Second Term Lesson Note for Week Four
Class : JSS 2
Subject : Basic Science
Topic : Transfer of Energy during Work
Duration : 40 Minutes
Period : Single Period
Reference Book :
- Basic Science and Technology for Junior Secondary School, JSS 2.
- Lagos State Unified schemes of work for Junior Secondary School, JSS 1 – 3.
- Online Resources
Instructional Material : Chart showing the transfer of Energy.
Learning Objectives : By the end of the lesson learners will be able to :
i. Explain the transfer of energy
ii. Identify the relationship between work, Energy and Power
iii. Solve problems on Work, Energy and Power.
Content :
Energy Transfer When Work Is Done
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.
In principle ,the quantity of potential energy stored in a body is always equal to kinetic energy produced when the body is released to do work. In other words, when energy changes ,for example from potential to kinetic, there is always an accompanying workdone.
EXAMPLE
What work is done when a mass of 6kg is raised through a vertical height of 3.5m (acceleration due to gravity is 10m/s2)
Mass = 6kg
Acceleration( due to gravity) = 10m/s²
Work done, W = Mass x acceleration x distance
= 6kg x 10 x 3.5
= 210 joules.
POWER
Power is also related to the concepts of energy and work. Power is defined as the rate of doing work, i.e workdone divided by time.
Power=Workdone/Time taken
The unit of power is Watt(w)
Example 1
What is the power of a child that has done work of 60 J in 20 seconds.
Solution
Power P=Work/Time
= ( 60/20) J/s
=3 watts
Example 2
Calculate the power of a pump which can lift 500kg of water through a vertical height of 12m in 0.3 minutes. Assuming g=9.8m/s2
Force = mass x acceleration(g)
Force = 500kg x 9.8N
Distance = 12m
Workdone=500 x 9.8 x 12m
Time taken = 0.3minutes x 60
= 18 secs
Work done = mass x acceleration x distance
Work done = Force x distance
Since force = mass x acceleration
Work done = 500kg x 9.8N x 12m
= 58800J
Power = workdone ÷ Time taken
= 58800/18
Power = 3266.67 watts
Convert this to kilowatts by dividing by 1000
= 3266.67/1000
= 3.27kilowatts
EVALUATION :
1. If the kinetic energy of a ball moving at a velocity of 5m/s is 60kj.What is the mass of the ball?
2. The mass of a ball is 10g and the height is 4m.Calculate the velocity of the ball just before it touches the ground.(g=10m/s)
3. If the mass of an object is 6kg,and the height is 5m.Calculate the potential energy.(assuming=10m/s).
4. If a force of 10 Newton is applied by a car over a distance of 4metres,calculate the work done by the car.
5. Calculate the work done by a ball of mass 40g which falls freely from a height 0.8m above the ground.(g = 10m/s²)
6. Calculate the power of a pump which can lift 500kg of water through a vertical height of 12m in 0.3minutes . (Assuming g-9.8m/s²)
Conclusion : The teacher summarizes the lesson all over again, for a clear understanding.