Lesson Notes By Weeks and Term - Senior Secondary School 2

SUBJECT: PHYSICS

CLASS:  SS 2

DATE:

TERM: 1st TERM

REFERENCE TEXT

• New School Physics by M.W Anyakoha
• SSCE WAEC Past Questions
• UTME Past Questions

 
WEEK NINE AND TEN

TOPIC: CONTENT

• Definition
• Types of Machines
• Mechanical Advantage of Machines
• Velocity Ratio of Machines
• Efficiency of Machines

FOR WEEKS NINE AND TEN

MACHINES 

Machines make our work simpler .It is a force producing device by which a large force called load can be overcome by a small applied force called effort 

Terminologies Used in Machines

1. Force ratio (mechanical advantage)  
2. Velocity ratio 
3. Efficiency

MECHANICAL ADVANTAGE 

We define effort as the force applied to a machine and load as the resistance overcome by the machine. The ability of a machine to overcome a large load through a small effort is known as its mechanical advantage .It is  given by 

M.A = Load/ Effort    = L/E    …………………………. 1 

The mechanical advantage  of a machine  is influenced by  friction  in parts 

VELOCITY RATIO (V.R) 

The  velocity ratio is the ratio of  distance  moved  by  the effort  and load  in  the same  interval 

V.R =  Distance   moved by effort         ……………………….. 2

Distance  moved by the load

The velocity  ratio  depends on the  geometry  of the  machine 

EFFICIENCY (E)

The efficiency of a machine is defined as 

Ef =Usefulwork done by the machine  x  100   …………………………………… 3

       Work put into the machine 

Work = force  x  distance 

Ef=  load x distance moved by load    x 100   ………………………………………….. 4

        Effort x distance moved by effort 

 Then V.R =M.A 

TYPES OF MACHINES

1          LEVER 

This is the simplest form of machine . It  consist  of a rigid rod  pivoted about a  point  called the fulcrum F with a small  effort applied at one end  of the  lever to overcome  a large  load L  at the other end . There are various types of lever depending on the   relative positions of the  load, effort and fulcrum. 

      L

                F

                      E

Examples of first class lever are the  crowbar.pair  of scissors or pincers,  claw hammer , see-saw ,pliers etc

In second order lever , the load is between the fulcrum and effort 

            E

        F

                                           L

Examples are wheel barrow, nut cracker tarp door , an oar etc .

In the third order lever, the effort is between the fulcrum and the load .Human  fore arm ,  laboratory  tong etc.

    E

                          F

                                                        L

WHEEL AND AXLE 

It consists of  a large  wheel  to which a rope or string is attached and an axle  or small wheel  with the rope  or string  wound round  it  in  opposite direction . The load to be lifted is hung at the free end of the rope on the axle  while  the effort  is applied at the end of the rope on the wheel . For each complete  rotation  the load and the effort  move through distance equal  to the  circumference of the wheel and axle  respectively. 

                 V.R = R/r  …………………………………………. 5 

The principle of wheel and axle is used in brace  screw driver but spanner windless and gear-boxes 

In  gear boxes , there  are toothed wheels of different  diameter interlocked  to give turning force  at low speed  depending on which  gear  is the driver  and whih is the driven

V.R = No of teeth on driven wheel ( A)      ……………………………. 6

         No of teeth on driving wheel (B)

belt

                    A                    shaft                   B

THE HYDRAULIC PRESS 

The  machine is  widely used  for  compressing  waste  paper and cotton  into  compact bales  forging different alloys into  desirable shape etc .It s work is based on Pascal’s principle which  states  that  pressure is transmitted equally in fluid Oil is  the liquid  normally  used in hydraulic press 

                        L

       E                                             

                             R    V.R =  R2/r2  ………………… 7

     r

THE WEDGE

The  wedge  is  a combination of two inclined planes >It is used  to separate bodies which  are held together by large  force .Examples of wedge type of machines are axes chisels knives etc

                Xo

θ

                             X1

M.A =  X1 =   Slant height of wedge 

           X0      Thickness of wedge …………………………………………… 8

PULLEY

A simple pulley is a fixed wheel hung on a suitable support with a rope  passing round its groove. For a set of pulley, the velocity ratio is the number of pulley  in the system.

             E                L

BLOCK AND TACKLE (PULLEY)

This is   the  more practical  system  of pulleys in which one or  more  pulley  are mounted on  the same axle  with  one continuous rope  passing  all-round the pulleys 

INCLINED  PLANE : This  is in form of a sloping  plank commonly used to raise heavy load such as  barrels of oil with  little applied effort than  by lifting  it vertically .

   V.R =   Distance moved by force

Distance  moved by load 

 = x /h 

V.R = 1/ sin θ   ……………………………………….. 9

THE SCREW 

Geometrically speaking the screw is an inclined plane wrapped round  a  cylinder to form  a   thread . The distance between successive threads on a screw is called its pitch. For one complete revolution  of  screw  through  an  effort ,  the load  moves a distance equal  to its  pitch  e.g  screw  jack  nut  and bolt 

In a screw  jack where length of the operating  handle  is a , the effort  moved a distance equal to the pitch P. If frictional forces are negligible

Thus V.R= 2πa  = 2πr    …………………….. 10

                    P       P

EFFECTS OF FRICTION ON MACHINE

Work is always wasted in machines to overcome the frictional forces present  between the moving parts and also  to lift  to part of the machine. The  greater  the friction  , the  greater the effort  required and the smaller the M.A  . M.A depends on  friction but  depends  on the geometry of moving parts. 

In practical machines  the efficiency is usually  less than 100% because of friction  in  the moving parts of the machine. 

GENERAL EVALUATION QUESTION

1. A body travels from rest over a distance x in time t. if it has a uniform acceleration a, the value of t expressed in terms of a and x is?
2. A uniform meter rule  of mass 0.42Kg is balanced at the 60cm mark when a mass of m is placed at the 90cm mark. Calculate the value of m.

WEEKEND ASSIGNMENT

1. A machine with a velocity ratio of 30 moves a load of 3000N when an effort of 200N is applied. The efficiency of the machine is (a) 30% (b) 50% (c) 60% (d) 75%.
2. The efficiency of a wheel and axle system is 80% and the ratio of radius of wheel to radius of the axle is 4 : 1, In order to lift a mass of 20kg,the effort required is (a) 60N (b) 62.5N (c) 32.5N (d) 250 (e) 50N.
3. The velocity ratio of an inclined plane whose angle of inclination is Ɵ is 

(a) sinƟ    (b) cosƟ (c) tan Ɵ (d) 1/sin Ɵ (e) 1/cosƟ.

4. Which of the following is not an example of levers of the first order        (a) crow bar (b) Nutcracker (c) scissors (d) pliers (e) claw hammer.
5. A plane inclined at 30о to the horizontal has an efficiency of 50%. Calculate the force parallel to the plane required to push a load of 120N uniformly up the plane.    (a) 50.0N (b) 120.0N (c) 200.0N (d) 240.0N 

THEORY

1. Show that efficiency E, the force ration (MA) and the velocity ratio (VR) of a machine are related by the equation   E = MA/VR X 100.
2. (a) What is meant by a machine?

    (b) Explain why a machine can never be 100% efficient.