Pressure and Hydrostatic Pressure

Pressure

Pressure is the force acting on a unit area of a surface.

P = F / A

Where,

P is Pressure (Pa)

F is Perpendicular Force applied (N)

A is Area (m²)

Units :

Pascal (Pa), Newton per square metre (N/m²)

Relationship between units : 1 Pa = 1 Nm⁻²

Figure 1. Perpendicular Force Acting on a Surface Area

Easy Memory Trick

More Force → More Pressure

More Area → Less Pressure

Applications

Sharp Knife

• A knife has a very small edge area.

• Therefore it produces high pressure and cuts easily.

  
  

Needle

• The pointed tip has a very small area.

• Therefore it easily pierces cloth and skin.

  
  

Tractor Tyres

• Large tyres increase contact area.

• This reduces pressure and prevents sinking into mud.

Figure 2. Effect of Surface Area on Pressure: Knife, Needle, and Tractor Tyre

Pressure in Liquids

Liquids also exert pressure. This pressure is called Hydrostatic Pressure.

What Causes Hydrostatic Pressure?

Hydrostatic pressure is caused by the weight of the liquid above a point. Imagine swimming underwater.

The deeper you go, the more water is above you. Therefore, the pressure increases.

Hydrostatic Pressure

Pressure exerted by a liquid at rest is called hydrostatic pressure.

P = hρg

Where,

P is Hydrostatic Pressure (Pa)

ρ is Density of liquid (kg m⁻³)

g is Gravitational field strength (m s⁻²)

h is Depth of liquid (m)

Factors Affecting Hydrostatic Pressure

1. Depth of Liquid (h)

   
   

   Greater depth → Greater pressure

   
   

   Example:

   
   

   Divers feel greater pressure when they go deeper into the sea.

   
   

2. Density of Liquid (ρ)

   
   

   Higher density → Higher pressure

   
   

   Example:

   
   

   Salt water exerts more pressure than fresh water at the same depth.

   
   

3. Gravity (g)

   
   

   Higher gravity → Higher pressure

   
   

   Example:

   
   

   Jupiter exerts greater hydrostatic pressure than Earth at the same depth and in the same liquid.

Factors That Do Not Affect Hydrostatic Pressure

• Shape of the container

• Volume of liquid

• Surface area of liquid

Look at a water tank with holes at different heights. Water from the lowest hole shoots the farthest because the pressure is greatest at the bottom.

Figure 3. Water Jets from Holes at Different Depths

One Minute Revision

Pressure

• Force per unit area

• Unit - Pascal (Pa)

• More force → More pressure

• More area → Less pressure

  
  

Hydrostatic Pressure

• Pressure exerted by a liquid at rest

• Increases with depth

• Depends on density, gravity, depth

• Does not depend on container shape