# Lesson Notes By Weeks and Term - Junior Secondary School 2

CALCULATION INVOLVING WORK DONE

TERM: 2nd TERM

SUBJECT: BASIC SCIENCE

CLASS:  JSS 2

REFRENCE

• Precious seed BASIC SCIENCE FOR JUNIOR SECONDARY SCHOOLS BOOK 2

WEEK THREE                                                                                    DATE:………………..

TOPIC: CALCULATION INVOLVING WORK DONE

Worked Example 1

How much work is done when a force of 5 kN moves its point of application 600mm in the direction of the force.

Solution

W.D = (5×103)×(600 × 10-3)

= 3000J

= 3KJ

Worked Example 2

Find the work done in raising 100 kg of water through a vertical distance of 3m.

Solution

The force is the weight of the water, so

W.D = (100 × 9.81) × 3

=294J

Work done by a variable force

Worked example 3

What is the potential energy of a 10kg mass: 100m above the surface of the earth at the bottom of a vertical mine shaft 1000m deep.

Solution

1. P.E = mgh

= 10 × 9.81 × 100

=9810J

= 981KJ

1. P.E = mgh

= 10 × 9.81 × (-100)

= -98100J

=-98.1KJ

Worked example 4

A car of mass 1000 kg travelling at 30m/s has its speed reduced to 10m/s by a constant breaking force over a distance of 75m.

Find:

1. The cars initial kinetic energy
2. The final kinetic energy
3. The breaking force

Solution

1. Initial K.E =1/2MV2

= 500 ×302

= 450000J

= 450KJ

1. Final K.E = 1/2MV2

= 500 × 102

= 50000J

= 50KJ

1. Change in kinetic energy = 400 kJ

Equation 4

The SI unit for power is the watt W.

A power of 1W means that work is being done at the rate of 1J/s.

Larger units for power are the kilowatt kW (1kW = 1000 W = 103 W) and the megawatt MW (1 MW = 1000000 W = 106 W).

If work is being done by a machine moving at speed v against a constant force, or resistance, F, then since work doe is force times distance, work done per second is Fv, which is the same as power.

Work done

Ques I; If a force of one Newton  is applied by a car over a distance of 5metres , what is the work done?

Work done =Force x distance, therefore

W= f  x d

200 x 6= 1200joules

Ques 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/s

Solution

Force                                      =500kg x 9.8N

Distance                                 =12m

Work done                            =500 x  9,8 x 12j

Time taken                            = 0.3 x 60 = 18 sec

Power = Work done   =     500 x  9.8  x  12  j

Time taken                    18

=3266.67watts

=3.27Kilowatts

Ques 3: A student  whose mass is 60kg   runs up a height  of 7.2 m in 10.4  second . Find the power  used  by the student (given that g = 9.8m/s)      .

Solution

Force            = 60kg x  9.8N

Distance        =7.2m

Work done   = F x D

= 60 x  9.8 x 7.2 j

Power == Work done           =     60 x  9.8  x   7.2

Time taken                      10.4sec

=407.076j/sec

=0.40Kilowatts(KW)

EVALUATION

1. A constant force of 2kN pulls a crate along a level floor a distance of 10 m in 50s. What is the power used?
2. What is the work done by a force of 20N pushing an object 5m in the direction of force?

GENERAL EVALUATION

1. What is the potential energy of a 10kg mass: 100m above the surface of the earth at the bottom of a vertical mine shaft 1000m deep
2. A constant force of 2kN pulls a crate along a level floor a distance of 10 m in 50s. What is the power used?
3. What is the work done by a force of 20N pushing an object 5m in the direction of force?
4. Calculate the power of a pump which can lift 500kg of water through a vertical height of 10m in 0.5 minutes ,assuming g =9.8m/s

Precious seed BASIC SCIENCE FOR JUNIOR SECONDARY SCHOOLS BOOK 2 PAGE 117

WEEKEND ASSIGNMENT

1. How much work is done when a force of 5 kN moves its point of application 600mm in the direction of the force. A. 3KJ B. 4KJ C. 3.8KJ D. 2.67KJ
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/s A. 3.27Kilowatts B. 3.7Kilowatts
1. 3.2Kilowatts D. 3Kilowatts.
1. A student  whose mass is 60kg   runs up a height  of 7.2 m in 10.4  second . Find the power  used  by the student (given that g = 9.8m/s)  A. 0.40KW B. 0.04KW C. 0.42KW D. 0.004KW
2. If a force of 200 Newton  is applied by a car over a distance of 6 metres ,the workdone is A. 1200joules B. 120joules C. 12joules D. 1.20joules
3. Find the work done in raising 100 kg of water through a vertical distance of 3m.
1. 294J B. 24J C. 94J D. 29J

THEORY

1. Define the term energy.
2. The distance  from   boys classroom  to the edge  of a football field is 120 metres .He runs  with a force of 100 Newtons  across the field. calculate the work done.