#### Energy

Energy is defined as the ability to do work. Energy can take many forms. In mechanics the two most common forms of energy are **Kinetic Energy **and** Potential Energy. **The general symbol for all energy forms is** E. **Energy is measured in Joules** (J).**

#### Potential Energy

Potential Energy is the energy stored in an object when it has been lifted off the ground. The symbol for Potential Energy is **E _{P}**

In the figure above, an object with mass m (in kilograms) is lifted up to a height of h meters above the ground, against gravity. This object will have stored gravitational potential energy **E _{P}**

To calculate this stored (or sometimes called "latent" or "cached") energy we use the equation below:

**E _{P}**= m x g x h

The units of Potential Energy, are the same as the unit of work. **Work** and energy are both calculated in (J), "Joules".

**Example 1:**

In the picture above, the ball on the cliff has a mass of 300 g and the cliff is 20 meters above the ground. What is the potential energy of the ball?

**Given:**

m = 300

h = 20 m

g = 9.8 m/s^{2} or 9.8 N/Kg

g = 0.300 Kg

** Find:**

**E _{P}**

**Solution:**

**E _{P}** = m x g x h

= (0.300 Kg)((9.8 N/Kg)(20 m)

= 58.8 Nm

Therefore the mass will have 58.8 J of stored (Potential Energy) at the top of the cliff.

#### Kinetic Energy

Kinetic Energy is the energy of a moving object. The symbol for Kinetic Energy is **E**_{k}

The kinetic energy of an object depends on the mass (m) of the object and on the speed (v) of the object.

**Example 2:**

A 200 g soccer ball is moving with a speed of 20.0 m/s. Calculate its kinetic energy.

**Given:**

m = 200 g = 0.200 Kg

v = 20 m/s

**Find:**

**E**_{k}

**Solution:**

Therefore the soccer ball has a Kinetic Energy 40 J