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.

When the wood was at the wooden ramp so it was in static friction and 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 rolss down the ramp

Hey there!

If there isnt wood , the force of sliding friction would balances the gravitational force pulling the block down the ramp. So there will be no net force on the block, so it doesn´t slide down the incline. When the wheels are attached, the gravitational force pulling the block down the ramp is now bigger than the friction force between the block and the ramp. Eventually, there is a net force on the block pulling the block down the ramp, so the block begins to move in the direction of this unbalanced force.

Hope this helps

Tobey

3. dark matter

empty space could also be a simple vacuum