Start a ball rolling down a bowling alley and you’ll find

Step 1 of 2

First of all, Newton’s law of inertia states that:

Until an external force acts on the system, the motion of the system will be unaffected. This means the object in rest will be at rest and the object in motion will continue its motion with the same speed and same direction until an external force acts on it.

But, for the case given in the question, after rolling the ball, the speed of the ball decreases slightly with time. But there was a condition in Newton’s law of inertia, which is, until an external force acts. As we can easily verify that, there is friction between the surface of the bowling alley and the surface of the ball.

As there is a frictional force, so the velocity has to be decreased. Because frictional force acts in the opposite direction of motion.