# Selina Concise Class 6 Physics Chapter 4 Simple Machines Solution

## Selina Concise Class 6 Physics Solution Chapter No. 4- ‘Simple Machines’ For ICSE Board Students.

A. Objective Questions

(1) A boy does work while pushing a wall.

Ans. False.

(b) A machine performs work by itself.

Ans. False.

(c) In an ideal machine, work done on load is equal to the work done by effort.

Ans. True.

(d) All levers are force multipliers.

Ans. False.

(e) A pulley changes the diresction of force.

Ans. True.

(f) An inclined plane always has the mechanical advantage more than 1.

Ans. True.

(2) Fill in the blanks:

(a) The useful work done by an actual machine is always is always ______ than the work done on the machine.

Ans. Less.

(b) In class II levers, the _____ is in between fulcrum and ______.

(c) The mechanical advantage of class ____ lever is always less than 1.

Ans. III

(d) A pulley is used to change ______

Ans. the direction of effort.

(e) Mechanical advantage of an inclined plane is always _____

Ans. greater than 1.

(3) Match the following columns:

 Column A Column B (a) Needle (i) class II lever (b) Door knob (ii) inclined plane (c) Ramp (iii) class I lever (d) Lemon crusher (iv) wheel; and axle (e) See-saw (v) wedge

Ans. (a) – (v), (b) – (iv), (c) – (ii), (d) – (i), (e) – (iii).

(4) Select the correct alternatives:

(a) For an ideal machine, the efficiency is

(i) greater than unity

(ii) less than unity

(iii) equal to unity

(iv) depends on the  value of load.

(b) Mechanical advantage of a machine is defined as:

(c) The mechanical advantage of a lever is equal to:

(iii) Load arm + Effort arm

(iv) Load arm – Effort arm

(d) A pulley is used because it

(i) has the mechanical advantage greater than one

(ii) has 100% efficiency

(iii) helps to apply the force in a convenient direction

(iv) requres more effort t6o raise a less load.

(e) Wheel is used with axle because

(i) sliding friction is less than the rolling friction

(ii) rolling friction is less than the sliding friction

(iii) they work as the inclined plane

(iv) they help us to change the direction of force.

### Short / Long Answer Questions Solution:

1) WHEN IS WORK SAID TO BE DONE BY A FORCE?

ANS: Work is said to be done when a force is applied on a object and then moves through a distance in its own direction.

2) WHAT IS ENERGY?

ANS: The ability or capacity to do work is called Energy.

3) WHAT DO YOU UNDERSTAND BY A MACHINE?

ANS: A machine is a device which help us to do work more easily by applying less force and spending less energy. Machines make our work easier to do. Machines have made our lives comfortable and faster.

4) WHAT IS THE PRINCIPLE ON WHICH A MACHINE WORKS?

ANS: Principle of Machine:-  The work output of a machine is equal to the work input.

5) STATE TWO FUNCTIONS OF A MACHINE?

ANS:- Machine has the following functions:-

1.It decreases the magnitude of the force required i.e the effort is less than the load.

Example:- A jack is used to lift a car, a bar is used to push a heavy stone, a spade is used to turn the soil. In these examples the effort needed is much less than the load.

2.Changing the speed of an object:-

While riding a bicycle, a force is applied on pedals which multiplies the speed.

6) NAME SIX SIMPLE MACHINES. GIVE AN EXAMPLE OF EACH MACHINE.

ANS:- The name of the six simple machines and examples are:

1.Screw:- Examples- A screw.

2.The Lever:- Examples- A crow bar,claw hammer etc.

3.The wedge:- Examples- Knife, Axe, Plough, etc.

4.The inclined Plane:- Examples- staircase,hilly roads etc.

5.The Pulley:- Examples- A pulley is used in raising a load.

6.The wheel and axle:- Examples- biocycle pedal, steering wheel of a car.

7) DEFINE THE ‘WORK INPUT’ AND ‘WORK OUTPUT’ IN RELATION TO A MACHINE.

ANS:- Work input is work done on a machine equal to the effort force times the distance through which the forceis applied. Its called work input.

And the Work output is work that is done by a machine equal resistance force times the distance through which the force is applied.For an ideal machine the work output is equal to the work input i.e the efficiency.

8) Explain the term mechanical advantage of a machine.

ANS:-  The mechanical advantage of a machine is the ratio of theb load to  tie effort. And w e will say in other words

9) DEFINE THE TERM EFFICIENCY OF A MACHINE.

ANS:- The ratio of the work done by the machine to the work done on tie machines called efficiency of a machine.

Efficiency= (Output energy/Input energy)

10) WHAT IS AN IDEAL MACHINE?

ANS:-  A machine is that has no part of the work done on the machine is wasted, called  an idal machine. So for an ideal machine, the work output is equal to the work input that is the efficiency of an ideal machine is 1.

11) CAN A MACHINE HAVE AN EFFICIENCY OF 100%? GIVE A REASON TO SUPPORT YOUR ANSWER.

ANS:- NO, Because a machine is always less than 100% as output energy is always less than the input energy , and some energy is lost to overcome the friction.

12) A MACHINE IS 75% EFFICIENT. WHAT DO YOU UNDERSTAND BY THIS STATEMENT.

ANS:- If a machine is 75% efficient that means 75% of the work input to the machine is obtained as the useful work output. The remaining 25% of the work input has been lost in overcoming the friction.

13) WHAT IS A LEVER?

ANS:- A lever is a simple rigid bar that is free to move around a fixed point called the fulcrum. A lever is a simple machine.

(14) 15) WHAT DO YOU MEAN BY THE MECHANICAL ADVANTAGE?

ANS:- The Mechanical advantage of a lever is equal to the ratio of the effort arm to the load arm. This is called the principle of lever.

16) WHICH CLASS OF LEVER HAS THE MECHANICAL ADVANTAGE ALWAYS MORE THAN 1? GIVE AN EXAMPLE.

ANS:- The class 2 lever has the mechanical advantage always more than 1.     Example- Bottle opener,nail cutter etc.

17)WHICH CLASS OF LEVER HAS THE MECHANICAL ADVANTAGE ALWAYS LESS THAN 1?GIVE AN EXAMPLE.

ANS:- The mechanical advantage of class 3 lever is always less than 1.    Example- Knife, forceps etc.

18) GIVE ONE EXAMPLEOF CLASS 1 LEVER IN EACH CASEWHERE THYE MECHANICAL ADVANTAGE IS   1) MORE THAN 1  2) Equal to 1  3) less than 1.

ANS:- CLASS 1 LEVER-

1.More than 1- Load arm of pillers.

1. Equal to 1- A See saw
2. Less than 1- The load arm of a pair of scissors.

19) NAME THE CALSS TO WHICH THE FOLLOWNG LEVERS BELONG:-

a) A pair of scissors- Class 1 lever.

b) A lemon squeezer- Class 2 lever.

c) a nut cracker- Class 2 lever.

d) A pair of sugar tongs- Class 3 lever.

e) A beam balance- Class 1 lever.

f) An oar rowing a boat- Class 1 lever.

g) A wheel barrow- Class 2 lever.

h) A see saw- Class 1 lever.

i) A pair of pillers- Class 1 lever.

j) A cow bar- Class 1 lever

(20) 22) HOW CAN YOU INCREASE THE MECHANICAL ADVANTAGE OF A LEVER?

ANS:-The mechanical advantage of a lever can be increased by increasing the effort arm or reducing the load arm.

23) HOW DOES THE FRICTION AT THE FULCRUM AFFECT THE MECHANICAL ADVANTAGE OF THE LEVER?

ANS:- We know that Friction at the fulcrum affect the mechanical advantage.

24) 25) WHAT IS A PULLEY?

ANS:- Pulley is a flat circular disc with a groove in its edge and a rope passing through the groove. It is capable of rotating around a fixed point passing through its  central axis called axle.

26) WHAT IS THE MECHANICAL ADVANTAGE OF AN IDEAL PULLEY?

ANS:- In an ideal pulley the effort applied is equal to the load to be lifted.

27) THE MECHANICAL ADVANTAGE OF AN ACTUAL PULLEY IS LESS THAN 1. GIVE A REASON. WHAT IS THE JUSTIFICATION FOR USING THE PULLEY THEN?

ANS:- Due to friction the mechanical advantage is less than 1. The reason for using the pulley when its mechanical advantage is equal to 1 or less than 1 is that the pulley allows us to apply the effort downwards  that is in a convenient direction. To raise a load directly upward is difficult. But with the help of a pulley, the effort can be applied in the downward direction to move the load upwards. One can hang on it to make use of his own weight also in order to apply the effort.

(28) 29) WHAT IS AN INCLINED PLANE? WHAT IS ITS USE? GIVE TWO EXAMPLES WHERE IT IS USED?

ANS:-  An inclined plane is a rigid sloping surface over which heavy loads can be raised or lowered to a certain height or depth. The mechanical advantage of an inclined plane is the ratio o the length of the plank to the vertical height of the load raised. Its value is greater than 1. Therefore an inclined plane acts as a force multiplier. Thus it can be used to lift heavy loads.      Example- If a heavy box needs to be loaded on a lorry, it is far easier to push it over an inclined planethan to lift up. Steeper the inclined plane grater will be the effort required to push up the load. Sloping ramps, roads on hills and staircase are all examples of inclined plane.

30) WHAT IS A SCREW? GIVE TWO EXAMPLES.

ANS:- A Screw is a simple machine which appeas like an inclined plane wound around a rod with a pointed tip.    Examples- A drill.

31) WHAT IS WHEEL AND AXLE? GIVE TWO EXAMPLES.

ANS:-The Wheel and Axle is a simple machine with having a wheel and axle. The linear motion of axle is obtained by rotating the wheel so as to reduce friction.

Example- Screw drivers, Water tap etc.

32) HOW DOES A WHEEL HELP IN MOVING THE AXLE?

ANS:- Wheel and axle arrangement consists of two cylinders of different diameters joined together such that if one of  is made to rotate and other also rotates. The axle is a cylindrical rod fixed to the centre of a circular disc like object called the wheel.

33) WHAT IS A WEDGE? GIVE TWO EXAMPLES.

ANS:- A wedge is a double inclined plane such that the two sloping surfaces taper to form either a sharp edge or a pointed edge.

Examples-  A knife, a chisel.

But some special cases the number of inclined planes used can be more than two as well.  In such case the sloping surfaces generally taper to form a either a very sharp or a pointed edge to split. Pins and needles are the examples of pointed wedges. The wedge works on a principle of an inclined plane.

34) NAME THE MACHINE TO WHICH THE FOLLLOWING BELONG:-

1. BEAM BALANCE- A class 1 lever.
2. LEMON CRUSHER- A class of 2 lever.

3.SUGAR TONGS- A class 3 lever.

1. RAMP- An inclined plane.
2. DOOR KNOB- Wheel and axle.
3. Needle- a wedge.

35) WHAT CARE WOULD YOU TAKE TO INCREASE THE LIFE SPAN OF A MACHINE WHICH YOU USE?

ANS:- The procedure is –

1. Machine made of iron should be protected from rust by coating them with paint.
2. Machine should be kept in a clean environment which is free from dust and moisture.
3. When machine is not use should be kept covered to prevent collection of dust on them.

36) a) A wheel barrow is a lever of class 1- Incorect.

b)The efficiency of a machine is always 100%- Incorrect.

1. c) Friction is moving parts of a machine reduces its efficiency- Correct.
2. d) No lever has the mechanical advantage greater than 1- Incorrect.
3. e) It is easier to lift a load vertically up than to push it along an inclined plane- Incorrect.
4. f) A screw is made by two inclined planes polaced together- Incorrect.
Updated: April 9, 2020 — 4:33 pm