(a) how much helium (density 0.18 kg/m3) is needed to lift a balloon carrying two people, if the total mass of people, basket, and balloon (but not gas) is 280 kg?
(b) repeat for a hot-air balloon whose air density is 10% less than that of the surrounding atmosphere.

Two cars collide, lock bumpers and move together after the collision.this collision is known as perfectly inelastic collision.

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