First question: A balloon filled with 2 m3 of Helium is attached to a rope with mass per unit length 0.14 kg / m. The mass of the balloon fabric is 0.24 kg. The rope is first attached to the ground. The balloon rises when the rope is uncoiled as shown in the figure (Please visit here) (a) The forces acting on the balloon are named A, B and C as shown at the right hand side of the figure. What are the forces A, B and C? (b) Calculate the length of rope which has been uncoiled when the balloon has stopped rising. (Density of Helium = 0.13 kg / m3, Density of air = 1.30 kg / m3) (c) If the rope is now allowed to fall away from the balloon,the balloon again rises and eventually bursts. Explain why the balloon bursts. Answers to (a). A is upthrust, B is total weight of the balloon, C is tension of the rope. (b). Don't know how to do, please help. (c). The pressure inside is greater than the pressure outside. And another question, A big balloon, the ballon can carry people to the sky. When it is filled with hot air, it will rise and go into the sky. Why it will stop rising when the air inside it is cooled down? Is it true that when the air is hot, hot air rises and makes the balloon rise, and when the air is cooled down, cool air descends and makes the balloon stop rising.