Machines

ELATE PHYSICS

UNIT 3

TOPIC : Machines

SUBTOPIC : Efficiency of machines

CLASS : Senior Three

NUMBER OF LEARNERS: 50

DURATION: 120 minutes minimum – 240minutes maximum

INTRODUCTION:

Learners must have been introduced to machines and covered the following subtopics; Work, Energy Power, Force (effort, load), Mechanical Advantage (MA) and Velocity Ratio (VR).

Machines are devices that make work easier though not smaller. This is achieved by the application of a force (effort) at one point in order to overcome another (load) at another point.

People and machines cannot do work without a supply of energy. We get energy from the food that we eat. Machines are fed with energy in many other forms e.g. fuels such as coal, oil and gas, solar energy, hydro electricity, nuclear energy etc.

A machine converts the energy supplied into another form. The total amount of energy input the machine is equal to the total amount of energy output. This implies that a machine doesn’t consume energy; however the useful energy or work done is usually less than the total energy input.

The amount of time taken to do a piece of work may vary from machine to machine depending on how powerful the machine is. A faster machine uses more power in doing a piece of work.

ACTIVITY 1:

The teacher will guide the class in reviewing the previously taught areas that relate to machines. Review the following;

• Work, Energy, Power, Force, MA and VR.
• Examples of simple machines
  • Levers
  • Pulleys
  • Inclined plane
  • Hydraulic machine
  • Petro engines.

ACTIVITY 2:

The teacher organizes the class into groups of 5 to 10. Each group will carry out the following assignments.

Record examples of commonly used machines in the following categories;

1. Levers

Machine	Note where the pivot is found	Identify the class of lever
Wheel barrow
Human fore arm
Bottle Opener
Pair of Scissors
Pair of pliers

2. Pulleys

Draw diagrams to show the following pulley systems

• Single fixed
• Single moving
• Block and tackle system of VR=4

3. The inclined plane. A slope or ramp which allows a load to be raised more gradually and by using a smaller force (effort) than if it was lifted vertically upwards.

As we climb up a stair case we use the principle of the inclined plane.

h

Find the formula for the work input and the useful work output.

ACTIVITY 3:

Efficiency = =××100%

=MA××100%

PRACTICAL EXERCISE OF MEASURING THE MA AND EFFICINCY OF A BLOCK AND TACKLE PULLEY SYSTEM AND SHOWING THE VARIATION OF MA WITH LOAD AND EFFICIENCY WITH LOAD

INSTRUCTIONS

1. Assemble the block and tackle system as shown in the diagram above.
2. Suspend known masses between 1.0kg to 6.0kg on the system in turn and record readings of the spring balance for the effort needed to just raise the loads.
3. Work out the MA and efficiency for each load.
4. Record your readings in the table below.

Mass/kg	Load/N	Effort/N	MA=Load/effort	Efficiency
1.0
2.0
3.0
4.0
5.0
6.0

5. Plot graphs of MA against load and efficiency against load.
6. What deductions can you make from the graphs?

ACTIVITY 4

Mr. Kapere is not keen at taking his car for service when the mileage is due. He has always reasoned that “after all the car still moves perfectly well.” In a small note to kapere, advise him as a friend on the disadvantages of continual use of the car without servicing it.

• List down as many causes of inefficiency in machines and state how they can be minimized.
• Prepare a group’s presentation to the whole class.

• inary level Physics by AF Abbott

PHYSICS

SAMPLE LESSON PLAN

DATE	CLASS	SUBJECT	NO.OF LEARNERS	DURATION	TIME
…./…./…	SENIOR THREE	PHYSICS	50	80 MINS	8.00 – 9.20am

Topic: Machines

Subtopic: Efficiency

Time: 80 minutes

Objectives: by the end of this lesson, learners should be able to:

• Define work input, work output, velocity ratio, mechanical advantage and efficiency.
• Derive the relationship between Efficiency, velocity ratio and mechanical advantage.
• Identify the causes of inefficiency in a simple machine such as a pulley system.

Teaching aids:

• Pulleys and pulley strings
• Loads (masses of 100g)
• Inclined plane
• Spring balance

References:

• The world of Physics by John Avison
• Ordinary level Physics by AF Abbott

Time/Minutes	Teacher’s activity	Learners’ activity	Comments
10	Introduces lesson by defining work, energy and power. Finds out what Learners know about work and different types of energy.	Suggest different forms of energy and its transformations from one form to another.
20	Introduces mechanical advantage as the ratio of load to effort Velocity ratio as the ratio of distance moved by effort to that moved by load in the same time. Simple illustrations in form of calculations	May ask questions for clarity Try out the numeric questions given by teacher
15	Shows class how simple pulleys of different velocity ratios may be assembled (demonstration) Asks learners for any observations and conclusions made Responds to learners’ queries.	Make observations and draw conclusions Reactions Make notes
35	The inclined plane. Use of spring balance to find effort that pulls load up the incline. Organizes class into groups and provides necessary apparatus for inclined plane.	Measurement of effort, load, distance by effort and distance covered by load. Calculation of Work input, work output and efficiency for different loads. (group work) Relationship between efficiency and load(graph) Relationship between MA and load.
5	Supplements learners’ observation and conclusions Concludes lesson.

Lesson Evaluation:

------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------