Work And Energy
Class 9th Science CBSE Solution
- When do we say that work is done?
- Write an expression for the work done when a force is acting on an object in the direction…
- Define 1 J of work.
- A pair of bullocks exerts a force of 140 N in a plough. The field being ploughed is 15 m…
- What is the kinetic energy of an object?
- Write an expression for the kinetic energy of an object.
- The kinetic energy of an object of mass m moving with a velocity of 5 m s-1 is 25 J. What…
- What is Power?
- Define 1 watt of power.
- A lamp consumes 1000 J of electrical energy in 10 s. What is its power?…
- Define average power.
- Look at the activities listed below. Reason out whether or not work is done in the light…
- An object thrown at a certain angle to the ground moves in a curved path and falls back to…
- A battery lights a bulb. Describe the energy changes involved in the process.…
- Certain force acting on a 20 kg mass changes its velocity from 5 m s-1 to 2 m s-1.…
- An object of mass 10 kg is at a point A on a table. It is moved to a point B. If the line…
- The potential energy of a freely falling object decrease progressively. Does this violate…
- What are the various energy transformations that occur when you are riding a bicycle?…
- Does the transfer of energy take place when you push a huge rock with all you might and…
- A certain household has consumed 250 units of energy during a month. How much energy is…
- An object of mass 40 kg is raised to a height of 5 m above the ground. What is its…
- What is the work done by the force of gravity on a satellite moving round the earth?…
- Can there be displacement of an object in the absence of any force acting on it? Think.…
- A person holds a bundle of hay over his head for 30 minutes and gets tired. Has he done…
- An electric heater is rated 1500 W. How much energy does it use in 10 hours?…
- Illustrate the law of conservation of energy by discussing the energy changes which occur…
- An object of mass m is moving with a constant velocity v. How much work should be done on…
- Calculate the work required to be done to stop a car of 1500 kg moving at a velocity of 60…
- In each of the following a Force F is acting on an object of mass m. The direction of…
- Soni says that the acceleration in an object could be zero even when several forces are…
- Find the energy in kWh consumed in 10 hours by four devices of power 500 W each.…
- A freely falling object eventually stops on reaching the ground. What happens to its…
In Text Questions-pg-148
Question 1.A force of 7 N acts on an object. The displacement is, say 8 m, in the direction of the force (see Figure below). Let us take it that the force acts on the object through the displacement. What is the work done is this case?
Answer:
We know that,
Work done = Force × Displacement
W = F × s
Given,
Force, F = 7 N
Displacement, s = 8 m
W = 7 × 8
= 56 J
Therefore, the work done is 56 J
In Text Questions-pg-149
Question 1.When do we say that work is done?
Answer:
Work is said to be done whenever the given conditions are satisfied:
(i)A force acts on a body.
(ii) There is a Displacement in the body caused by the applied force along the direction of the applied force.
Question 2.
Write an expression for the work done when a force is acting on an object in the direction of its displacement?
Answer:
When a force F displaces a body through a distance s in the direction of the applied force, then the work done W is given by the expression:
Work done = Force × Displacement
W = F × s
Question 3.
Define 1 J of work.
Answer:
1 Joule is the energy used to accelerate a body with a mass ofone kilogram using one newton of force over a distance of one meter.The SI unit of energy is joule (J)
Question 4.
A pair of bullocks exerts a force of 140 N in a plough. The field being ploughed is 15 m long. How much work is done in ploughing the length of the field?
Answer:
Given,
Force, F = 140 N
Distance, s = 15 m
Work done, W =?
W = F × s
= 140 × 15
= 2100 J
In Text Questions-pg-152
Question 1.What is the kinetic energy of an object?
Answer:
The kinetic energy of an object is the energy that it possesses due to its motion. It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity.
The heavier a thing is, and the faster it moves, the more kinetic energy it has.
Question 2.
Write an expression for the kinetic energy of an object.
Answer:
If a body of mass m is moving with a velocity v, then its kinetic energy
is given by the expression:
K.E. = 
mv2
Where, m = Mass of the object
v = velocity of object
Its SI unit is Joule (J).
Question 3.
The kinetic energy of an object of mass ‘m’ moving with a velocity of 5 m s-1 is 25 J. What will be its kinetic energy when its velocity is doubled? What will be its kinetic energy when its velocity is increased three times?
Answer:
Given:
velocity of the body, v=5 m/s
kinetic energy of the body, K.E.=25 J
Formula Used:
(1) 
k.E. is the kinetic energy of the body(J)
m is the mass of the body(Kg)
v is the velocity of the body( m/sec)
putting the value in the equation(2):
We get,
In the first case, the velocity of is doubled
hence, the velocity becomes 5×2 = 10 m/s and mass is 2 kg
putting the values in the equation(1) we get
Therefore, when the velocity is doubled, then its kinetic energy becomes 100 J which is 4× the Kinetic energy when speed is 5 m/s
In the second case, the velocity of is increased three times.
hence, the velocity becomes 5×3 = 15 m/s and mass is 2 kg
putting the values m and v in equation (1)
Therefore, when the velocity of object is increases three times, then its kinetic energy becomes 225 J which is 9 × the kinetic energy when speed is 5 m/s
In Text Questions-pg-156
Question 1.What is Power?
Answer:
Power is defined as energy consumed or work done per unit time.
Power = Work done / time
Or,
Energy consumed / time
Its SI unit is watts or joules/ sec
Question 2.
Define 1 watt of power.
Answer:
When the work of 1 joule is done in a time of 1 s, the power is said to be one watt.
Question 3.
A lamp consumes 1000 J of electrical energy in 10 s. What is its power?
Answer:
We know that,
Power = 
Power = 
= 100
= 100 W
Question 4.
Define average power.
Answer:
When a machine or person does different amounts of work or uses energy in different intervals of time, the ratio between the total work or energy consumed to the total time is average power.
Average Power = 
Exercise-pg-158
Question 1.Look at the activities listed below. Reason out whether or not work is done in the light of your understanding of the term ‘work’:
(a) Suma is swimming in a pond.
(b) A donkey is carrying load on its back.
(c) A wind-mill is lifting water from a well.
(d) A green plant is carrying out photosynthesis.
(e) An engine is pulling a train.
(f) Food-grains are getting dried in the sun.
(g) A sailboat is moving due to wind energy.
Answer:
(a) Work is done because the displacement of swimmer takes place in the direction of applied force.
(b) A donkey is not doing work against gravity. No work is done as the displacement of load does not take place in the direction of applied force.
(c) Work is done, as the displacement takes place in the direction of force.
(d) No work is done, because no displacement takes place.
(e) When an engine pulls a train the angle between distance and force becomes 0° and work will be done.
(f) No work is done in this process as the grains are not moving or covering some distance when being dried in the sun, hence no work is done.
(g) When wind energy is applied to the boat it starts moving in the direction of the force applied by the wind. So the angle between distance and force becomes 0° and some work will be done.
Question 2.
An object thrown at a certain angle to the ground moves in a curved path and falls back to the ground. The initial and the final points of the path of the object lie on the same horizontal line. What is the work done by the force of gravity on the object?
Answer:
Since, the body returns to a point which is on the same horizontal line through the point of projection, hence no displacement has taken place against the force of gravity, therefore, no work is done by the force due to gravity.
Question 3.
A battery lights a bulb. Describe the energy changes involved in the process.
Answer:
Within the electric cell of the battery the chemical energy changes into electrical energy. The electric
Energy on flowing through the filament of the bulb, first changes into heat energy and then into the light energy
Question 4.
Certain force acting on a 20 kg mass changes its velocity from 5 m s-1 to 2 m s-1. Calculate the work done by the force.
Answer:
When the Velocity, v = 5 m/s
Mass, m = 20 kg
Then,
K.E. = mv2
= × 20 × (5) 2
= × 20 × 25
= 250 J
When the Velocity, v = 2 m/s
Mass, m = 20 kg
K.E. = mv2
= × 20 × (2) 2
= × 20 × 4
= 40 J
We know that,
Work done by force = Change in kinetic energy
= 250 - 40
= 210 J
Question 5.
An object of mass 10 kg is at a point A on a table. It is moved to a point B. If the line joining A and B is horizontal, what is the work done on the object by the gravitational force? Explain your answer.
Answer:
The work done is zero. This is because the gravitational force and displacement are perpendicular to each other.
Question 6.
The potential energy of a freely falling object decrease progressively. Does this violate the law of conservation of energy? Why?
Answer:
It does not violate the law of conservation of energy. Whatever, is the decrease in Potential energy due to loss of height, same is the increase in the Kinetic energy due to increase in velocity of the body.
Question 7.
What are the various energy transformations that occur when you are riding a bicycle?
Answer:
The chemical energy of the food changes into heat and then to muscular energy.
On paddling, the muscular energy is converted into kinetic energy of the pedal.
The kinetic energy of pedal is converted into kinetic energy of the bicycle.
The kinetic energy of cycle is converted to kinetic energy of the cyclist.
The mechanical energy is sum of potential energy and kinetic energy ,since potential energy is zero.
Thus ,we can say that the muscular energy of the cyclist is directly converted to the mechanical energy of the cyclist.
Question 8.
Does the transfer of energy take place when you push a huge rock with all you might and fail to move it? Where is the energy you spend going?
Answer:
Energy transfer does not take place as no displacement takes place in the direction of applied force. The energy spent is used to overcome inertia of rest of the rock.
Question 9.
A certain household has consumed 250 units of energy during a month. How much energy is this in joules?
Answer:
Given,
Energy consumed = 250 units
= 250 kWh
We know that,
1 kilowatt-hour = 3.6 × 106 J
250 kilowatt-hours = 3.6 × 106 × 250
= 9 × 108 J
Question 10.
An object of mass 40 kg is raised to a height of 5 m above the ground. What is its potential energy? If the object is allowed to fall, find its kinetic energy when it is half way down (g = 10 m/s2)
Answer:
Given,
Mass, m = 40 kg
Acceleration due to gravity, g = 10 m/s2
Height, h = 5 m
We know that,
Potential energy = m × g × h
= 40 × 10 × 5
= 2000 J
When the object is fall to its half way down, then its height above the ground = 
=2.5 m
Now, Potential energy = m × g × h
= 40 × 10 × 2.55 J
= 1000 J
Now, we know that according to the law of conservation of energy:
Total potential energy = Potential energy at half way down + Kinetic energy at half way down
2000 = 1000 + Kinetic energy at half way down
Kinetic energy at half way down = 2000 – 1000
= 1000 J
Question 11.
What is the work done by the force of gravity on a satellite moving round the earth? Justify your answer.
Answer:
When a satellite revolves around the earth in a circular path, its displacement in any short interval of time is along the tangent to the circular path of the satellite. The force of gravity acting on the satellite is along the radius of the earth at that point. Since a tangent is always at right angles to the radius, therefore, the motion of a satellite and force of gravity are at right angles (90 o angle) to each other.
Now, work done W = F Cos 90 o × s.
Since Cos 90 o = 0, therefore,
Work done W = F × 0 × s = 0
Thus, the work done by the force of gravity on a satellite round the earth is zero.
Question 12.
Can there be displacement of an object in the absence of any force acting on it? Think. Discuss this question with your friends and teacher.
Answer:
Yes, the given situation is true as we known that F = m × a
Now, when force F = 0,
Then, m × a = 0
Since ‘m’ cannot be zero, therefore, when force F = 0 then acceleration ‘a’ = 0
In such a situation, either the object is in uniform motion in a straight line or it is at rest (not moving). In the first case, there is a displacement of the object without any force acting on it.
Question 13.
A person holds a bundle of hay over his head for 30 minutes and gets tired. Has he done some work or not? Justify your answer.
Answer:
The person does not do work because no displacement takes place in the direction of applied force as the force acts in the vertically upward direction.
Question 14.
An electric heater is rated 1500 W. How much energy does it use in 10 hours?
Answer:
Given,
Power, P = 1500 W
= 
KW
= 1.5 kW
Time, t = 10 h
We know that,
Power = 
1.5 = 
Energy used = 1.5 × 10
= 15 kWh
Question 15.
Illustrate the law of conservation of energy by discussing the energy changes which occur when we draw a pendulum bob to one side and allow it to oscillate. Why does the bob eventually come to rest?
Answer:
When the pendulum bob is pulled (say towards left), the energy supplied is stored in it is the form of potential energy on account of its higher position. When the pendulum is released so that it starts moving towards right, then its PE changes into kinetic energy such that in mean position, it has maximum kinetic energy, and Zero potential energy. As the pendulum moves towards extreme right, its kinetic energy changes into potential energy such that at the extreme position, it has maximum potential energy and zero Kinetic energy. When it moves from this extreme position to mean position, its potential energy again changes to kinetic energy.
This illustrates the law of conservation of energy. Eventually, the bob comes to rest, because during each oscillation a part of the energy possessed by it transferred to air and is used in overcoming friction of air . Thus, the energy of the pendulum is dissipated during oscillation in the air.
Question 16.
An object of mass ‘m’ is moving with a constant velocity ‘v’. How much work should be done on the object in order to bring the object to rest?
Answer:
Initial kinetic energy of the object, K.E. (Initial) = 
mv2
When the object comes to rest, final kinetic energy of the object, K.E. (Final) = 0
Change in kinetic energy = mv2 - 0
= mv2
Work done on the object = Change in its kinetic energy
= mv2
Question 17.
Calculate the work required to be done to stop a car of 1500 kg moving at a velocity of 60 km/h?
Answer:
The kinetic energy of the car is zero as it stops. So, the work done that is required to stop the car will be equal to its kinetic energy.
Given,
Mass of car, m = 1500 kg
Velocity of car, v = 60 km/h
We know that, when car is moving the car energy is in form of kinetic energy but to stop the car kinetic energy should be zero.Thus Work done= Kinetic energy
And ,
= 208333.3 J
Therefore, the value of the work required to stop the car is equal to kinetic energy of the car is 208333.3 J
Question 18.
In each of the following a Force F is acting on an object of mass m.
The direction of displacement is from west to east shown by the longer arrow. Observe the diagrams carefully and state whether the work done by the force is negative, positive or zero.
Answer:
Work is done whenever the given two conditions are satisfied:
(a) A force acts on the body.
(b) There is a displacement of the body by the application of force in or opposite to the direction of force.
(i) In this case, the direction of force acting on the block is perpendicular to the displacement. Therefore, work done by force on the block will be zero.
(ii) In this case, the direction of force acting on the block is in the direction of displacement. Therefore, work done by force on the block will be positive.
(iii) In this case, the direction of force acting on the block is opposite to the direction of displacement. Therefore, work done by force on the block will be negative.
Question 19.
Soni says that the acceleration in an object could be zero even when several forces are acting on it. Do you agree with her? Why?
Answer:
Yes, I agree with her because when the object is at rest, its velocity is zero which in turn means that its acceleration is also zero. Several forces may act on it but they cancel each other. When the object is in motion and moving with a constant velocity, its acceleration is zero. Even in this situation, several forces may act on the objects which balance each other.
Question 20.
Find the energy in kWh consumed in 10 hours by four devices of power 500 W each.
Answer:
Given,
Power of one device = 500 W
Power of four devices = 500 × 4
= 2000 W
= 
kW
= 2 kW
Time = 10 h
We know that,
Power = 
2 kW =
Energy consumed = 2 × 10
= 20 kWh
Question 21.
A freely falling object eventually stops on reaching the ground. What happens to its kinetic energy?
Answer:
When an object falls freely towards the ground, its potential energy decreases and kinetic energy increases. As the object touches the ground, all its potential energy gets converted into kinetic energy. As the object hits the hard ground, all its kinetic energy gets converted into heat energy and sound energy. It can also deform the ground depending upon the nature of the ground and the amount of kinetic energy possessed by the object.