4.4 Pressure in liquids
Summary: How do you squash compress a polystyrène cup without touching it? Take it deep below the sea and the pressure in the water will do it for you, like in Figure 1. Tags: science oxford-science student-book-7 physics Created: 2026-05-19 Last Updated: 2026-05-19
After this topic, you will be able to:
- describe how liquids exert a Pressure in all directions
- describe how liquid pressure changes with depth
- explain why some everyday objects float and some sink.
Think back
- State what produces gas pressure.
- State what happens to atmospheric pressure as you go down a mountain.
- Describe upthrust.
Key idea Liquid pressure occurs when water Molecules push against each other and on surfaces. Liquids cannot be compressed like gases.
Key words liquid pressure, incompressible, upthrust
How do you squash (compress) a polystyrène cup without touching it? Take it deep below the sea and the pressure in the water will do it for you, like in Figure 1**.**
Figure 1At 3000 m, water exerts a big pressure.
What is liquid pressure?
When you swim underwater, the water exerts a pressure on you. The water molecules are pushing on each other and on surfaces, and this liquid pressure acts in all directions.
Look at Figure 2. When you squeeze (press) a bag with holes in it, the water is pushed out of all the holes because of liquid pressure. The water comes straight out of each hole and then falls because of Gravity.
Figure 2The water comes out in all directions.
A State the angle between the flow of water leaving the bag and the surface.
If you put water in a syringe, cover the end, and try to compress the liquid, you will find it impossible. Liquids are incompressible. This is because the particles in a liquid are touching each other and there is very little space between them. Liquids pass on any pressure applied to them.
B Explain why a gas in a syringe is compressible, but a liquid is not.
How does pressure change with depth?
Look at Figure 4. The wall of a dam is not straight. It is wider at the bottom than at the top. The pressure at the bottom of the lake is bigger than the pressure at the top. The pressure at a particular depth in a liquid depends on the Weight of water above it. The water pressure at the bottom of the Atlantic Ocean is the same as the weight of eight cars pushing on an area the size of your thumb. Therefore, submarines need to be made of strong materials (see Figure 3).
Figure 3Ships that can travel underwater, such as submarines, need to be very strong.
Figure 4A dam is wider at the bottom. Full text alternative to the content
Why do some things float and others sink?
Upthrust actson any objectthat is floating, or is submerged (underwater) in afluid like a liquidor gas. Figure 5 explains upthrust.
Figure 5The arrows show the forces acting on an object because of its weight or the pressure of the water.
A plastic duckfloats becausethere are lotsmore water molecules hitting the bottom of the duck than there are air molecules hitting the top of the duck. This produces the upthrust. The duck sinks (goes down) until there is enough upthrust to balance the weight.
If the area in contact with the water is too small, there is not enough upthrust to make the object float. The object will sink.
C Explain why a very heavy ship can float.
When a submarine is underwater, there is a difference in pressure between the top and bottom of the submarine. That difference produces the upthrust.
Literacy skills A young child says that “heavy things sink and Light things float”. Use the example of a passenger boat to explain to the child why this is not true.
Stretch zone
- Copy and complete thesentences. The pressure in a liquid acts two/three/all directions. The pressure decreases/increases as you go deeperbecause the Energy/weightof the water above you getsbigger/smaller. Thedifference in pressureexplains why there is a forcecalled downthrust/upthruston a floating object.
- Explain, in terms of pressure, why: a boat made of modelling clay floatsa he same mass of modelling clay shaped into a ball sinks.
Stretch zone
- You fill a bucket with water. Describe how the water pressure changes from the top to the bottom of the bucket. You push a table tennis ball to the bottom of the bucket of water.Explain why the ball moves up when you let it go.