- #1
greypilgrim
- 515
- 36
Hi.
If I fill a glass with water up to the top and close it with a coaster, I can turn it upside down without the coaster falling off and spilling everything. Usually, this is explained with the atmospheric air pressure being bigger than the pressure of the water inside the glass (hence, it should also work with a tube up to a height of about 10 m).
However, I noted that the glass doesn't need to be full up to the top at all, in fact it works with any water level. Why is that? Shouldn't the pressure of the air-filled part still be atmospheric and add up with the pressure of the water-filled part to something bigger than the outside atmospheric pressure?
A wet coaster also sticks to a completely empty glass, I assume due forces from the surface tension of the water. But I don't think they are strong enough to explain the above.
If I fill a glass with water up to the top and close it with a coaster, I can turn it upside down without the coaster falling off and spilling everything. Usually, this is explained with the atmospheric air pressure being bigger than the pressure of the water inside the glass (hence, it should also work with a tube up to a height of about 10 m).
However, I noted that the glass doesn't need to be full up to the top at all, in fact it works with any water level. Why is that? Shouldn't the pressure of the air-filled part still be atmospheric and add up with the pressure of the water-filled part to something bigger than the outside atmospheric pressure?
A wet coaster also sticks to a completely empty glass, I assume due forces from the surface tension of the water. But I don't think they are strong enough to explain the above.