A balloon seller uses a portable burner to heat air inside a hot-air balloon before launch. Fig. 1 shows the balloon envelope, basket and open lower mouth. ...
Question 1 Report
A balloon seller uses a portable burner to heat air inside a hot-air balloon before launch. Fig. 1 shows the balloon envelope, basket and open lower mouth. The heated air can move in and out of the balloon, so it is not trapped in a sealed container. The surrounding air is cooler.
(a) State how heating changes the speed of the air molecules inside the balloon. [1]
(b) Explain why the heated air becomes less dense than the cooler air outside. [3]
(c) State the direction of the upthrust on the balloon. [1]
(d) Explain why the balloon rises when the upthrust is greater than its weight. [2]
(e) Describe what the pilot should do to descend slowly. [2]
(f) Calculate the weight of a basket and equipment with mass 180 kg. Take gravitational field strength as 10 N/kg. [2]
(g) State which gas property is reduced when the heated air spreads out: density, mass, or temperature? [1]
(h) Explain why the burner must be switched off before the balloon is packed away. [2]
Answer Details
(a) Heating makes the air molecules move faster. [1]
(b) The molecules gain kinetic energy. They spread further apart and occupy a greater volume. The same mass of air therefore has less mass per unit volume, so its density is lower than that of the surrounding cooler air. [3]
(c) Upthrust acts upwards. [1]
(d) If upthrust is greater than weight, there is a resultant upward force. The balloon therefore accelerates upwards and rises. [2]
(e) The pilot should reduce or turn down the burner. The air then cools, becomes denser, and the upthrust decreases, allowing a slow descent. [2]
(f)
\[W=mg=180\times10=1800\text{ N}\]
Weight = \(1800\text{ N}\). [2]
(g) The property reduced when the heated air spreads out is density. [1]
(h) The burner must be switched off so hot parts can cool, reducing the risk of burns or fire damage to the fabric. [2]
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