• When a force acting on a body produces a change in the position of the body, work is said to be done by the force. Work done on an object is defined as the magnitude of the force multiplied by the distance moved by the object in the direction of the applied force. The unit of work is joule: 1 joule

= 1 newton × 1 metre. Work done on an object by a force would be zero if the displacement of the object is zero.

• **Power** is defined as the rate of doing work. Power = (work done) / (time taken). The SI unit of power is watt. 1 W = 1 Joule/second. The unit of power is also horse power. It is the power of an agent which can work at the rate of 550 foot pounds per second or 33,000 foot pounds pwe minute.

• An object having capability to do work is said to possess energy. Energy has the same unit as that of work.

• An object in motion possesses what is known as the kinetic energy of the object. An object of mass, m moving with velocity v has a kinetic energy of (1/2) mv2.

• The energy possessed by a body due to its change in position or shape is called the potential energy. The

gravitational potential energy of an object of mass, m raised through a height, h from the earth’s surface is given by mgh.

• According to the **law of conservation of energy**, energy can only be transformed from one form to another; it can neither be created nor destroyed. The total energy before and after the transformation always remains constant.

• Energy exists in nature in several forms such as kinetic energy, potential energy, heat energy, chemical energy etc. The sum of the kinetic and potential energies of an object is called its mechanical energy.

• **Pressure**: Pressure is defined as force acting per unit area. Pressure = force/ area. The SI unit of pressure is newton per meter squared or Pascal.

• The same force acting on a smaller area exerts a larger pressure, and a smaller pressure on a larger area. This is the reason why a nail has a pointed tip, knives have sharp edges and buildings have wide foundations.

• All liquids and gases are fluids. A solid exerts pressure on a surface due to its weight. Similarly, fluids have weight, and they also exert pressure on the base and walls of the container in which they are enclosed. Pressure exerted in any confined mass of fluid is transmitted undiminished in all directions.

• All objects experience a force of buoyancy when they are immersed in a fluid.Objects having density less than that of the liquid in which they are immersed, float on the surface of the liquid. If the density of the object is more than the density of the liquid in which it is immersed then it sinks in the liquid.

•** Archimedes’ Principle**: When a body is immersed fully or partially in a fluid, it experiences an upward force that is equal to the weight of the fluid displaced by it.

• Archimedes’ principle has many applications. It is used in designing ships and submarines. Lactometers, which are used to determine the purity of a sample of milk and hydrometers

used for determining density of liquids, are based on this principle.

• **Density and Relative Density**: The mass per unit volume of a substance is called its density. The SI unit ofdensity is kilogram per meter cubed. Density= mass/volume.

• The relative density of a substance is the ratio of its density to that of water: **Relative density** = Density of a substance/Density of water. Since the relative density is a ratio of similar.