If a pebble is being transported in a stream by rolling, how does the velocity of it compare to the velocity of the stream?

Streams carry sediment, like pebbles, in their flows. The pebbles can be in a variety of locations in the flow, depending on it's size, the balance between the upwards velocity on the pebble (drag and lift forces), and it's settling velocity.

Unbalanced forces can cause an object to change its motion.

Unbalanced forces can also change the speed or direction of an object that is already in motion. ... If the players on one side of the rope use more force than other side, they will win the game. Another good example is when you and your dog play with a tug toy.