# KSEEB Solutions for Class 9 Science Chapter 11 Work and Energy

Students can Download Science Chapter 11 Work and Energy Questions and Answers, Notes Pdf, KSEEB Solutions for Class 9 Science helps you to revise the complete Karnataka State Board Syllabus and to clear all their doubts, score well in final exams.

## Karnataka State Syllabus Class 9 Science Chapter 11 Work and Energy

### KSEEB Class 9 Science Work and Energy Intext Questions and Answers

Question 1.
A force o*f 7 N acts on an object the displacement is say 8 in in the direction of the force, what is the work done in this case?
F = 7 N,
S = 8 M
W = F x s
= 7 x 8
= 56 joules Question 2.
When do we say that work is done?
The body displaces in the direction of the applied force.
W = F x s

Question 3.
Write the expression for the work done when a force is acting on an object in the direction of its displacement.
W = force x displacement
W = F x S

Question 4.
Define 1 J of work.
When one newton of force is applied on a body, body displaces 1 meter in the direction of the applied force, then the amount of work done is said to be 1 joule.
Work = force x displacement
W = F x S
One joule = 1 newton x 1 meter Question 5.
A pair of bullocks enters a force of 140 N on a plow, the field being plowed is 15 m long. How much work is done in plowing the length of the field?
W = f x s
= 140 x 15
= 2100 Joules (newton – meter)

Simple Kinetic Energy Calculator to Compute, Mass, or Velocity. Just enter in your values.

Question 6.
What is the kinetic energy of an object?
The energy possessed by a moving body by virtue of motion is called kinetic energy.

Question 7.
Write on expression for the kinetic energy of an object.
K.E = $$\frac{1}{2}$$ mv2 Question 8.
The kinetic energy of an object of mass in moving with a velocity of 5 ms-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?
K.E. = 25 joules
V = 5 m/s
KE = $$\frac{1}{2}$$mv2
∴ $$\mathrm{m}=\frac{2 \mathrm{KE}}{\mathrm{mv}^{2}}=\frac{2 \times 25}{5}$$ = 2kg

Kinetic energy when its velocity is doubled.
V = 10 m/s
$$=\frac{1}{2} m v^{2}=\frac{1}{2} \times 2 \times 10^{2}=100 \mathrm{J}$$

Kinetic energy when its velocity is increased by three times.
$$=\frac{1}{2} m v^{2}=\frac{1}{2} \times 2 \times 15^{2}=225 \text { joules }$$

Question 9.
What is power?
The rate of doing work is called power.
$$Power =\frac{\text { Work done }}{\text { Time taken }}$$ Question 10.
Define 1 watt of power.
1 watt = $$\frac{1 \text { joule of workdone }}{1 \text { sec }}$$

Question 11.
A lamp consumes 1000 J of electrical energy in 10 s what is its power.
$$P=\frac{W}{E}=\frac{1000 \mathrm{J}}{10 \mathrm{sec}}=100 \mathrm{watts}$$

Question 12.
Define average power.
Average power
$$=\frac{\text { total energy consumed }}{\text { total time taken }}$$

### KSEEB Class 9 Science Work and Energy Textbook Exercise Questions and Answers

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’.

1. Suma is swimming in a pond.
2. A donkey is carrying a load on its back.
3. A windmill is lifting water from a well.
4. A green plant is carrying out photosynthesis.
5. An engine is pulling a train.
6. Food grains are getting dried in the sun.
7. A sailboat is moving due to wind energy. 1. The sum is doing work.
2. No work is done (only force without displacement)
3. Work is done. Work is done against gravity.
4. No work is done (no force and no displacement)
5. The engine pulls the train here force and displacement exist work is done.
6. No work is done (no force and no direction)
7. Work is done (wind energy and displacement)

Question 2.
An object was 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 I the path of the object lie on the same horizontal line. What is the work done by the force of gravity on the object?
The displacement of the object is in the horizontal direction. The force of gravity on the object acts vertically downwards. Work done by this force is zero as the force is perpendicular to the displacement.

Question 3.
A battery’ lights a bulb. Describe the energy changes involved in the process.
A battery converts chemical energy into electrical energy’. This is converted into light energy.

Question 4.
The certain force acting on a 20 kg mass changes its velocity from 5 ms-1 to 2 ms1. Calculate the work done by the force.
Initial Kinetic energy
$$=\frac{1}{2} \mathrm{mu}^{2}=\frac{1}{2} \times 20 \times 5^{2}=250 \mathrm{J}$$

Final kinetic energy = $$\frac{1}{2}$$mv2 Work done = change in Kinetic energy
= -210 J = 250 – 40 = 210 J

Question 5.
A mass of 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.
Work done on the body zero. Question 6.
The potential energy of a freely falling object decreases progressively. Does this violate the law of conservation of energy? Why?
Now we can say that the law of conservation of energy is not violated, because of the decrease in potential energy results in the increase of kinetic energy.

Question 7.
Wliat are the various energy transformations that occur when you are riding a Bicycle?
In the case of bicycle, muscular energy is converted into the kinetic energy of the bicycle.

Question 8.
Does the transfer of energy take place when you push a huge rock with all your might and fail to move it? Where is the energy you spend going?
When we push a huge rock, then the rock also exerts a huge force on us according to Newton’s third law of motion. The muscular energy spent by us in the process is used to oppose the huge force acting on us due to the rock. Question 9.
A certain household has consumed 250 units of energy’ during a month. How much energy is this in joules?
Energy consumed = 250 units
= 250 kwh
= 250 x 1000 we have .
= 250 x 1000 x (3600 S)
= 900 x 106 = 9 x 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.
V2 – u2 = 2gh
V2 = 2 x (10) x (2.5)
V2 = 50 m/s2
K.E = $$\frac{1}{2}$$ mv2 = $$\frac{1}{2}$$ x 40 x 50 = 1000 J
OR
After P.E. half – way down
E = m x g x h = 400 x 10 x 2.5
= 1000 J Question 11.
What is the work done by the force of gravity on a satellite moving round the earth? Justify your answer.
$$\mathrm{f}=\frac{\mathrm{mv}^{2}}{\mathrm{r}}$$
It satisfies centripetal force.

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.
No work

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.
Muscular fatigue, no displacement hence no work a done.

Question 14.
An electric heater is rated 1500 W. How much energy does it use in 10 hours?
W = p x t = 1500 x 3600
= 15000 Wc have = 15 kwh
= 15 units

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? What happens to its energy eventually? Is it a violation of the law of conservation of energy?
When pendulum oscillates in extreme ends. There is a continuous exchange between potential energy and kinetic energy and the sum of two types of energy remains constant. According to the law of conservation of energy. This is true in case there is no loss of energy due to external force.

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?
Initial K.E = $$\frac{1}{2}$$mv2
Comes to rest KE = 0
Charge m K.E. = $$\frac{1}{2}$$mv2
Work done on the object
= charge in its K.E. = $$\frac{1}{2}$$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?
Initial velocity = 60 km/h Find kinetic energy = $$\frac{1}{2}$$mv2 = 0
Charge in kinetic energy
= charge in kinetic energy
= 20833.3 J

Question 18.
In each of the following a force, F is acting on an object of mass, in. 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. (a) no displacement
(b) force is positive (work is done)
(c) work done is 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?
The acceleration of an object can be zero even if several forces are acting on its provided the resultant force is ” zero.

Question 20.
Find the energy in kW h consumed in 10 hours by four devices of power 500 W each.
Total power = P = 4 x 500 W = 2000 W
t = 10 h
W = p x t = 2 kinetic energy x 10 = 20 units

Question 21.
A freely falling object eventually stops on reaching the ground. What happens to its .kinetic energy?
When a freely falling object eventually stops on reaching the ground, then sound and heat are produced. Thus kinetic energy is converted into sound and heat.

### KSEEB Class 9 Science Work and Energy Additional Questions and Answers

Question 1.
Can there be displacement of an object in the absence of any force acting on it?
In the absence of any force on the object i.e., F = 0, ma, (as F = ma) since m ≠ 0 and a = 0. In such a case the object is either at rest or in a state of uniform motion in a straight line in the latter case there is a displacement . of the object without any force acting on it. Question 2.
Is a pension doing any work by holding a suitcase?
No. As there is no displacement.

Question 3.
How does a bullet pierce a target?
A bullet moves with a large velocity and as such possesses a lot of kinetic energy the work in piercing the target is derived from the kinetic energy of the bullet.

Question 4.
How is energy stored in a clock?
The energy stored in the clock is in the form of the elastic potential energy of its coiled spring the spring is coiled by winding the clock.

Question 5.
Why do some engines require fuels like petrol and diesel?
Internal combustion heat engine use the chemical energy of fossil fuel (petrol and diesel) for their operation. These engines first convert the chemical energy of the fuels into heat energy. Which is later on converted into mechanical energy.

Question 6.
Express 2 kwh in units.
2 kwh
= 2 x 1000 x w x h
= 2 x 1000 x 3600 x w
= 72 x 10C6 watts Question 7.
Calculate the work done by a body. By the force of 5 N makes to move through a distance of 12 m.
W = F x s = 5 x 12 = 60 joules

Question 8.
Express 35 kwh into watts.
35 kwh
35 x 1000 x w x 3600.
= 35 x 3600 x 1000 x 3600

Question 9.
When force 6 N applied on a wall, the wall remains in the same position, calculate the work done.
W = F x s
W – 6 x s = 6 x 0 = 0 Joules No work is done on the body

Question 10.
What do you mean by the total energy of a system? The sun of both kinetic and potential energy.
Total energy = kinetic energy + potential energy

Question 11.
Electricity is the most convenient form of energy. Why?
It can be converted into other forms of energy easily.
It can be produced by different means. It is ecofriendly Question 12.
Minimum use of fossils arc preferred, justify.
Non-renewable sources of energy. Produces more pollution.

Question 13.
Calculate the power experienced by a source can do work of 50 joules in 5 seconds.
Power = $$\frac{\text { Workdone }}{\text { Time taken }}$$
$$=\frac{50}{5} \frac{\text { Joules }}{\text { Second }}$$ = 10 J/s = 10 watts

Question 14.
Calculate the force experienced oh a body of mass 5 kg moves with an acceleration of 4 m/s-1.  