1. If two coins are placed on a rotating disc (constant speed), what forces act on the coins, at what direction and in what proportions? (The coins aren't sliding) Gravity is one, and the normal force is the one counteracting on that. But what forces could be expected in a horizontal plane? A centripetalforce is crucial for the coins to obtain a uniform circular motion, and this is obviously pointing to the centre at all times. I'm guessing the centripetal force through friction between the coin and the disc.. But is there another friction force that makes it possible for the coins to spin along with the disc? Thanks for any help or comments.