Useful frictions and unhelpful frictions

Sometimes friction can be helpful. Friction can stop things that are moving from moving. For example if your pencil is rolling along the desk, friction could make it stop before it falls off.

However, friction can also be unhelpful. It can stop things from moving that should be moving. For example when playing sports, friction might slow down the ball when really you would like the ball to keep going.

Jet streams are relatively narrow bands of strong wind in the upper levels of the atmosphere. the winds blow from west to east in jet streams but the flow often shifts to the north and south. jet streams follow the boundaries between hot and cold air. the earth's rotation is responsible for the jet stream as well.