# Chapter Unit - 1 : Force, Work, Energy and Power - Frank Solutions for Class 10 Physics ICSE

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## Chapter Unit - 1 - Force, Work, Energy and Power Excercise 47

Question 1

Work is the application of a ....... through a distance.

Solution 1

Force

Question 2

A boy does work when he pushes against a brick wall. (yes/no).

Solution 2

Yes

Question 3

What is the SI unit of work?

Solution 3

Joule

Question 4

Nm is the unit of .......

Solution 4

Work

Question 5

One joule is the amount of work done when a force of ......... moves a body through a distance of .......

Solution 5

1N, 1m in its own direction

Question 6

What is the work done when no net force is applied on the body?

Solution 6

When no net force is applied, the work done which is the dot product of force and displacement is zero.

## Chapter 1 - Force, Work, Energy and Power Excercise 48

Question 1
What is the work done when the object acted upon by a force remains at rest?
Solution 1
The work done is zero because the displacement is zero.
Question 2
What is the work done when the force on the object and the displacement of the object are perpendicular to each other?
Solution 2 Question 3
Is work done a scalar or a vector physical quantity?
Solution 3
Work is a scalar quantity because it is a measure of transfer of energy without indicating any direction.
Question 4
What is the work done when the displacement of the body is in the opposite direction to that of the applied force?
Solution 4 Question 5
What is the work done by the gravitational force of the earth on a satellite revolving around the earth?
Solution 5
The work done by the gravitational force of the earth on a satellite revolving around the earth is zero because the motion of the satellite is perpendicular to the force at every point.
Question 6
What is the work when a load of 500 kg is lifted vertically by 5 m? Given, g = 10 ms-2. Express your answer in kilo joule.
Solution 6 Question 7
A man lifts a mass of 10 kg from the floor to a shelf 4 meter high. If g = 10 ms-2, what is the work done?
Solution 7 Question 8
What is the work done against gravity when a body is moved horizontally along a frictionless surface?
Solution 8
The work done against gravity is zero when a body is moved horizontally along a frictionless surface because the force of gravity is perpendicular to the displacement in this case.
Question 9
What do you mean by the term 'work'?
Solution 9
'Work' is said to be done when the applied force makes the body move i.e., there is a displacement of body.
It is equal to the product of force and the displacement of the point of application of the force in the direction of force.
Question 10
What are the quantities on which the work done depends?
Solution 10
Work done depends upon:
(i) the magnitude and direction of the applied force, and
(ii) the displacement it produces.
Question 11
When you move upstairs, do you perform some work?
Solution 11
Yes, we perform work against gravity.
Question 12
What should be the angle between the directions of the displacement and the applied force so that the work done is zero?
Solution 12
The angle should be 90o.
Question 13
Why is the work done on an object moving along a circular path zero?
Solution 13
This is because at each point of the circular path, the displacement is perpendicular to the force, which is directed towards the centre, along the radius.
Question 14

In which one of the following cases is the work done more?  When the angle between the direction of motion and that of the force is (i) 90°     (ii) 0°.

Solution 14

When the angle between the direction of motion and that of the force is 90°;
W = Fd cos 90° = 0
When the angle between the direction of motion and that of the force is 0°;
W = Fd cos 0° = Fd
Hence in the second case, when the angle is 0°; the work done is more.

Question 15

A man carrying a suitcase in his hand is walking horizontally. What is the work done against gravity?

Solution 15

The displacement of the man and suitcase is along the horizontal direction. Thus, the angle between the displacement and the force of gravity is 90°;
Thus, W = Fd cos 90° = 0
Hence, no work is done against gravity in this case.

Question 16
What is the work done on a body by the gravitational force towards the centre of the path acting when it moves along a circular path?
Solution 16
When a body moves along a circular path, work done by the gravitational force towards the centre of the path is zero, because the displacement in this case is normal to the gravitational force.
Question 17
What is the work done on the earth by the gravitational force of the sun during its motion around the sun?
Solution 17
The work done by the gravitational force of the sun on earth during its motion around the sun is zero because at every point, the displacement of earth is perpendicular to the gravitational force of sun i.e.,
W = Fd cos 900 = 0
Question 18
What do you mean by a kilo joule?
Solution 18
A kilojoule of work is said to done when a force of 1 newton displaces a body through 1000 metres in its own direction.
1 kJ = 103 joules
Question 19
How many joules are there in 1 mega joule?
Solution 19
1 MJ = 106 joules
Question 20
Define 'joule'.
Solution 20
The SI unit of work is joule.
1 joule of work is said to be done when a force of 1 newton displaces a body through 1 metre in its own direction.
Question 21
What is the ratio of SI units to CGS units of 'work'?
Solution 21
The SI unit of work is 'joules' and the CGS unit is 'erg'.
1 joule = 107 erg
Thus the ratio is 107: 1
Question 22
An engine does 54,000 J of work by exerting a force of 6000 N on it. What is the displacement in the direction of the force?
Solution 22 Question 23
How much force is applied on a body when 150 J of work is done in displacing the body through a distance of 10 m in the direction of the force?
Solution 23 Question 24

Work done by a force is equal to the product of ....... and ....

Solution 24

Applied force, displacement in the direction of the applied force.

Question 25
Give two examples of work done.
Solution 25
Examples of work done:
(i) In free fall of a body of mass m, under gravity from a height h, the force of gravity (F=mg) is in the direction of displacement (=h) and the work done by the gravity is mgh.
(ii) A coolie lifting a load does work against gravity.
Question 26
On what factors does the work done by a force depend?
Solution 26
Work done depends upon:
(i) the magnitude and direction of the applied force, and
(ii) the displacement it produces.
Question 27
Write an expression for work done by a force depends?
Solution 27 Question 28
Write an expression for the work done against gravity.
Solution 28
Work done against gravity = mass x acceleration due to gravity x height
Or, W = mgh

Question 29
No work is done by a man moving on a horizontal road while carrying a box on his head. Explain.
Solution 29
The displacement of the man and box is along the horizontal direction. Thus, the angle between the displacement and the force of gravity is 900;
Thus, W = Fd cos 900 = 0
Hence, no work is done against gravity in this case; however some work is done against friction.
Question 30
Is power a scalar quantity?
Solution 30
Yes, power is a scalar quantity.
Question 31
Can every force produce work?
Solution 31
No, every force cannot produce work. Force can produce work if the applied force cause displacement in the direction of the force.
Question 32
Distinguish between work and power.
Solution 32
Work is said to be done only when the applied force on a body makes the body move but power is the rate of doing work.
The SI unit of work is 'joules' and that of power is 'watt'.
Question 33

Complete the following sentences:
(a)    The SI unit of work is ....... and of power is .......

(b)    Kilowatt is the unit of ....... and kWh is the unit of

(c)    Joule is the unit of .......

(d)    1 J = ........ Erg.

(e)    1 H.P. = ........ W.

Solution 33

(a) joule, watt
(b) power, energy
(c) work
(d) 107
(e) 746

Question 34
A weight lifted a load of 200 kgf to a height of 2.5 m in 5 s. calculate: (i) the work done, and (ii) the power developed by him. Take g= 10 N kg-1.
Solution 34 ## Chapter 1 - Force, Work, Energy and Power Excercise 49

Question 1
A machine raises a load of 750 N through a height of 16 m in 5s. calculate:
(i)    work done by machine,
(ii)    power at which the machine works.
Solution 1 Question 2
Name the physical quantity whose MKS units are kgm2s-3.
Solution 2
Power
Question 3
A boy of mass m climbs up a staircase of vertical height h. What is the work done by the boy against the force of gravity? What would have been the work done if he uses a lift in climbing the same vertical height?
Solution 3
Work done depends upon the vertical height and not the path taken, hence if the boy uses a lift to reach the same vertical height, work done will be mgh.
Question 4
Can work done be zero even if force acts on the body?
Solution 4
Yes, for e.g. if you push a wall, you apply force on it but no work is done since the displacement is zero.
Question 5
What is the relation between 1 H.P. and 1 kW?
Solution 5
1 H.P. = 0.746 kW
Question 6
It takes 20 s for A to climb up the stairs, while B does the same in 15 s. compare the (i) work done, and (ii) power developed by A and B.
Solution 6 Question 7
State and define the SI unit of power.
Solution 7 Question 8
Write the SI and CGS units of power. How are they related?
Solution 8 ## Chapter 1 - Force, Work, Energy and Power Excercise 72

Question 1
What do you mean by a balanced force?
Solution 1
When the resultant of a group of forces acting on the same object is zero, the forces are said to be balanced. Balanced forces do not change the speed of stationary objects. They may deform objects.
Question 2
Is force a vector quantity?
Solution 2
Yes, force is a vector quantity.
Question 3
How is the unit of force kgf related to newton?
Solution 3
1 kgf = force due to gravity on 1 kg mass
= 1 kg mass x acceleration due to gravity g in ms-2
= g newton
1 kgf = 9.8 newton
Question 4
What is the ratio of SI to CGS units of force?
Solution 4
The SI unit of force is 'newton'.
In CGS system, the unit of force is 'dyne'.
1 newton = 105 dyne
Therefore, ratio of SI to CGS unit of force is 105 : 1.
Question 5
Is weight a force?
Solution 5
Yes, weight is a force.
Question 6
Give one example where a force causes motion of a body.
Solution 6
When we kick a football at rest, it starts moving.
Question 7
Give one example of a force producing change in size of the body.
Solution 7
When a balloon is inflated the force of air inside changes it shape or size.
Question 8
State the main effects that a force can produce. Give one example of each effect.
Solution 8
Effects a force can produce and examples:
1. Change the state of rest; e.g. pushing a door to open it or close it.
2. Change the state of motion; e.g. applying a force to stop the cricket ball.
3. Change the direction of motion and not speed; e.g. when a force is applied to move a body in a circular path with uniform speed there is only a change in direction of motion but speed remains constant.
4. Change both speed and direction of motion; e.g. when a body is swirled in the vertical circle its direction of motion and speed changes at every point.
5. Change the dimension; when a balloon is inflated the force of air inside changes its shape or size.
Question 9
Is 1 gf cm = 9.8 Nm?
Solution 9
No
Question 10
Is 1 Nm = 10-7 dyne cm?
Solution 10
No
Question 11
What do you mean by the turning effect of a force?
Solution 11
The turning effect produced by a force on a rigid body about a point, pivot or fulcrum is called the moment of force or torque. It is measured by the product of force and the perpendicular distance of the pivot from the line of action of force.
Moment of a force = Force x perpendicular distance of the pivot from the force.
Its SI unit is newton-metre (Nm).
Question 12
Name the physical quantity whose SI unit is Nm.
Solution 12
The physical quantity is 'torque'.
Question 13
What is meant by a torque?
Solution 13
The turning effect produced by a force on a rigid body about a point, pivot or fulcrum is called the moment of force or torque. It is measured by the product of force and the perpendicular distance of the pivot from the line of action of force.
Question 14
A body is pivoted at a point. A force of 10 N is applied at a distance of 30 cm from the pivot. Find the moment of force about the pivot.
Solution 14 Question 15
Name the SI and CGS units of couple.
Solution 15
SI unit of couple = newton
CGS unit of couple = dyne
Question 16
Is moment of a force a scalar quantity?
Solution 16
No, the moment of force is a vector quantity.
Question 17
Is it easier to turn a steering wheel of large diameter than that of small diameter? Why?
Solution 17
A larger diameter provides a greater torque (= force x perpendicular distance); hence, it is easier to turn a steering wheel of a large diameter than that of a small diameter.
Question 18
Define the centre of gravity of a body.
Solution 18
The point through which the resultant of the weights of all the particles of the body acts is called its centre of gravity
Question 19
What is meant by equilibrium?
Solution 19
A body is said to be in equilibrium under the action of a number of forces, if the forces are not able to produce any change in the state of rest or of uniform motion or uniform rotation.
Question 20
What is meant by the principle of moments?
Solution 20
Principle of moments: If a body is in equilibrium under the action of number of force, then the sum of clockwise moments is equal to the sum of anticlockwise moments.
Question 21
What is the difference between mass and weight?
Solution 21 Question 22
The moment of a force of 20 N about a fixed point 0 is 10 Nm. Calculate the distance of the point 0 from the line of action of the force.
Solution 22 Question 23
What is meant by the term 'center of gravity of a body'?
Solution 23
The point through which the resultant of the weights of all the particles of the body acts is called its centre of gravity
Question 24
Where is the centre of gravity of a uniform ring situated?
Solution 24
The centre of gravity of a uniform ring is situated at the centre of the ring.
Question 25
State the factors on which the centre of gravity of a body depends.
Solution 25
The centre of gravity of a body depends upon:
(i) Body's weight
(ii) Body's shape
Question 26
Can the centre of gravity be situated outside the material of the body? Give an example.
Solution 26
Yes, the centre of gravity of a body can be outside it. The CG of a uniform ring is at its centre, a point which is not on the body.
Question 27
Fig. 1 shows five pieces of cardboard of uniform thickness cut into different shapes. On each diagram draw two lines to indicate the position of centre of gravity G. Solution 27 Question 28
What is meant by a centripetal force? Is it same as centrifugal force?
Solution 28
Centripetal force: Whenever a body is moving in a circular path with a uniform speed, its velocity is continuously changing due to change in its direction. The body thus possesses acceleration and this acceleration is called centripetal acceleration. The force which produces this acceleration is called centripetal force. It acts along the radius towards the centre of the circular path.
It is not the same as the centrifugal force.
Question 29
Can force be used to change the size and shape of the body? Give an example.
Solution 29
Yes, force can be used to change the size and shape of the body.
Example: On squeezing toothpaste tube, its size as well as shape changes.
Question 30
State the general characteristics of non-contact forces.
Solution 30
Characteristics of non-contact forces:
(i) Forces at distance are equal and opposite.
(ii) Depend upon the distance between the two objects.
(iii) Depend upon the medium between the two objects for electrical and magnetic forces but not gravitational forces.
Question 31
How can force start or stop the motion of a body? Give two examples in each case.
Solution 31
Examples where force can start the motion of a body:
(i) The pulling of a cart by a rope.
(ii) Pushing a door to open it.
Examples where force can stop the motion of a body:
(i) Applying a force to stop a cricket ball.
(ii) Applying the brakes of car to stop it.
Question 32
Name the physical quantity whose unit is kgfm. Define it.
Solution 32
The physical quantity is 'torque'.
Torque may be defines as the turning effect produced by a force on a rigid body about a point, pivot or fulcrum. It is measured by the product of force and the perpendicular distance of the pivot from the line of action of force.
Question 33
What do you mean by a couple? Write its SI unit.
Solution 33
Two equal and opposite parallel forces acting along different lines on a body constitute a couple. Its SI unit is 'newton'.
Question 34
What is meant by the turning effect of force? Give two examples.
Solution 34
The turning effect produced by a force on a rigid body about a point, pivot or fulcrum is called the moment of force or torque. It is measured by the product of force and the perpendicular distance of the pivot from the line of action of force.
Examples of turning effect of force:
(i) Turning a steering wheel
(ii) Tightening a cap
Question 35
What is meant by the state of equilibrium of a body?
Solution 35
A body is said to be in equilibrium under the action of a number of forces, if the forces are not able to produce any change in the state of rest or of uniform motion or uniform rotation.
Question 36
What is the difference between static and dynamic equilibrium?
Solution 36
Equilibrium in any case requires the ? forces acting on an object = 0, i.e. that there is
Fnet = 0.
Static equilibrium is the situation where the object upon which the forces act is no moving.
The object is "static" hence the state of equilibrium gets its name.
Dynamic equilibrium is the situation where an object is in constant velocity motion.
{This object can't experience an acceleration which means Fnet >0}

Question 37
State the conditions of a stable equilibrium of a body.
Solution 37
When the centre of gravity is nearer to the base of a body, the body is in stable equilibrium.
Conditions for stable equilibrium:
(a) The body should have a broad base.
(b) Centre of gravity of the body should be as low as possible.
(c) Vertical line drawn from the centre of gravity should fall within the base of the support.
Question 38
A body is acted upon by two forces, each of magnitude F, but in opposite directions. State the effect of the forces when
(a) Both forces act at the same point of the body.
(b) The two forces act at two different points of the body at a separation d.
Solution 38
(a) If both the forces act at the same point of the body, they have the same line of action, and then the moment becomes zero.
(b) If both the forces act at two different points of the body at a separation d then they constitute a torque whose value is given F x d.

## Chapter 1 - Force, Work, Energy and Power Excercise 73

Question 1
A wheel of diameter 3 m is shown in fig. 2 with axle at 0. A force F = 8 N is applied at Q in the direction shown in figure. Calculate the moment of force about:
(i)    centre o, and
(ii)    point p. Solution 1 Question 2
Give one example in each case where:
(i)    the force is of contact, and
(ii)    the force is at a distance.
Solution 2
(i) Pulling of a cart.
(ii) A ball falls down when it is dropped from a height
Question 3
Calculate the resultant moment of forces about O and state its direction in fig. 3. Solution 3 Question 4
State the principle of moments. A meter scale is pivoted at 30 cm mark and it is in equilibrium when a mass of 40 g is suspended from 10 cm mark. Calculate the mass of the ruler.
Solution 4 Question 5
A meter scale is provided at its mid-point when the various masses are suspended on it as shown in fig. 4. from which point will you suspend a 50 g mass in order to keep the ruler in equilibrium? Solution 5 Question 6
Fig. 5 shows a uniform meter scale weighing 200 gf. Provided at its centre. Two weights 300 gf and 500 gf are suspended from the ruler as shown in the diagram. Calculate the resultant torque of the ruler and hence calculate the distance from mid-point where a 100 gf should be suspended to balance the meter scale. Solution 6 Question 7
A meter scale is pivoted at its mid point and a 50 g mass suspended from the 20 cm mark. What mass balances the ruler when suspended from 65 cm mark?
Solution 7 Question 8
What do you mean by the state of equilibrium? What are the conditions for stable equilibrium?
Solution 8
A body is said to be in equilibrium under the action of a number of forces, if the forces are not able to produce any change in the state of rest or of uniform motion or uniform rotation.
Conditions for stable equilibrium:
(a) The body should have a broad base.
(b) Centre of gravity of the body should be as low as possible.
(c) Vertical line drawn from the centre of gravity should fall within the base of the support.
Question 9
Fig. 6 shows the dimensions of an acute angled triangle. By geometrical construction mark the C.G. of the triangle. Solution 9 Question 10
A right-angled triangle cardboard piece is placed as shown in fig. 7. Redraw the diagram showing the relative position of the vertices of the triangle when it is suspended by a pin from the hole A. Explain why the position changes? Solution 10 Question 11
Give scientific reasons for the following:
(i) It is easier to push a boy standing on one leg than on both legs.
(ii) When a man climbs a slope he bends forward.
(iii) There are chances of toppling when a truck takes a sharp turn   especially when it is not fully loaded.
(iv) A man runs in the direction of train while getting down from a moving train.
(v) Passengers in a bus are pushed backward when it starts suddenly.
Solution 11
(i) We keep our body balanced on two feet by keeping the center of gravity of our body between our feet. It acts normal to the sea level vertically downwards. If COG goes out we fall or we get unbalanced.
A boy standing on both legs has his COG in balanced position and is thus in stable equilibrium but a boy standing on one leg has his COG in unbalanced position which makes him quite unstable and hence it is easier to push him.
(ii) A man bends forward in order to keep himself in a stable equilibrium while climbing up a slope. By bending forward he increases the base of the support, so that the vertical line passing through his centre of gravity may still fall within the base.
(iii) When a truck is not fully loaded, its COG is at a high point and hence the turning moment of the weight is much greater, thus, the truck will be quite unstable and there are chances of toppling, when a truck takes a sharp turn.
(iv) When a man gets down from a moving train, his feet come to rest immediately, while the upper part of his body due to inertia of motion still remains in motion and consequently he leans in forward direction. The person while getting down of a train should run forward in the direction of the moving train to avoid fall.
(v) This is due to the fact that the body of the passenger is in the state of rest as long as the bus is at rest. When the bus starts, his feet acquire the velocity of the bus and come to motion with the moving bus, while the upper portion of his body due to inertia of rest tends to remain in the state of rest, resulting in his tendency to fall backwards.

## Chapter 1 - Force, Work, Energy and Power Excercise 74

Question 1
What are the different methods by which you can increase the stability of a body?
Solution 1
To increase the stability of a body, its base should be made broad and heavy, and the centre of gravity of the body should be lowered.
Question 2
(i) What do you understand by the term couple of forces?
(ii) Calculate the moment of a couple shown in fig. 8. Solution 2
(i) Two equal and opposite parallel forces acting along different lines on a body constitute a couple.(ii) Question 3
Give scientific reasons for the following:
(i) Even though the Tower of Pisa is leaning through an angle it does not fall.
(ii) While climbing a hill you will try to bend your body forward.
(iii) In a moving bus the standing passenger stands keeping both his legs apart.
(iv) In a doubled decker bus passengers are not allowed to stand in the upper deck.
Solution 3
(i) Leaning tower of Pisa is stable because a line through the centre of gravity falls within the structure's base. If the line falls outside the structure's base then there is a possibility that overturning will occur. This structure could be classified as unstable.
(ii) We bend forward in order to keep ourselves in a stable equilibrium while climbing up a hill. By bending forward we increases the base of the support, so that the vertical line passing through our centre of gravity still falls within the base.
(iii) By keeping the legs apart, the base of the body broadens, thus the C.G. lowers and the body attains a more stable equilibrium.
(iv) Passengers are usually advised not to stand in the upper deck of the double deck bus. When the passengers are standing, the C.G. rises. This decreases the stability of the bus. When the passengers are sitting, the C.G. gets lowered and stability of the bus increases.
Question 4
Three forces A, B and C are acting on a rigid body which can turn about O in fig.9. If all the three forces are applied simultaneously, in which direction will the body move? Explain. Solution 4 Question 5
Fig. 10 shows a uniform meter scale weighing 100 N pivoted at its centre. Two weights of 500 N and 300 N are hung from the ruler as shown in fig. 10. (i)    Calculate total clockwise and anticlockwise moments.
(ii)    Calculate difference in clockwise moment and anticlockwise moment.
(iii)    Calculate the distance from O where a 100 N weight should be suspended to balance the meter scale.
Solution 5 Question 6
A meter scale is provided at 10 cm mark and is balanced by suspending 400 g from 0 cm mark (fig. 11). Calculate the mass of meter scale. Solution 6 Question 7
(a) Define the term work.
(b) Name the CGS and SI unit of work.
Solution 7
'Work' is said to be done when the applied force makes the body move i.e., there is a displacement of body.
It is equal to the product of force and the displacement of the point of application of the force in the direction of force.
The SI unit of work is 'joules' and the CGS unit is 'erg'.
Question 8
The following are some of the energy transformations.
A. Electrical to light
B. Work to heat
C. Chemical to light
D. Electrical to sound
E. Mechanical to electrical
Identify the energy transformation that takes place in the following by inserting the corresponding letter in the shape provided.
(i)    A candle flame
(ii)    A torch is lighted
(iii)    A microphone is used in a meeting
(iv)    A cycle dynamo
(v)    A piece of metal is being filed
Solution 8
(i) [C]
(ii) [A]
(iii) [D]
(iv) [E]
(v) [B]

Question 9
A boy pulls a box up to 10 m with a force of 5 kgf. Calculate the work done by him.
Solution 9 Question 10
Calculate the amount of work done by a child carrying a bag of 20 kg when he moves a distance of 40 m in
(a) Vertical direction, and
(b) Horizontal direction
Solution 10 Question 11
(a) Define power and name its unit.
(b) A girl weighting 50 kg climbs up 60 steps each of 20 cm height in 5 minutes. Calculate the power developed.
Solution 11 Question 12
Define energy and name its unit.
Solution 12
The energy of a body is its capacity to do work.
The SI unit of work is 'joules' and the CGS unit is 'erg'.
Question 13
When an elevator starts to move down suddenly we experience 'weightlessness'. Explain.
Solution 13
When an elevator begins to move downwards in an accelerated mode, the forces acting on the body are the following:
a.Weight of the body acting downwards
b.Normal reaction of the floor acting upwards
c.The centrifugal force acting on the body, acting upwards.
Weight of the body is due to gravitational force on the body acting downwards.
Normal reaction is the force that is exerted by the elevator floor in response to the force with which the body presses itself against the floor.
The centrifugal force here  is fictious force that acts on the body in the direction opposite to the acceleration of the reference frame, here it is, the elevator floor. It is given by ma, where m is the mass of the body & a is the accelaration of the elevator floor. Centrifugal force is directed opposite to the acceleration of the elevator floor.
Weightlessness is the condition of the zero apparent weight.
When the acceleration of the elevator is such that the upward centrifugal force  Fc completely balances the downward weight Wt. of the body, the resultant normal reaction (N =Fc - Wt.) of the body is reduce dto zero. That's whene the body on the elevator floor will experience the state of weightlessness.
Question 14
A block of mass 20 kg is pulled up a slope (fig.12) with a constant speed by applying a force of 500 N parallel to the slope. A and B are initial and final positions of the block.
(a)    Calculate the work done by the force in moving the block from A and B.
(b)    Calculate the potential energy gained by the block. Solution 14 ## Chapter 1 - Force, Work, Energy and Power Excercise 75

Question 1
How fast should a boy weighting 30 kg run so that his kinetic energy is 375 joule?
Solution 1 Question 2
A girl of mass 50 kg runs up a flight of 40 steps in 20 seconds. If each step in 20 cm high, calculate the power developed by the girl. (g= 10 ms-2).
Solution 2 Question 3
A water pump raises 80 kg of water through a height of 20 m in 10 s. Calculate the power of the pump.
Solution 3 Question 4
The truck has to apply a force of 3000 N to overcome friction while moving with a uniform speed of 36 kmhr-1. What is the power developed by the truck?
Solution 4 Question 5
Define energy and state the unit of energy and the law of conservation of energy.
Solution 5
The energy of a body is its capacity to do work.
The SI unit of work is 'joules' and the CGS unit is 'erg'.
According to the law of conservation of energy, energy can neither be created nor be destroyed but can be transformed from one form to another. In other words, energy can be transformed from one form to another but the total amount of all the energies remain the same.
Question 6
List any six forms of energy and write a short note on each of them.
Solution 6
Six forms of energy:
1. Solar energy: The energy radiated by the sun is called the solar energy. Inside the sun, energy is produced by nuclear fusion reaction. Solar energy cannot be used to do work directly, because it is too diffused and is not always uniformly available. However, a number of devices such as solar panels, solar cells etc. have been invented to make use of solar energy.
2. Heat energy: The energy released on burning coal, oil, wood or gas is the heat energy. The stem possesses heat energy it has capacity to do work.
3. Light energy: It is the form of energy in presence of which other objects are seen. The natural source of light energy is sun. Many other sources of heat energy also give light energy.
4. Chemical or fuel energy: The energy possessed by fossil fuels such as coal, petroleum and natural gas is called chemical energy or fuel energy. These fuels are formed from the decayed remains of dead plants and animals that lived millions of years ago. In the interior of earth, due to high pressure and temperature the remains slowly changed into fossil fuels.
5. Hydro energy: The energy possessed by fast moving water is called the hydro energy. This energy is used to generate electricity in hydroelectric power stations. For this, dams are built across the rivers high up in the hills to store water. Water is allowed to run down the pipes and the energy of running water is used to turn a turbine. The turbine drives generators to produce electrical energy.
6. Nuclear energy: The energy released during the processes of nuclear fission and fusion is called nuclear (or atomic) energy. In both these processes, there is loss in mass which converts into energy in accordance with Einstein's mass-energy relation, E =mc2.
Question 7
What do you understand by 'potential energy' and 'Kinetic energy'? Give three examples of each to illustrate your answer.
Solution 7
The energy possessed by a body by virtue of its position, shape or change of configuration is called potential energy.
Examples of potential energy:
(i) Water stored at a height in a reservoir.
(ii) A stretched spring.
(iii) A bent bow.
The energy possessed by a body by virtue of its motion is called kinetic energy.
Examples of kinetic energy:
(i) Air in motion has kinetic energy.
(ii) A swinging pendulum.
(iii) Moving hands of a clock.
Question 8
A bullet is of mass 'm' g and is moving with a velocity 'v' m/s. Find the kinetic energy of the bullet when
(a) The mass is doubled,
(b) The velocity is tripled.
Solution 8 Question 9
A block of iron of mass 400 kg is used as a pile driver. If it is raised to a height of 20 meters, calculate the potential energy possessed by the iron block. (Assume g= 10 ms-2).
Solution 9 Question 10
Identify the type of energy possessed by the body in each of the following:
(a) A coiled spring of the toy car
(b) A hammer which is raised
(c) A stone shot from a catapult
(d) Water stored in the overhead tank
(e) A tadpole moving in water.
Solution 10
(a) Potential energy
(b) Potential energy
(c) Kinetic energy
(d) Potential energy
(e) Kinetic energy
Question 11
Fill in the respective boxes (Fig. 13) to show the corresponding energy transformation. Solution 11 Question 12
State the energy changes which take place when
(a) Water stored in the dam is used to turn turbine in dynamo.
(b) An electric bulb glows when it is connected to a source of electric current.
(c) A piece of magnesium wire is burnt in a jar of oxygen.
(d) A stone dropped from the top of a cliff reaches the ground after some time.
(e) A toy car with a wound spring moves on ground.
Solution 12 (e) P.E (Wound spring) to K.E. (motion)

## Chapter 1 - Force, Work, Energy and Power Excercise 76

Question 1
Give one example of each of the following:
(a) Electrical energy changes of sound energy.
(b) Chemical energy changes to heat energy.
(c) Chemical energy changes to electrical energy.
(d) Light energy changes to electrical energy.
(e) Electrical energy changes to heat energy.
Solution 1
(a) Electric bell
(b) Candle flame
(c) Dry cell
(d) Solar cell
(e) Electric iron
Question 2
Find the kinetic energy of a car of mass 1000 kg traveling at 72 km/h.
Solution 2 Question 3
A bullet of mass 25 g has a velocity of 600 ms-1. What is the kinetic energy of the bullet? If it penetrates 50 cm into a target, find the resistive force offered by the target.
Solution 3 Question 4
A lead bullet moving at 70 ms-1 is brought to rest on hitting a target. If 80% of its energy is converted into heat energy, find the rise in temperature (Sp. Heat cap. Of lead is 140 J/kgK).
Solution 4 Question 5
If 60% of the potential energy available in a waterfall is converted into heat energy, find the height of the waterfall, when the temperature difference between the top and the bottom of the fall is 0.210C (sp. Heat cap. Of water = 4200J/kgK).
Solution 5 Question 6
Define the following:
(a) Simple machine           (b) Lever
(c) Mechanical advantage    (d) Velocity ratio
(e) Efficiency
Solution 6
(a) Simple machine: A machine is a device by which we can either overcome a large resistive force at some point by applying a small force at a convenient point and in a desired direction or by which we can obtain a gain in speed.
(b) Lever: A lever is a rigid, straight or bent bar which is capable of turning about a fixed axis.
(c) Mechanical advantage (M.A.): The ratio of the load to the effort is called the mechanical advantage of the machine.
(d) Velocity ratio (V.R.): The ratio of the velocity of effort to the velocity of load is called the velocity ratio of the machine.
It is also defined as the ratio of the displacement of effort to the displacement of load.
(e) Efficiency: Efficiency of a machine is the ratio of the useful work done by the machine to the work put into the machine by the effort. In other words, it is the ratio of the work output to the work input.
Question 7

Fig. shows a spade. It is being used to lift soil weighing 30N from the ground. (a)    Mark the direction of the least force on the handle necessary to keep the spade balanced.
(b)    Calculate the least force on the handle necessary to keep the spade balanced (the weight of the spade is negligible).
(c)    If the left hand was move towards the soil on the spade, would the force on the handle necessary to keep the soil balance the greater or less? Give a reason for your answer.
(d)    To which class of lever does the spade belong?

Solution 7 Question 8
A crowbar 4 m long has its fulcrum 50 cm from one end. What minimum effort is required to displace a weight of 500 kgf? Calculate the M.A. of the crowbar.
Solution 8 Or, E = 71.4 kgf
Question 9

A pair of nut crackers is 12 cm long. An effort of 10 gf is required to crack a nut which is passed at a point 3 cm from the finger. Calculate the load. Solution 9 Question 10

(a)  Fig. represents an incomplete diagram of a simple string pulley      system. Copy this diagram on a new page and complete it and mark where the effort must be applied to lift the load.

(b) What is the velocity ratio of this system?
(c) If the pulley system is 80% efficient and the load is 720 N, then
(i) What effort must be applied to lift the load?
(ii) What work must be done must be done in lifting the load through    a distance of 2 m using this machine? Solution 10 Question 11
(a) What makes a balance faulty?
(b) A faulty balance of equal arms but pans of unequal weight is used to find the weight of a body. By the method of double weighing the weights are found as 8 kg and 8.2 kg. Find the actual weight of the body.
(c) The arms of a beam balance are 20 cm and 21 cm, but the pans are of equal weight. By the method of double weighing the weights are found to be 1000 g and 20 g. Find the actual weight of the body.
(d) A faulty balance of unequal arms and pans of unequal weights is used to find the true weight of a metal. By double weighing the weights are found to be 1210 g and 1000 g. Calculate the true weight of the metal.
Solution 11 Solution 1

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