- Books Name
- Science Made Easy Science Book

- Publication
- Science Made Easy

- Course
- CBSE Class 9

- Subject
- Science

**Energy**

Energy is defined as the ability to do work. Its unit is the same as that of work.

SI unit of energy or work = Joule (Nm)

Energy has different forms: Light, heat, chemical, electrical or mechanical.

Mechanical energy is the sum of:

(i) Kinetic energy (K.E)

(ii) Potential energy (P.E)

**Kinetic Energy**

Objects in motion possess energy and can do work. This energy is called Kinetic Energy.

F = ma.

Also W = F.s

⇒ From the 2nd equation of motion v^{2}−u^{2}=2as,

⇒ we get s = v^{2}−u^{2} / 2a

Substituting equation for work done by a moving body,

⇒ we get W =m.a * v^{2}−u^{2} / 2a

Or

⇒ Kinetic Energy = K.E= 1/2 mv^{2 }(taking initial velocity u=0)

When two identical bodies are in motion, the body with a higher velocity has more K.E.

**Work-energy theorem**

**The work-energy theorem states that the net work done by a moving body can be calculated by finding the change in KE.**

**⇒ ****W _{net }**

**=**

**KE**

_{final}**−**

**KE**

_{initial}**⇒ ****W _{net }**

**= 1/2 mv**

^{2}