Thrust and Pressure

Chapter:- 4


Thrust and Pressure

Thrust is the force or push acting on an object perpendicular to its surface.  Effect of thrust depends on the area on which it acts. The unit of thrust is the same as that of force, i.e., the SI unit of thrust is newton (N). It is a vector quantity.


Pressure is the force acting perpendicularly on a unit area of the object.

The SI of pressure is N/m2, which is also called pascal (Pa) named after the scientist Blaise Pascal. It is a scalar quantity.

1 Pa = 1N/m2

Some Daily Life Applications of Pressure

  • The handles of bags, suitcase, etc . are made broad, so that the small pressure is exerted on the hand.
  • Buildings are Provided with broad foundations. so that the pressure exerted on the ground becomes less.
  • Railway tracks are laid on cement or iron sleepers, so that the pressure exerted by train is spread over the larger area and thus, pressure becomes less.
  • Cutting tools have sharp edges to reduce the area so that with lesser force more pressure is exerted.
  • The tractors have broad tyres so that there is less pressure on the ground and the tyres do not sink into comparatively soft ground in the field.

Pressure in Fluids

Those substances which can flow easily are called fluids. All the liquids and gases are fluids. Water and air are two most common fluids. Solids exert pressure on a surface due to their weight.

Fluids also exert pressure on the base and walls of the container in which they are enclosed. Fluid (liquid or gas) exerts pressure in all directions, upwards also.


The tendency of liquid to exert an upward force on an object immersed in it, is called buoyancy.

Buoyancy Force is an upward force which acts on an object when it is immersed in a liquid: It is also called upthrust. It is the buoyant force due to which heavy object seems to be lighter in water. As we lower the position of object into a liquid, the greater upward pressure of liquid underneath it, provides an upward force.

For Example, a piece of cork is held below the surface of water.

When we apply a force by our thumb, the cork immediately rises

to the surface. This is due to  the fact that every liquid

exerts an upward force on the objects immersed in it.

Factors Affecting Buoyant Force

Factors Affecting Buoyant Force

(i) Density of the Fluid

The liquid having higher density exerts more upward buoyant force on an object than another liquid of lower density. This is the reason, why it easier to swim in sea water in comparison to normal water, because sea water has higher density and hence, exerts a greater buoyant force on the swimmer than the fresh water having lower density.

(ii)Volume of Object Immersed in the Liquid

As the volume of solid object immersed inside the liquid increases, the upward buoyant force also increases.  The magnitude of buoyant force acting on a solid object does not depend on the nature of the solid object. It depends only on its volume.

For Example, when two balls made of different metals having different weights but equal volume when fully immersed in a liquid they will experience an equal upward buoyant force as both the balls displace equal weight of the liquid due to their equal volume.

Floating or Sinking of Objects in Liquid

When an object is put in a liquid, then following two forces act on it.

  • Weight of object which acts in downward direction. i.e., it tends to pull down the bject.
  • Buoyant Force (upthrust) which acts in upward direction.i.e., it tends to push up the object.

There are three conditions of floating and sinking of objects

(i):- If the buoyant force or upthrust exerted by the liquid is less than the weight of the object, the object will sink in the liquid.

(ii):- If the buoyant force is equal to the weight of the object, the object will float in the liquid.

(iii):- If the buoyant force is more than the weight of the object, the object will rise in the liquid.

Note:- So far an object floating in a liquid the weight of object should be equal to buoyant force acting on it.

Law of Floatation

According to this law, an object will float in a liquid if the weight of object is equal to the weight of liquid displaced by it. Mathematically, it is expressed as

Weight of object = weight of liquid displaced by it


The density of a substance is defined as mass per unit volume.

The SI unit of density is kilogram per metre cube (kg/m3). It is a scalar quantity.

Archimedes’ Principle

Archimedes’ Principle

When an object is fully or partially immersed in a liquid, it experiences a buoyant force or upthrust, which is equal to the weight of liquid displaced by the objects.

Buoyant force or upthrust acting on an object = Weight of liquid displaced by the object

Even gases like air, exert an upward force or buoyant force on the objects placed in them. It is buoyant force or upthrust due to displaced air which makes a ballon rise in air.

Application of Archimedes’ Principle

  • Designing ships and submarines
  • Lactometer (used to determine the purity of milk)
  • Hydrometer (used for determining the density of liquid)
  • Determining the relative density of a substance

How does a Boat Float in Water?

A boat floats in water due to upward force called buoyant force (or upthrust) which is caused by the pressure of water pushing up the bottom of the boat. When this buoyant force becomes just enough

to support the weight of boat, the boat stops sinking down in water.

Relative Density

The relative density of a substance is the ratio of its density to that of water.

Since,  the relative density is a ratio of similar quantities it has no unit. The relative density of a substance expresses the heaviness of the substance in comparison to water. By saying that relative density of iron is 8.7, we mean that iron is 8.7 times as heavy as an equal volume of water.

Note:- Density of water is 1000kg/m3  or 1g/cm3.