motion will continue at the same speed and the same direction unless a force acts upon it. Every object has inertia. How much inertia any given object has depends on its mass. The more mass something has, the more difficult it is to get it moving or to change its speed or direction once it is moving.
In a wheel, inertia also depends on how the mass is distributed. A wheel has more inertia if its mass is concentrated near its outside edge than if the mass is concentrated around the wheel’s center. The thicker the rim of a wheel, the more 
resistant it is to slowing down. This is because the outside of a wheel is traveling faster than the center, so with more mass there, the inertia is greater. To understand this, think of spinning around and swinging a towel around and around your body. Now imagine doing the same thing with a heavy object tied to the end of the towel or string. Can you imagine the difference? The heavier the object tied to the end, the more powerful your spin would be, and the harder it would be to stop spinning!
