Describe two ways to change the frictional force between two solid surfaces.

-- To increase the frictional force between the two surfaces, press them together
harder, or rough up the area of contact with sandpaper or dirt.

-- To decrease the frictional force between two surfaces, decrease the force
with which they press together, or smooth (polish) the area where their surfaces
contact, or make the area seem smoother by lubricating it with grease, oil, air,
or smooth leaves.