Gravity pulling the mass of the atmosphere toward the center of the
earth. Atmosphere includes air, water vapor, etc.
Answer:
When plastics decomposed, they produce methane gas, which contributes to global warming (not questioning beliefs on the matter). Plastic debris also is an issue affecting the ecology of an environment and can cause issues for the affected organisms. But the greenhouse gases released from biodegradable plastics is likely a greater concern.
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Hot air balloons have hot air inside them, which is less dense as the air around it. Thus it rises. A cold air balloon would sink because the air would be more dense than the air around it. Also, the balloon’s volume would vary in size depending on the weight of the balloon.