Pilots of hot air balloons are able to steer them somewhat by moving them up or down to enter different air currents that move in different directions. Explain how the flame can be used to do that.

Hot air balloons flies by means of it being lighter than air

The flame can be used to move the pilot to different directions by increasing the temperature of the air in the balloon and therefore, the volume, therefore, decreasing the density of the air which changes the elevation of the balloon to the appropriate elevation having winds moving in the desired direction

The principle by which the above process works is as follows:

According to Charles law, the volume of a mass of gas is directly proportional to its Kelvin temperature

V ∝ T

Therefore, as the temperature is increased, the volume occupied the mass gas increases, and therefore, the density decreases

The principle on which the hot air balloon operates on is the principle that hot air is less dense than and therefore, rises above cool air

Therefore, the pilot of an hot air balloon heats up the air in the balloon to allow the balloon to rise and the pilot switches off the burner so that the air in the balloon cools and the balloon looses altitude

The mechanism of the hot air balloon does not have a steering, and therefore, the wind direction is used to steer the balloon

The direction of the wind is different at different altitude, such that by obtaining the direction of the wind at a given altitude, the pilot can raise or lower the balloon up or down to the altitude having the desired wind direction, by increasing the temperature of the air in the balloon by switching on the burner to go to a higher altitude, or by switching off the burner so the air in the balloon cools and the balloon loses some of its altitude to reach the wind going in the direction of the pilot

Learn more about hot air balloon here:

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To understand this first we must understand what is happening at the molecular level. When we heat any substance it changes its state of matter because its molecules get excited. For example, when we heat water it turns into gas so the molecules get excited. The hot air balloon will move faster if we add heat because the gas molecules will get excited and push against the balloon, making it faster. If we start removing heat, the balloon's acceleration will slow down because the molecules will get less excited.

c) should be the answer.