**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

- P.E = mgh

= 10 × 9.81 × 100

=9810J

= 981KJ

- 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:

- The cars initial kinetic energy
- The final kinetic energy
- The breaking force

Solution

- Initial K.E =1/2MV2

= 500 ×302

= 450000J

= 450KJ

- Final K.E = 1/2MV2

= 500 × 102

= 50000J

= 50KJ

- 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**

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

**GENERAL EVALUATION**

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

**READING ASSIGNMENT**

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

**WEEKEND ASSIGNMENT**

- 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
- 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

- 3.2Kilowatts D. 3Kilowatts.

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

- 294J B. 24J C. 94J D. 29J

**THEORY**

- Define the term energy.
- 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.

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