Simple machine

C9 Simple machine

1. Define simple machine with examples.
Ans: The mechanical devices or tools which are use to make our work easier and faster by changing the direction or magnitude of a force applied is known as simple machine. E.g: scissors, nut cracker, screw driver,etc.
2. What is perfect machine? Why there is no perfect machine in practice?
Ans: Ans: The machine that wastes no part of work done on it is called perfect or ideal machine i.e in a perfect machine, no work is wasted and all the input work is converted into output work. Such a machine is frictionless and the efficiency is 100%.

There is no perfect machine in practice because the output of a machine is always less than the input since there is always loss of energy in overcoming the force of friction between moving parts of the machine.

3. Explain the terms mechanical advantage, velocity ratio and efficiency of a machine.
Ans: Mechanical Advantage (MA)= Mechanical Advantage is define as the ratio of load to the effort.

Velocity Ratio (VR)= Velocity ratio is defined as the ratio of distance travelled by effort to the distance travelled by load.

Efficiency=The ratio of output work to the input work expressed in percentage is called efficiency.

4. Derive the relation among mechanical advantage, velocity ratio and efficiency.
Ans:Let us consider a machine overcomes a load L by an effort E. The efficiency of machine is $\eta $ with mechanical advantage MA and velocity ratio VR. The distance moved by load is Ld and distance moved by effort is Ed.By the definition of efficiency:

\[\text{ }\!\!\eta\!\!\text{ =}\frac{\text{Output work}}{\text{Input work}}\text{ }\!\!\times\!\!\text{ 100% }\]\[\text{ }\!\!\eta\!\!\text{ =}\frac{\text{L }\!\!\times\!\!\text{ Ld}}{\text{E }\!\!\times\!\!\text{ Ed}}\text{ }\!\!\times\!\!\text{ 100 %}\]\[\text{ }\!\!\eta\!\!\text{ =}\frac{\text{L}}{\text{E}}\text{ }\!\!\times\!\!\text{ }\frac{\text{Ld}}{\text{Ed}}\text{ }\!\!\times\!\!\text{ 100 %}\]\[\text{ }\!\!\eta\!\!\text{ =}\frac{\text{L}}{\text{E}}\text{ }\!\!\times\!\!\text{ }\frac{\text{1}}{\frac{\text{Ed}}{\text{Ld}}}\text{ }\!\!\times\!\!\text{ 100 %}\]\[\text{ }\!\!\eta\!\!\text{ =MA }\!\!\times\!\!\text{ }\frac{\text{1}}{\text{VR}}\text{ }\!\!\times\!\!\text{ 100% } \!\! [\!\!\text{ }\because \text{ MA=L/E and VR=Ed/Ld }\!\!]\!\!\text{ }\]\[\text{ }\!\!\eta\!\!\text{ =}\frac{\text{MA}}{\text{VR}}\text{ }\!\!\times\!\!\text{ 100%}\]This is the required expression for the relation among MA, VR and $\eta $.
5. Velocity ration is always greater than mechanical advantage in a simple machine, why?

Ans: Velocity ratio indicates the maximum mechanical advantage that can be gained in a simple machine when the machine is frictionless. But is practice, it not possible to have mechanical advantage equal to the velocity ratio. So, the velocity ratio is always greater than the mechanical advantage.
6. What do you mean by mechanical advantage of a machine is 10?
Ans: The mechanical advantage of machine is 10 means load moved by effort is 10 time the effort.
7. What are the factors that affect the mechanical advantage of a machine?
Ans: The factors affecting the mechanical advantage of a machine are:
i.Friction
ii.Weight of a simple machine.
8. Define load and effort.
Ans:Load is a heavy or a bulky object that requires effort to move or apply a force on an object or load is a resistance offered by an object on a machine that is to be overcomed by effort.
Effort is an applied force to bring desired change to the position(push or lift)of the load or effort is defined as the external force applied on the machine to overcome the load.
9. What do you mean by the statement that efficiency of a machine is 90%?
Ans: The efficiency of a machine is 90% means that the work output is 90% of The work Input.This also indicates that 90% of the input work is converted into useful work and the remaining 10% energy is converted into other form energy heat or sound while overcoming the friction and the weight of the machine.
10. Define input work and output work.
Ans:  The work done on a machine by applying some effort is called Input work.
The work done by a machine after applying some effort on it is called Output work.
11. Why output work is always less than the input work?
Ans: The output work of a machine is always less than the input work because there is always loss of energy in overcoming the force of friction between moving parts of the machine due to this the efficiency of a machine is always less than 100%.
12. What is lever? On which principle does it work?
Ans: A lever is a rigid straight or bend bar which is capable of rotating about a fixed point called fulcrum. It works in the principle of moments.
13. Derive MA in case of first class, second class lever and third class lever.

14. Which class of lever always has mechanical advantages less than one? Why?
Ans: Third class lever always has the mechanical advantages less than one because the effort arm is always smaller than load arm which is neither used to change the direction of the force nor to multiply the force but it is used to accelerate the work. Since MA is also equal to the ratio of effort arm to the load arm.
15. Which class of lever always has mechanical advantages more than one? Why?
Ans: Second class lever always has the mechanical advantages more than one because in this lever the effort arm is always greater than the load arm which is neither use to change the direction of the force nor to accelerate the work but is used to multiply the force.
16. Explain three types of lever with figure. Write two examples of each class of lever.
Ans: First Class lever: First class lever is the type of lever in which fulcrum lies in between effort and load. E.g: crow bar, see-saw, scissors, pliers etc.

In first class lever fulcrum can be shifted anywhere between load and effort due to which its mechanical advantage also changes according to the position of fulcrum i.e load arm and effort arm changes according to the position of fulcrum. So, the MA of first class lever ma be less, greater or equal to one.
Second class lever: If the load is in between effort and fulcrum, it is called the second class lever. E.g:  Nut cracker, lemon squeezers, wheel barrow etc.

For second class lever effort arm is always greater than load arm. So, mechanical advantage of second class lever is always greater than one.
Third class lever: If the effort is in between the load and fulcrum, it is called third class lever.

For third class lever, effort arm is always less than load arm, Hence the mechanical advantage of this class of lever is always less then one.
17. Draw a neat diagram of lever with mechanical advantage greater than one.

18. Why the handle of a scissors used to cut metal sheets are made longer?

19. Why the load is kept near the wheel in a wheel barrow?

20. What is pulley?
Ans: A pulley is a metallic or wooden disc with a grooved rim which change the direction of applied force and used to raise heavy weights.
22. Derive MA, VR and Efficiency of two pulley system.

23. Calculate the MA of a single movable pulley with two pulleys.

24. There is no gain in MA of a single fixed pulley, yet the pulley is used in practice. Explain              why?
Ans: There is no gain in MA of a single fixed pulley, yet the pulley is used in practice because of following reason:
i.To raise a load directly upward is difficult, so applying effort in the downward direction can move
   the load upwards conveniently.
ii.Used to change the direction of force applied to a more convenient direction.
25. Draw the diagram of single fixed pulley, single movable pulley and four pulley system.

26. What do you mean by wheel and axle? Give some examples.
Ans: A wheel and axle is a two co-axial cylinders of different radii fitted one into other the large one being called a wheel and the smaller one,the axle. Rope is coiled around both the wheel and the axle in opposite direction. When effort is applied in one end of the rope load is lifted in another end of the rope. E.g. screw driver, door knob, pedal of bicycle, wheel of sewing machine etc.
27. Derive MA, VR and efficiency of wheel and axle.

28. Draw the labelled diagram of wheel and axle.

29. Why wheel and axle is considered as continuous lever?
Ans: The wheel and axle is considered as continuous lever because there is load, effort and fulcrum, it works as a lever.When the wheel and axle is in use, the points of load and effort can vary continuously for 360 degrees on the circumference of the wheel and axle. Hence,it is considered as a continuous lever.
30. Define inclined plane. Give some examples.

31. Draw a labelled diagram of wheel and axle.

32. Why is inclined plane considered as a machine?

33. Why the roads in hilly regions are made longer?

34. Why inclined plane is also called second class lever?

35. Calculate MA, VR and efficiency of inclined plane.

36. Explain why the mechanical advantage of an inclined plane is greater than one.

37. What do you mean by screw? Give some examples.
Ans: Screw is a simple machine that consists of spirally moving inclined plane wrapped around a cylinder.Some examples of screw are bottle caps, the end of light bulbs, jar lids, etc.
38. Define thread and pitch of a screw.

39. Draw the labelled diagram of a screw.

40. Write down VR of a screw.

41. What do you mean by wedge? Give some examples.
Ans: Wedge is a simple machine consisting of a double or single inclined planes.Some examples of wedge are knife, axe, chisel, scissors, etc.
42. Draw the labelled diagram of a wedge.

43. Write down VR of a wedge.