Energy transfer when work is done.







Energy transfer when work is done.



BEHAVIOURAL OBJECTIVES : At the end of the lesson , pupils should be able to


  • Explain energy transfer when work is done


Week 3

Topic: Energy Transfer When Work is Done

Work is the force acting on an Object to cause a displacement. Work is done on an object when you transfer energy to that object. If one object transfers (gives) energy to a second object, then the first object does work on the second object.When an object is dropped from above the ground, work is done as the object is pulled to the ground. As the object is falling and work is done, the potential energy of the body is changed to kinetic energy. Work done and energy transferred are measured in joules (J). The work done on an object can be calculated if the force and distance moved are known. A change in momentum happens when a force is applied to an object that is moving or is able to move. In principle, the quantity of potential energy stored in a body is always equal to the kinetic energy produced when the body is released to do work. In order words, when energy changes, for example from potential energy to kinetic energy, there is always an accompanying work done.

Work-Energy Principle –The change in the kinetic energy of an object is equal to the net work done on the object.


  1.  A force of 20N pushing an object 5 meters in the direction of the force. How much work is done?
  2. The work done on an object is 5 Kilo joules and the object moved a distance of 800cm. Calculate the force acting on the object.


  1. Workdone = Force x distance
    20N x 5m = 100Nm or joules
  2. Workdone = Force x distance
    Force = Workdone/distance
    work done = 5 kilo joule convert to joules = 5 x 1000 = 5000 joules
    distance = 800 cm.
    100cm = 1m
    800 cm = 8m
    Force = 5000/8 = 625N


Further discussion on Energy transfer when work is done

If you apply a force over a given distance – you have done work.  Work = Change in Energy. If an object’s kinetic energy or gravitational potential energy changes, then work is done. The force can act in the same direction of motion. Or, the force can act against the motion. (Drag and friction do that.) Forces can act when objects touch.

In general, the energy transferred depends on the amount of force and the distance over which that force is exerted.

If the man pushes the rock in the direction of the force, he has done work. If the rock rolls back and pushes him, then the rock does work on the man.

No work: If the net force is perpendicular to the motion then no work is done. If you push on an object and it doesn’t move, then no work is done.  If an object’s kinetic energy doesn’t change, then no work is done.

Another Equation for Calculating Work:

Work = Mass * Gravity * Height and is measured in Joules. Imagine you find a 2 -Kg book on the floor and lift it 0.75 meters and put it on a table. Remember, that “force” is simply a push or a pull.

Work = Mass X Gravity X Height

= 2 X 10 X 0.75

= 14.7 Joules

Energy is defined as the ability to do work. If you can measure how much work an object does, or how much heat is exchanged, you can determine the amount of energy that is in a system.  As with work, energy is also measured in Joules. Energy is not created nor destroyed according to the Law of Conservation of Energy. Energy only changes form. It is transformed from one kind of energy to another. In fact, the energy that makes your body work can be traced back to the sun. Solar energy is transformed to chemical energy in the plants. We get chemical energy from the plants and animals we eat.

In science, we say that work is done on an object when you transfer energy to that object. If you out energy into an object, then you do work on that object (mass). If a first object is the agent that gives energy to a second object, then the first object does work on the second object. The energy goes from the first object into the second object. At first we will say that if an object is standing still and you get it moving, then you have put energy into that object. The object has kinetic energy as a result of your work. You pushed it through a displacement, you did a work on the object.

For example, a golfer uses a club and gets the stationary golf ball moving when s/he hits the ball. The club works on the golf ball as it strikes the ball. Energy leaves the club and enters the ball, this is a transfer of energy. Thus we say that the club did work on the ball, and before the ball was struck, the golfer did work on the club. The club was initially standing still, and the golfer got it moving when he or she swung the club.

So, the golfer does work on the club, transferring energy into the club, making it move. The club does work on the ball, transferring energy into the ball, getting it moving.


  1. How much work is done if a force of 20N is used to displace an object 3m?

Work done = Force X distance

W = 20N X 3m = 60Nm or 60 Joules

  1. A force of 15.73N acts on an object over a displacement of 16.93m. The force and displacement are in the same direction. How much work does the force do on the object?

Work done = Force X distance

W = 15.73N X 16.93m = 266.31J

  1. How much work is done by a force of 25N that operates over a displacement of 6.2m?

Work done = Force X distance

W = F X D = 25 X 6.2 = 155J





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