Energy is defined as the capacity or the ability to do work. The S.I unit of energy is joule (J).

1. Potential Energy (P_e)

Potential energy is defined as the energy a body has because of it position which allow it to do work when released. Potential energy can be calculated from the formula below;

P_e=mgh

Where m = mass

g = acceleration due to gravity

h = height

Gravitational potential energy is the energy a body has because of it position in the gravitational field.

Examples include;

• A girl on the roof of the church.
• A straight string.
• A ripped mango on a tree.

Example: Calculate the potential energy of a boy of mass 65kg on top of a tall tree 20m long.

Solution

P_e=mgh

But m=65kg,           g=10m/s²,    h=20m

P_e=13,000J 

2. Kinetic Energy (K_e)

Kinetic energy is defined as an energy a moving body has. If a body of mass m kg is moving with a velocity v m/s, then its K_e can be calculated as shown below;

K_{e= mv²2}

Example: Calculate the kinetic energy of a body of mass 2000g moving with a velocity of 5m/s.

Solution

K_{e= mv²2}

M = 2000g = 2kg

v = 5 m/s

K_{e= 2kg5ms22=25J} 

K_e = 25J

The law of Conservation Of Energy

The law states that “In a closed system, the total amount of energy is always the same”. Where ever energy changes from one form to another. That is, energy can neither be created nor destroyed.

ENERGY TRANSFORMATION

Potential energy and kinetic energy are forms of mechanical energy and can be transformed from one form to another. A polling object would convert P.e to k_e alt the point of contact with the ground it P.e is entirely converted to K.e. When the object finally touches the ground all it K.e is converted to sound energy.

The total energy of the ball can be calculated from the formulae bellow.

Example: A following object of mass 2kg is 20m above the ground and it is moving Witt velocity 20m/s. Calculate the ET.

Solution

ET=P.e + K.e

  =mgh + 1/2mv²

  m=2kg, h=20m, g=10m/s, v=20m/s²

  = ET =2kg x 10m/s² x 20m + 1/2(2kg) (20m/s) ²

  =400J + 400J =800J

ET = 800J