**Notes For All Chapters Science Class 9**

**Facts that Matter**

**Work:** When a force acts on an object and the object shows displacement, the force has done work on the object.

**Two conditions need to be satisfied for work to be done:**

(i) A force should act on object

(a) The object must be displaced

Work = Force x Displacement Unit of workdone = Joule = Newton x metre 1 Joule work is said to be done when 1 Newton force is applied on an object and it shows the displacement by 1 meter.

**Energy**

The capacity of a body to do work is called the energy of the body.

Unit of energy = Joules 1KJ = 1000 J

**Forms of Energy:** The various forms of energy are potential energy, kinetic energy, heat energy, chemical energy, electrical energy and light energy.

**Kinetic Energy:** Energy possessed by a body due to its motion. Kinetic energy of an object increases with its speed.

Kinetic energy of body moving with a certain velocity = work done on it to make it acquire that velocity

**Derivation**

Let an object of mass m, move with uniform velocity u, let us displace it by s, due to constant force F, acting on it

**Potential Energy**

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

**shape**

**Gravitational Potential Energy: (GP)**

When an object is raised through a height, work is said to be done on it against gravity.

The energy possessed by such an object is called the gravitational potential energy.

GPE = work is done in raising a body from the ground to a point against gravity.

**Derivation**

Consider a body with mass m, raised through a height h, from the ground, Force required to raise the object = weight of object mg.

The object gains energy to the work done on it.

Work done in both the cases (i) and (ii) is same as a body is raised from position A to B, even if the path taken is different but the height attained is the same.

**Mechanical Energy:** The sum of kinetic energy and potential energy is called mechanical energy.

**Law of Conservation of Energy:**

Energy can neither be created nor destroyed, it can only be transformed from one form to another. The total energy before and after transformation remains the same.

Potential energy + Kinetic energy = Constant (Mechanical energy)

A body of mass ‘m’ is raised to height ‘h’ at A its potential energy is maximum and kinetic energy is 0 as it is stationary.

When body falls at B, h is decreasing hence potential energy decreases and V is increasing hence kinetic energy is increasing.

When the body is about to reach the ground level, h = 0, v will be maximum hence kinetic energy –> potential energy

Decrease in potential energy = Increase in kinetic energy

This shows the continual transformation of gravitational potential energy into kinetic energy.

**Power**

**Commercial Unit of Energy**