MACHINES

MACHINES are device that simplifies work.

How machines simplify work.

1. Reducing the force needed to do work.
2. Increasing the speed at which work is done.
3. Changing the direction n which force is applied.

2.      Machines are grouped into  7  groups. These are referred to as simple machines.

Examples of simple machines:-

1. Levers
2. Inclined planes
3. Wedges
4. Screws
5. Wheels and axles
6. Gears
7. Pulleys

LEVERS

1. A lever is a rigid rod moving freely at a fixed point.
2. A fixed point at which the lever moves is called a pivot or fulcrum.
3. In order for a lever to function force is applied. The applied force to the lever is called effort.
4. The objects to be lifted or resistance to be overcome is known as the load.

i.        The side which has the load is known as load arm.

ii.       The side with the effort is known as effort arm.

• The distance from the pivot to the effort is known as effort distance.
• The distance from the pivot to the load is known as load distance.

CLASSES OF LEVERS.

1. Levers are divided into three classes.
2. The classes depend on the position of the load, pivot and effort.
3. If the pivot is in between the load and effort that is a first class lever.
4. If the load is in between the pivot effort it is a second class lever.
5. If the effort is in between the load and pivot it is a third class lever.

(PLE  –  HELPS TO KNOW THE ORDER OF LEVERS.)

FIRST CLASS LEVER

These are leavers whose pivot is between the load and the effort.

Examples of first class lever machines

Advantages of using a first class lever.

1. It changes the direction of effort. i.e  Effort moves in the opposite direction to that of the load.
2. Small effort moves a large load.

NB:    Work done is the same b’se the effort moves a longer distance than the load.

SECOND CLASS LEVER

This is the class of levers in which the load is between the effort and the fulcrum.

THIRD CLASS LEVER.

1. This is the class of levers in which the effort is between the load and the fulcrum.
2. Effort and load move in the same direction.

Illustration.

A fishing rode

USING LEVERS TO MAKE WORK EASY.

1. Levers make work early by using less effort to lift the heavy load.
2. In order to use less effort the effort arm should be longer than the load arm.

Example I

1. When held at  C,  the least effort is used to lift the load.
2. A smaller boy seated at the end of the seesaw will be able to balance with the big girl seated near the pivot.

MOMENTS

1.       A moment is a turning force of a lever.

The Laws of levers/ Moment.

The law of levers states that

1. “For a lever to balance, effort multiplied by the effort arm distance equals to the load, multiplied by the load arm distance, and vise versa.

E x EA = L x LA

OR

L x LA = E x EA

Example I:

If the lever is to balance, find the length of X.

Solution.

E  x  ED        =       L  X  LD

15kg  x  2m  =       10kg  x  X

30                =       10x

10                                   10

X                 =       3m

• The longer the effort arm from the fulcrum, the smaller the effort applied.

L  x  LD        =       E  X  ED

ykg  x  3m    =       5kg  x  6m

3y                =       30

3                           3

y                 =       10

• The shorter the effort arm from the fulcrum, the more effort is applied.

L  x  LD        =       E  X  ED

5kg  x  6m    =       ykg  x  3m

30                =       3y

3                           3

y                 =       10

NOTE:

Compare the effort applied in b and c. Effort applied in c is more than effort applied in b because the effort arm in c is shorter than the effort arm in b. i.e. the longer the effort arm, the smaller the effort applied and the shorter the effort arm, the more effort is applied.

EXERCISE.

Remember to sketch the diagrams.

1.       Sarah sat 3m  from  the pivot at the seesaw. Her husband Musa who is 60kg sat 2m from the pivot. If the seesaw is balancing, what is Sarah’s weight? (Illustrate the diagram to show this information)

2.       Swabura who is  50kg sat on the seesaw and balanced with Tendo who is  70kg  and is seated  5m from the pivot. If the two are balancing, hos far is Swabura from Tendo. Use a diagram to illustrate this information.

3.       Wakida sits 4m away from the pivot of a sea saw and balance with his wife Mudondo on the other side. His wife weighs 90kg and sits 8m away from the pivot.

a) Draw a sketch to illustrate the information.

b) How heavy is Wakida?

4.       Katisi weighs 45 kg and sits fm away from the pivot and Hindu weighs 30 kg and sits 6m away from the pivot and they balance. Find the value of f.

5.       Study the diagram below and find the value of x

EFFICIENCY OF A MACHINE.

1.       Efficiency of a machine is the relationship between work input and work out put.

2.       if work input and output were the the same, the machine would be 100% efficient.

3.       There is no machine which is 100% efficient due to friction.

Efficiency =          workoutput x 100

Work input.

INCLINED PLANES

1.       An inclined plane is a sloping surface which connects a lower level to a higher one.

2.       It helps in raising the load by making the user apply less effort.

3.       The advantage of using an inclined plane is that less force is applied to raise the load.

4.       The disadvantage of using an inclied plane is that the effort moves a longer distance than the load.

WEDGES

1.       A wedge is a tool used for cutting or piercing.

2.       It is also called a double inclined plane.

Uses of wedges

a) For cutting

b) For piercing

c) For splitting wood

SCREWS

A screw is an inclined plane wound around the rod.

Examples of screws.

a) A car jack

b) Bolts and nuts

c) Some bottle lids

d) Some jerican lids

e) Drilling machines.

Uses of screws

a) Screws hold parts of machines together.

b) Screws are use4d for drilling holes.

c) Screws help to tighten lids of tins, bottles, and jericans.

WHEEL AND AXLE

1.       A wheel and axle is composed of two wheels rotating together.

2.       The bigger wheel is the wheel and the smaller one is the axle.

GEAR WHEELS.

1.       A gear wheel is a special wheel with teeth on its rim.

2.       gear wheels transmit motion/movement from one wheel to another.

• Gear wheels move in the opposite direction.
• If gear wheel A has 40 teeth and B has 20 teeth, in one revolution of A, b turns twice.

Uses of gear wheels.

a) They are used in vehicles to multiply on the speed.

b) They are used in machines to multiply effort.

c) They are used to change direction of movement i.e reversing in vehicles, rewinding tapes    in radios.

CONVEYOR BELTS/BELT DRIVES.

1.       A conveyor belt is used to transmit motion from one wheel to another.

2.       Wheels driven by conveyor belt turn in the same direction.

Examples of conveyor belts

a) Bicycle/motorcycle chains.

b) Sewing machine belts.

c) Luggage conveyors

d) Fan belts in car engines.

e) Motor belts in radios, Decks, Grinding mills etc.

PULLEYS

1.      A pulley is a free rotating wheel with a grooved rim.

2.      A rope or chain is passed through the groove and force is applied at one end to over come

• The frame which holds the pulley is called a Block.

Types of pulleys

a) Single fixed pulley.

b) Single movable pulley.

c) Block and Tackle/Fixed movable pulley

Single fixed pulley

1.       A single fixed pulley rotates but does not move.

2.       It is fixed to one side.

Advantage of a single fixed pulley.

It changes the direction of force by pulling down.

Mechanical advantage of a single fixed pulley.

The mechanical advantage of a pulley is one because the effort applied is equal to the load force.

Calculating the M.A. of a single fixed pulley.

Example.

Find the mechanical advantage of a single fixed pulley lifting the load of 45Kgf

Load = 45Kgf,        Effort = 45Kgf.

Effort

=       45Kgf

45Kgf

=       1

Example II

Find the effort force applied on a single fixed pulley to lift a load of 60Kg.

Effort

Effort  x  1             =       60kgf  x Effort

Effort

Effort                    =       60Kgf

EXERCISE

1.       Calculate the effort used to raise aq load of 90Kgf using a single fixed pulley.

2.       Find the load lifted by a single fixed pulley when an effort is of 63N is applied on the effort side.

3.       Calculate the mechanical advantage of a single fixed pulley on which an effort of 45Kgf is applied.

Single movable pulley.

1.       In a single movable pulley, the wheel moves along the rope or chain.

2.       Less force is used in a single movable pulley.

3.       It doesn’t change the direction of the force.

4.       The mechanical advantage of a single movable pulley is two.

5.       The effort applied in the single movable pulley is always half of the load.(1/2 of Load).

a) two sections of the rope/chain give support to the load, so we use less force than the load by half.

Calculating the Mechanical advantage of a single movable pulley.

Example

What force will be needed to raise a load of 50kgf using the  pulley shown below.

Difference between single fixed and single movable pulley

Block and Tackle (Fixed movable pulley)

1.       This has more than one pulley.

2.       It is made up of the fixed and the movable pulleys.

3.       The mechanical advantage of a block and tackle is three.

Calculating the Mechanical advantage of a single movable pulley.

Example:

1.       What force will be required to lift a load of 45kgf using a block and tackle with two wheels?

Effort

Effort x 3      =       45Kgf x E

3 E     =       45kgf

3                 3

Effort          =       15Kgf

EXERCISE

1.       Calculate the mechanical advantage of a fixed movable pulley with two wheels when an effort of 90kgf is applied to lift a load of 270kgf.

2.       What load force will be lifted by a block and tackle pulley if an effort force of 96kgf is applied?

USES OF PULEYS

1.       Pulleys are used in breakdown vehicles to pull stranded vehicles.

2.       They are used in lifts in tall buildings.

• They are used in hoisting the flood.
• They are used in by builders to lift blocks and other building material on top of high buildings.

EFFICIENCY OF MACHINES.

1.       Efficiency of a machine is the relationship between work input and work output.

2.       If two were the same, then the machine would be 100% efficient.

3.       There is no machine on earth which is 100% efficient because of friction.

Efficiency    =       Work output x 100

Work input

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