Experiment 1.0.1 CAN AIR HOLD UP WATER?
You need: the plastic container, one plastic cup, the plastic sheet
1. Fill the cup with water so that it is about half full.
2. Cover the cup with the plastic sheet.
3. Take the cup, covered with the sheet, and hold it above the container. Hold the cup at the bottom, since the sides are too soft.
4. Hold the sheet and turn the cup upside down, so that the sheet is below the cup. You do not have to hold the sheet any more. You can see that the water does not spill out of the cup. Do not squeeze the cup!
The explanation >>
Explanation 1: The layer of air above us is about 100 kilometres thick. The water should displace 100 kilometres of air above in order to flow out of the cup. The air may seem light, but displacing a 100 kilometres high column of air above the cup is quite difficult. The water in the cup cannot do this.
Explanation 2: Let‟s observe the plastic sheet in order to explain the experiment. Two forces act on it in the direction downwards: the force of water, which equals the weight of water, and the weight of plastic sheet. If only these two forces acted on the sheet, it would fall down and so would the water. Since this does not happen, we assume that there is one force more acting on the sheet in the direction upwards. This force acts on the sheet, because the air above us also has its weight. We know that the layer of air is 100 km thick on the surface of the Earth and pushes against every single object on the Earth. It pushes in every direction. Consequently, the air also pushes up on the sheet under the cup. This force is called the air pressure force.
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