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I was trying to solve the following problems, but I just don't understand what principles/laws/concepts I'm supposed to be using.

I hope someone can give me a hint as to where I should start.

Thanks alot for any replies.

31) Gayle runs at a speed of 4m/s and dives on a sled, initially at rest on the top of a frictionless snow-covered hill. After she has descended a vertical distance of 5m, her brother, who is initially at rest, hops on her back and together they continue down the hill. What is their speed at the bottom of the hill if the total vertical drop is 15m? Gayle's mass is 50kg, the sled has a mass of 5kg, and her brother has a mass of 30kg.

39) A 7g bullet, when fired from a gun into a 1kg block of wood held in a vise, penetrates the block to a depth of 8cm. This block of wood is placed on a frictionless horizontal surface, and a second 7g bullet is fired from the gun into the block. To what depth does the bullet penetrate the block in this case?

48) Consider a frictionless track. A block of mass m1 = 5kg is released from A, 5m in vertical height. It makes a head-on elastic collision at B, at our reference level, with a block of mass m2 = 10kg that is initially at rest. Calculate the maximum height to which m1 rises after the collision.