Question

a moving bicycle comes to rest after sometimes if we stop pedalling it. but newton's first law of motion says that a moving body should should continue to move forever, unless some external force acts on it. how do you explain the bicycle case?

Answer 1

According to Newton's first law, the only time the bicycle would stop, is if it is acted upon by an external force. This is the exact case that happens to make the bicycle to come to a stop. The external force in this case is the friction between the bicycle and the ground. The force of friction gradually reduces the moving force until it gets to zero and the bicycle stops moving.

Answer 2

There is friction force acting on the tyre of the wheel.  There is friction force also between moving and stationary parts on the axles inside the bicycle wheel.

Also there is wind opposing the movement of the person and bicycle by drag force.

These external forces slow down the speed of bicycle till it stops.