A rolling ball is hard to stop because it has rotational inertia. Rotational inertia is the resistance of an object to changes in its angular motion. In the case of a ball, its rotational inertia is due to the mass of the ball and its radius. The larger the mass and the larger the radius, the greater the rotational inertia.

When you try to stop a rolling ball, you are applying a force to the ball that is opposite to the direction of its motion. This force causes the ball to decelerate, but the rotational inertia of the ball resists this deceleration. The ball will continue to roll until the force you apply is greater than the force of friction between the ball and the surface it is rolling on.

The amount of force required to stop a rolling ball also depends on the speed of the ball. The faster the ball is rolling, the greater the force required to stop it. This is because the rotational inertia of the ball increases with its speed.

In addition to rotational inertia, the coefficient of friction between the ball and the surface it is rolling on also affects how hard it is to stop. The higher the coefficient of friction, the easier it is to stop the ball. This is because the coefficient of friction represents the amount of resistance between the ball and the surface, and the higher the coefficient of friction, the greater the resistance.