Everything Science Grade 12

CHAPTER 8. WORK, ENERGY AND POWER 8.3

DEFINITION: Work-Energy Theorem

The work-energy theorem states that the workdone on an object is

equal to the change in its kinetic energy:

W = ΔKE = KEf− KEi

The work-energy theorem is another example ofthe conservation of energy which you
saw in Grade 10.

Example 5: Work-Energy Theorem

QUESTION

A 1 kg brick is droppedfrom a height of 10 m. Calculate the work done on the

brick when it hits the ground assuming that there is no air resistance.

SOLUTION

Step 1 : Determine what is given and what is required

• Mass of the brick: m = 1 kg.


• Initial height of the brick: hi= 10 m.


• Final height of the brick: hf= 0 m.


• We are required to determine the work done onthe brick
  as it hits the ground.

Step 2 : Determine how to approach the problem

The brick is falling freely, so energy is conserved. We know that

the work done is equalto the difference in kinetic energy. The

brick has no kinetic energy at the moment it is dropped, because

it is stationary. Whenthe brick hits the ground, all the brick’s

potential energy is converted to kinetic energy.

Step 3 : Determine the brick’s potential energy at hi

PE = m· g· h

= (1 kg)(9,8 m· s−^2 )(10 m)

= 98 J

Step 4 : Determine the work done on the brick