Machines SS2 Physics Lesson Note

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 

1. 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 

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

2. Velocity Ratio (V.R)

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

V.R = Distance moved by effort 

         Distance moved by the load

The velocity ratio depends on the geometry of the machine 

3. Efficiency (E)

The efficiency of a machine is defined as:

Ef =Useful work done by the machine.                                 ×  100

Work put into the machine 

Work = force x  distance 

Ef =  load x distance moved by load 

        Effort x distance moved by  effort × 100

Then V.R =M.A 

TYPES OF MACHINES

1. LEVER

This is the simplest form of machine. It consists 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 levers depending on the relative positions of the load, effort and fulcrum. 

Taking a moment about F 

E x a = L x b   which is given   

L =  a  =M.A

E     b

 a/b = V.R 

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

 In the second order lever, the load is between the fulcrum and effort  Examples are a wheelbarrow, nutcracker tarp door, an oar etc. 

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

2. 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 around it in the 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 a distance equal to the circumference of the wheel and axle respectively.  

V.R = R/r

 The principle of wheel and axle is used in brace screwdrivers but spanner windlass and gear-boxes 

3. Gear Wheels

In  gearboxes, there  are toothed wheels of different diameters interlocked  to give turning force  at low speeds depending on which  gear  is the driver  and which is the driven

V.R = No of teeth on the driven wheel

         There are no teeth on the driving wheel                           

4. The Hydraulic Press 

The machine is widely used for compressing waste paper and cotton into compact bales forging different alloys into desirable shapes etc. Its 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 

V.R =  R2/r2

5. The Wedge

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

6. Pulley 

A simple pulley is a fixed wheel hung on a suitable support with a rope passing around its groove. 

i. Block And Tackle (Pulley)

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

EFFECTS OF FRICTION ON MACHINE 

1. 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. 
2. The efficiency of nearly all the machines varies with the load and the load and effort are related by: E = al + b ( a and b  are constant ). This is called linear law for a machine. It  follows  that E is proportional to L 
3. The  value  to give us the effort required to  operate  the  machine  moving part only  if no load is  present  while  A gives us  the measure  of the friction present

                      =  M.A x  100

                           V.R 

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

CLASSWORK

1(a) What is a machine? (b) Explain why a machine can never be 100% efficient.

2. Define the following terms as applicable to the machine (a) velocity ratio (ii) mechanical advantage (iii) efficiency
3. A pulley with a velocity ratio of 5 is used to lift a load of 400N through a vertical height of 8m by exerting an effort of 100N. Calculate the: (a) work done by the effort and (b) efficiency of the pulley system