# Pressure and Liquid Pressure

Force and Pressure is the 3rd chapter in SPM form 4 Physics. There are 6 main sub-topics in this chapter, namely pressure, liquid pressure, gas pressure, Pascal’s Principle, Archimedes Principle and Bernoulli’s Principle.

In pressure, we will discuss what pressure is, the formula of pressure and applications of high and low pressure.

In liquid pressure, we will discuss the formula of liquid pressure, characteristics of liquid pressure and applications of liquid pressure.

In gas pressure, we will discuss how to measure gas pressure. We will also discuss atmospheric pressure where we will focus our discussion on the characteristic of atmospheric pressure, proofs of existence of atmospheric pressure, how to measure atmospheric pressure and applications of atmospheric pressure.

In Pascal’s Principle, we will discuss the physics formula related to Pascal’s Principle and some applications of Pascal’s Principle. In Archimedes’ Principle, again we will discuss the formula and then its applications. In Bernoulli’s Principle, we will discuss what is stated in Bernoulli’s principle and its applications.

In SPM, there are a few things that we need to know about pressure. First, we need to know what pressure is? Pressure is defined as perpendicular force acting on a surface per unit area. The unit of pressure is Newton per meter square or Pascal, denoted by the symbol Pa.

Second, we need to know the formula of pressure. Pressure is given by the formula P = F/A, where P is pressure, F is the perpendicular force and A is the area. From the formula we know that pressure is directly proportional to the perpendicular force but inversely proportional to the area the force exerted on. In other words, the greater the perpendicular force, the greater the pressure; the greater the area, the lower the pressure.

Third, we also need to know some applications of high and low pressure. We are going to discuss this in the very next slide.

In SPM, you need to know some applications of high and low pressure. Applications of high pressure include knifes, spike shows or soccer shoes and ice skates. Applications of low pressure include ski, seatbelt, foundation of building, tyre of tractors and strap of handbag.

After this, we are going to discuss liquid pressure. There are a few points that we need to know about liquid pressure.

First, we need to know how liquid pressure is produced. Second, we need to know what is the equation related to liquid pressure and third we need to know what is the difference between liquid pressure and pressure in liquid? Click on the “play” or “next slide” button to start.

Liquid pressure is the pressure caused by a liquid. Liquid pressure exists due to the weight of the liquid. The pressure is caused by the weight of the liquid pressing on a surface.

In SPM, one of the most important things that you need to know about liquid pressure is the formula of liquid pressure. There are a lot of physics problems in this chapter are related to this formula. Liquid pressure is given by the equation P = hρg where P is the pressure, h is the depth from the surface, rho(ρ) is the density and g is the gravitational field strength, which is taken to be 10Nkg-1 in SPM.

One thing that you need to be careful about the formula of liquid pressure is the difference between liquid pressure and pressure in liquid. Liquid pressure is the pressure at a point in the liquid caused by the liquid wheareas liquid in pressure is the total pressure at a point in the liquid. Let’s see this diagram. There is a fish h metre below the pond. The weight of the water is pressing on the body of the fish, causing a pressure called liquid pressure. There is also another pressure caused by the atmospheric gases above the pond called the atmospheric pressure. As a result, the total pressure pressing on the fish is the liquid pressure + the atmospheric pressure. Therefore, the liquid pressure exerts on the fish is P = hρg  and the pressure in liquid is P = Patm + hρg  where Patm is the atmospheric pressure and g  is the liquid pressure.

Click on the example button to see the example question. If you think you have already well master this topic, you may skip the example and continue with “characteristic of liquid pressure”. Figure above shows a fish is at a place 2 metre below the surface of a lake. Find

1. the water  pressure exerts on the fish.
2. The pressure experiences by the body of the fish.

(Density of water = 1000kgm-2; Atmospheric pressure = 100,000 Pa)

For a., water pressure P = hρg, h is 2m, density is 1000 kgm-3, g is 10 N/kg. Therefore, P is 20,000 Pa.

In b., the question want us to find the pressure experiences by the body of the fish, it is the total pressure exerts on the body of the fish. In previous slide, we have learned that the total pressure in water is equal to the atmospheric pressure + the liquid pressure. Atmospheric pressure is equal to 100,000Pa.Therefore, the pressure exerts on the fish is 100,000  + 20,000 , which is equal to 120,000 Pa.

This example shows the difference between liquid pressure and pressure in liquid.