Yes — the force of friction. Muscular force is not the only contact force.
Friction acts between the two surfaces that are touching each other whenever one moves, or tries to move, over the other. It is a contact force because it arises only from two surfaces in contact — no touching surfaces, no friction. A ball rolling on the ground, a cycle coasting on a level road and a lunch box pushed across a table all slow down and stop because of it.
Q2.
A ball rolling on a flat ground stops on its own after some time … What causes the change in the speed of objects in such situations? … Is it possible that some force is indeed acting on them? Which force is that?
Answer
Yes, a force is certainly acting — the force of friction, and it acts in the direction opposite to the motion.
We already know that a force is essential to change the speed of an object. The ball and the coasting bicycle both lose speed, so a force must be acting on them, even though nobody appears to be touching them. That force acts between the surfaces in contact — the ball and the ground, the tyre and the road — and it acts backwards, against the motion. That is why the speed falls steadily until the object comes to rest.
Why it happens: notice the clue the book gives. On a rough road the cycle stops sooner than on a smooth one. Whatever this force is, it depends on the nature of the surfaces in contact — which is exactly what you would expect of a force born at the meeting of two surfaces.