gravitational Potential energy is given by

here we have
m = 200 kg
h = 21000 m
now we will have


so GPE of balloon will be 41160000 J above the given height from ground
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The units for mass are grams (g) and kilograms (kg)
the units for volume are millilitres (ml) and litres (l)
Answer:
Because a person may be pulled in the direction of the moving train. Thereby causing accident
Explanation:
According to Daniel Bernoulli's theorem, he was widely known as a Mathematician. He stated that due to the higher velocity of a moving train, there is higher kinetic energy in terms of volume around it, while the air pressure between the person and the train becomes lower.
As a result, a person near a moving train may be pulled in the direction of the moving train. Thereby causing accidents that may lead to death.