Ablock of wood is at rest on a wooden ramp. when wheels are attached to the block of wood, it moves down the ramp. explain each situation in terms of friction.

The wood was at the wooden ramp so it was in static friction. This is much more then rolling friction. That's why the wood doesn't move. When we attach wheels to it, it converts it into rolling friction which is 100 times less then static friction and that's why the block of wood rolls down the ramp.