a skateboard rolls horizontally off the top of a staircase and lands at the bottom of the stairs. the staircase has a horizontal length of 15.0 m and the jump last 1.15s. what's the skater's vertical velocity upon landing

When is this

Answer(a)   the matererials that are in contact, the two materials and the nature of their surfaces/// (b)/// the force pushing the two surfaces together, pushing the surfaces together causes the more of the asperities to come together and increases the surface area in contact with each other