FRICTION
Subject :
Basic Technology
Term :
Second Term
Week:
Week 6
Class :
JSS 3 / Basic 9
Topic:
FRICTION
Previous lesson :
The pupils have previous knowledge of
SOLDERING AND BRAZING
in Basic Science in their previous lesson
Behavoiural Objectives : At the end of the lesson , pupils should be able to
- Define FRICTION
- say the causes of friction
- mention the advantages of friction
- list the disadvantages of friction
- suggest ways by which friction can be reduced
Instructional Materials :
- Wall charts
- Pictures
- Related Online Video
- Flash Cards
Methods of Teaching :
- Class Discussion
- Group Discussion
- Asking Questions
- Explanation
- Role Modelling
- Role Delegation
Reference Materials :
- Scheme of Work
- Online Information
- Textbooks
- Workbooks
- 9 Year Basic Education Curriculum
- Workbook
CONTENT
Topic: MECHANICAL ENERGY TRANSMISSION SYSTEM (FRICTION)
Content:
- Friction and its effects
- Friction between two surfaces sliding on each
- Advantages and disadvantages of friction
- Uses of lubricants and bearings to reduce friction
Friction
Mechanical energy is a very useful form of energy. It is used to move objects from one place to the other. There are different ways of generating mechanical energy. Energy is converted from one form to the other. Through combustion the chemical energy in petrol fuel is converted into mechanical energy in the engine of a vehicle. In such a case, the mechanical energy exists in the form of kinetic energy of the moving pistons in the cylinders of the engine. Through a transmission system, the mechanical energy of the piston is transmitted to the tyres of the vehicle and use to rotate them. The friction between the rotating tyres and the road surface helps to propel the vehicle along the road.
Another example of mechanical energy transmission is seen in a bicycle. The mechanical energy generated from pedaling is transmitted to the rotating tyres. The friction between the rotating tyres and the road surface also helps to propel the bicycle along the road.
Here, we are interested in transmission systems that allow mechanical energy to be transmitted from one point the other. Mechanical energy transmission systems of interest are friction, belts and gears.
Friction and its effects
Frictional forces occur between any two surfaces which are in contact and move relative to each other, so that rubbing of the surfaces occur. The forces of friction tend to prevent motion; hence, work has to be done to overcome friction in order that motion may take place, for instance a bicycle rider while riding is working against friction as he pedals his bike.
Also one dragging something on the floor is working against friction, which occurs between the box and the floor. There is friction between your buttocks and your seat, between the sole of your shoes and the ground on which you stand, and so forth. Thus, friction is always present between any two surfaces which are in contact. It exists as a force that tends to stop movement between any two surfaces which are moving over each other. A force just sufficient to overcome friction must be applied to initiate or maintain motion. That is to say friction is a force that acts like a brake when a body moves relative to another body.
The magnitude of friction between any two surfaces in contact depends on the nature of the surfaces. It is difficult to slide on a highly smooth surface than it is on a rough surface. This is because friction is greater between two rough surfaces than between two smooth surfaces.
We reduce friction by lubricating the surfaces concerned. Friction is also reduced significantly by ensuring that the two surfaces roll over each other as opposed to sliding. To illustrate, consider the force it require to move a book across the table. The force required to cause sliding, is higher than that required to roll the book over round pencils. The force is reduced further if the book rolls over smooth steel balls as in bearings which are used in machines to reduce frictional force. For example, your bicycle has ball bearings at the wheel axles, the pedal cranks, and the steering column. Notice that the bearings are placed in areas where there are moving parts. Atypical ball bearing is shown below
Some of the properties of friction can be illustrated quite simply and interestingly. Bring both the palms of your hands together and rub them against each other slowly for about ten ties. Notice that they become warm. After a minute or two, bring the palms together again, but this time press them very firmly together while rubbing, also about ten times. Observe that they are much warmer than before, sometimes indeed hot.
Now smear both the palm with grease or any oil and repeat the experiment. What do you find? The palm tends to rub freely, and they do not generate any significant amount of heat. These and other findings lead us to the next important section.
Advantages of Friction
- It enables us to walk without slipping. To enhance friction, it is advisable to use shoes with rough sole while walking on slippery floors. For a similar reason, crutches are provided with rubber tips at their bottom to provide sufficient friction.
2. The breakers and tiers of our cars and bicycles depend on friction to function properly.
3. The ridges in the skin of our fingers and palms enable us to grasp and hold objects due to friction. - To prevent patients being uncomfortable in bed rubber sheets with spongy under surfaces are placed over mattresses. The friction between the spongy under surfaces and the mattress prevents the rubber sheet from slipping and wrinkling.
5. We cannot fix nail in the wood or wall if there is no friction. It is friction which holds the nail.
Disadvantages of Friction
- Production of heat, noise and wear in machine parts rubbing against one another.
2. Heat produce by friction may be sufficient to cause the abrasion of the skin, resulting in friction burn. Rubber tubes such as gastric and duodenal tubes, rectal tubes and catheters may burn or irritate the membrane over which they pass unless measure are taken to prevent friction.
3. Friction reduces the efficiency of engine and other machines. - Due to friction, engines of automobiles consume more fuel which is a money loss.
Lubrication
In a motor car, for example, there are many mating and moving metal surfaces. If these moving surfaces are not prevented from direct contact, a lot of heat will be generated. The surface will also wear themselves off easily and fast. To prevent these, we use lubricants. A lubricant is any substance, whether in form of oil or grease. It forms a very thin film between the mating metal surfaces so that the metal surfaces do not rub together. This prevents excessive heat generation. It also reduces the wearing of the metal surfaces.
Presentation
The topic is presented step by step
Step 1:
The class teacher revises the previous topics
Step 2.
He introduces the new topic
Step 3:
The class teacher allows the pupils to give their own examples and he corrects them when the needs arise
Conclusion
The class teacher wraps up or conclude the lesson by giving out short note to summarize the topic that he or she has just taught.
The class teacher also goes round to make sure that the notes are well copied or well written by the pupils.
He or she does the necessary corrections when and where the needs arise.
EVALUATION
- State the advantages and disadvantages of Friction
- Define friction
- mention two advantages of friction
- list three disadvantages of friction
- Define Soldering and state its types