OS8 Water Pressure

Perhaps you noticed the water flowing through the siphons slowed down as the top jar emptied out. Why would it do this?

Is this slowing related to less water in the jar so the mass is less making the water pressure on the siphon less?

When people go down to the bottom of the ocean, they go in small vehicles called bathyscaphes with very thick windows. Why is the glass so thick?

water pressure goes up as water mass goes up

Question: What is water pressure?



Thin plastic water bottle, empty



Place the empty water bottle on the scale to mass it

massing empty bottle

Massing the empty bottle is important as you can then subtract this mass to find the mass of the water you add later on.

Take the bottle off the scale and pour 1 cm water in the bottle

Mass the bottle and water

Take the bottle off the scale

Pour 1 cm water in the bottle

water pressure begins

The first centimeter is in the bottle. It distributes this mass over the bottom putting pressure on the scale.

Mass the bottle

Repeat this until the bottle is full or the scale can not have more mass on it


Masses of bottle and water


What happens to the mass of the bottle and water each time you add more water?

more mass is more water pressure

The bottle is filling up. Still all the mass of the water rests on the bottom of the bottle sitting on the scale.

If you were a bug standing on the bottom of the bottle, how much mass would be resting on your back when the first water landed in the bottle?

How much mass would be resting on your back when all the water is in the bottle?

Pressure is how much mass is resting on a certain area. As you add water mass to the bottle, what is happening to the water pressure on the bottom?

Water has a mass of 1 g for each cubic centimeter. If you were standing under a column of water a meter (100 cm) tall, how much mass or water pressure would be resting on you?

What if that column of water was 10 meters (1000 cm) tall?

Why are the windows of the bathyscaphes so thick?


What I Found Out:

Every time I added water to my bottle, the mass increased. Only 58.8 g of water would sit on my buggy back when the first centimeter of water arrived in my bottle. I’m glad I’m not a bug on the bottom of the bottle when the bottle was full because I would have 510.0 g of water sitting on my back.

the most water pressure in the bottle

The bottle is now full. All the mass is still resting on the bottom of the bottle giving that amount of pressure on the scale.

Since the area the water was resting on did not increase, the water column kept getting taller and heavier putting more pressure on that same area.

A column of water 100 cm tall would put 100 g of mass or pressure on my shoulder. But a column 1000 cm tall would put 1000 g of mass on my shoulder.

Bathyscaphes go to the bottom of the ocean, miles down. A mile is about 1.5 km or 1500 m or 150,000 cm which is 150,000 g or 150 kg of mass. [A kilogram is about 2.2 pounds so that is 330 pounds.] This would break a regular window. The thick glass is harder to break with the pressure.