Lesson Notes By Weeks and Term - Junior Secondary School 3





Reference Materials







  • Introduction   
  • Mechanical Properties of Air 
  • Experiments and Deductions
  • Air: Technological Applications



We feel the pressure of air, as it surrounds us like water surrounds the fish in the sea. The presence of air is felt as it blows over our body on windy days, or as it blows away light object thrown out of moving vehicle.

Air contains nitrogen, oxygen and other gases. Air thins out gradually as we go higher above the sea level. The blanket of air that covers the earth is about 72km thick. At high levels, air contains insufficient oxygen to support life. Hence, provision for the supply of more oxygen has to be made for aircraft flying at high altitude.   


Properties of Air 

The properties of air such as: its weight; pressure and variation of pressure; temperature and speed, are utilized in technology to produce machine.

Although air is very light, one cubic meter of it weight about 1.2kg

Atmospheric pressure is due to a weight of column of air about 72km above sea level. The pressure exerted by air at sea level is approximately 10,500kg/m2.



  1. Briefly describe the properties of air.


Experiments and Deductions


Air has weight

If two balloons (one inflated and the other deflated) are placed at ends of a lever balance. The end of the balance attached to the inflated balloon (containing air) goes down. This shows that air has weight    


Air exerts pressure   

 A plastic bottle that has been evacuated (removal of air from the bottle) is crushed by the atmospheric pressure. This shows that air exerts pressure.

Air pressure decreases the higher we go, hence, at high altitude, the inside of an aircraft is pressurized.

Atmospheric pressure can support about 10m column of water or 75cm column of mercury.

For fluid in a container to flow out freely through a punched hole in the container, the pressure inside the container must be equal to or greater than the external pressure of the surrounding air.

The pressure of air in a container can be increased by increasing the mass of air it contains or by squeezing the container to decrease the volume of air


Air speed’s relationship with Pressure and Temperature 

When air moves fast, it’s pressure decreases. Cold air is heavier than hot air; hence the surrounding cold air pushes a hot-air balloon up.

The surrounding air which is denser than the very light air in an airship punches the airship up. 

The lift of an aircraft is contained is obtained that air moves faster on the upper side of the wing (low pressure and slower on the under side (high pressure). The same principle applies to kite.



  1. State 3 experiments that describe properties of air.

Air: Technological Applications

Air, in the static condition as well as in motion, exhibits some properties as demonstrated above. Some of the properties are utilized in the following systems such as balloons, kites, airships, vacuum cleaners, aircraft propulsion and lift, wind power e.t.c.


When air is heated, it expands and becomes less dense i.e. lighter, as it increases in volume. This is the basis of operation of the hot-air balloons, which have in use since the eighteenth century. A balloon is filled with hot air, which is much lighter than the surrounding cold dense air. The denser outside air then lifts the balloon off from the ground. The balloon continues to rise until it gets to denser air up.

When the balloonist wants to descend, he lets air out of the balloon, which gradually descends to the ground.


Kite flying is one the favorite sports that people get involved in sunny windy days in open fields.

It is highly desirable that the string attached at a point such that the air has a longer path over the top of the kite than underneath. Hence, this will lead to a much higher speed and consequently low pressure on the top side, and on the underside a lower speed and much higher pressure. The effect is a lifting up of the kite into the air due to pressure difference   



Airship is basically a balloon in operation. It can be described as an elongated balloon. While the hot air balloon uses hot air to provide lift, airships use lighter-than-air gases, such as hydrogen and the inert helium gas, to fill the envelope.

Airship is fitted with propellers, and some control mechanism so that it can move in any direction as it floats in the air.

Vacuum Cleaners

A vacuum cleaner is a household gadget for cleaning carpets. It removes big and small dust particles, which are sucked into the bag or container.


An electric fan draws dust-laden air through the machine into the container through which the air passes, leaving the dust particles behind. As the fan blows air away into the container, air is continuously sucked from the other side through the nozzle; which is in contact with the carpet. The sucked air carries pieces of dust and dirt with it towards the fan. The cloth bag contains tiny holes through which only air can pass and later filtered until it leaves the machine finally as clean air.  



  1. Describe 3 Technological Applications of air.


Reading Assignment 



Reference Materials   

  1. NERDC, Introductory Technology for JSS, book 3, pages 170 -180
  2. EVANS, Introductory Technology for JSS, book 3, pages 139 - 157

Weekend Assignment

  • The presence of air is felt as it blows over our body on  (a) windy days (b) cold days (c) dry days (d) hot days
    1. Wind has the ability to blow away (a) light object (b) high rising building (c) sky scraper (d) heavy building
    2. The end of the balance attached to the inflated balloon (containing air) goes down. (a) air has weight (b) air is weak (c) air is strong (d) air flexible
  • The following are technologies from the application of air  EXCEPT (a) balloon  (b) airship (c) vacuum cleaner (d) printer
  • The pressure of air in a container can be increased by increasing the mass of air it contains or by squeezing the container to decrease the volume of air True or False.



  1. State 2 properties of air with proof
  2. Explain the following technologies from the application of air (a) balloon  (b) airship (c) vacuum cleaner

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