Class 9 - Physics - Work and Energy

Question1.

Look at the activities listed below. Reason out whether or not work is done in the light of your understanding of the term ‘work’.

• Suma is swimming in a pond.
• A donkey is carrying a load on its back.
• A wind-mill is lifting water from a well.
• A green plant is carrying out photosynthesis.
• An engine is pulling a train.
• Food grains are getting dried in the sun.
• A sailboat is moving due to wind energy.

• Suma is swimming in a pond: - She is pushing the water in the backward direction, which is an action performed by her.
• However, due to reaction, the water pushes her in the forward direction. Work is done by Suma.
• A donkey is carrying a load on its back: - In this case, force of gravity on the load is acting in the downward direction,
• whereas the displacement will be in the horizontal direction i.e., the force and displacement are perpendicular to each other.
• There is no displacement in the direction of the force of gravity, and therefore no is work done as there is no displacement.
• A wind-mill is lifting water from a well: - The work is done by the wind mill in lifting the bucket of water from the well. The work is done against the force of gravity.
• A green plant is carrying out photosynthesis: - No work done is done in this case. As both force and displacement are 0.
• An engine is pulling a train: - In this case, an engine is pulling a train parallel to the ground.
• The force exerted by the engine is in the direction of displacement of the train.
• Thus, the force and displacement are in the same direction. Therefore, work is done .
• Food grains are getting dried in the sun: - No work is done in this case as food grains remain at rest.
• A sailboat is moving due to wind energy: - The force exerted by the wind on the sail move the boat in the direction of force, hence, positive work is done by wind energy.

Question2.

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?

The force of gravity acts in the vertical downward direction and the displacement of the object acts along the horizontal direction. Angle between them is 900.

Therefore Work done W = Fscos900 =0   (as cos 900=00)

So the work done by the force of gravity will be 0.

Question 3.

A battery lights a bulb. Describe the energy changes involved in the process.

Battery converts chemical energy into electrical energy.

This electrical energy is further converted into light and heat energy. Energy changes involved in this process can be written as follows:-

Chemical energy à Electrical energy à Heat energy + Light energy

Question 4.

Certain force acting on a 20 kg mass changes its velocity from 5 ms–1 to 2 ms–1. Calculate the work done by the force.

Given:-

Mass of the body = 20kg

Initial velocity u=5 ms–1

Final velocity v = 2 ms–1

Initial kinetic energy Ei = (1/2) mu2 = (1/2) x 20 x (5 ms–1)2

=250kgms-2

= 250Nm = 250J

Final kinetic energy Ef = (1/2) mv2 = (1/2) x 20 x (2 ms–1)2

=40kgms-2

= 40 Nm

=40J

Therefore, Work done = change in kinetic energy

= Ef – Ei

=40J – 250J

Work done =-210J   where (-) ive sign shows force is acting opposite to the direction of motion.

Question5.

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 object by the gravitational force will be 0.

The force of gravity acting on the object of mass 10kg is in downward direction, whereas displacement is in horizontal direction.

Therefore, the force and displacement are at right angle to each other.

Question 6.

The potential energy of a freely falling object decreases progressively. Does this violate the law of conservation of energy? Why?

No, it does not violate the law of conservation of energy. During free fall there is continuous decrease in potential energy.

This decrease in potential energy appears as an equal amount of increase in kinetic energy.

As a result sum of potential energy and kinetic energy is same at all points. Therefore potential energy + kinetic energy = constant.

Question 7.

What are the various energy transformations that occur when you are riding a bicycle?

During riding a bicycle, the muscular energy of the rider is converted into heat energy and kinetic energy.

Kinetic energy provides velocity to the bicycle and heat energy heats our body.

Muscular energy -> Kinetic energy + Heat energy

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?

There will be no work done, as displacement is 0. The energy spent by us will be converted into the potential energy of the rock which may not be visible to us.

Question 9.

A certain household has consumed 250 units of energy during a month. How much energy is this in joules?

Given Energy consumed = 250 units

= 250 units = 250 × 3,600,000 J (1 unit = 3,600,000 J)

So, Energy consumed = 900,000,000 J

= 9 × 108J

Hence, the energy consumed = 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.

Given:-

Mass of the object m = 40kg, height h = 5m

Acceleration due to gravity g= 10ms-2

Potential energy = mgh

= 40kg x 10ms-2 x5m

=2000kg ms-2 m

=2000Nm = 2000J

According to law of conservation of energy, the total mechanical energy (Kinetic and potential energy) of an object remains constant.

Therefore, when the object is half-way down, its potential energy becomes half the original energy and remaining half converted into kinetic energy.

Hence, the potential energy =mg (h/2) = (mgh)/ (2)

(2000J)/ (2) = 1000J

Decrease in potential energy =2000J – 1000J = 1000J.

Therefore, kinetic energy when it is half-way down = Decrease in potential energy = 1000J.

Question 11.

What is the work done by the force of gravity on a satellite moving round the earth? Justify your answer.

When a satellite moves around the Earth, its displacement for 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.

As tangent is always at right angle to the radius, motion of the satellite and force of gravity are perpendicular to each other.

Net work done = Fs cos900 =0 (because cos 900 =0)

Therefore work done by the force of gravity on a satellite moving round the earth is 0.

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.

When an object moves with constant velocity .The net force acting on it will be 0. But there will be displacement along the motion of the object. Therefore there can be displacement without force.

We know that,

F = mx a.

Using F=0, then m x a=0. As mass cannot be zero, therefore acceleration is 0 when force is 0.

Either the object is at rest or it is in uniform motion.

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.

A person holding a bundle of hay over his head for 30 minutes has done no work as he has not moved any distance.

=> Displacement s=0.

Work, W = Force x displacement = Force x 0.

Question 14.

An electric heater is rated 1500 W. How much energy does it use in 10 hours?

Given:-

Power, P = 1500W = (1500/1000) = 1.5KW

And

Time, t = 10hours = 10h

We know that Power = (Energy) / (Time taken)

Therefore, 1.5 = (Energy)/10h

Energy =1.5KW x 10h

=15KWh or (15 units).

Therefore the energy consumed is 15KWh or 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 do the bob eventually come to rest? What happens to its energy eventually? Is it a violation of the law of conservation of energy?

According to law of conservation of energy can neither be created nor be destroyed. It can only be converted from one form to another.

Consider the motion of an oscillating pendulum.

1. When a pendulum moves from its mean position (P) to either of its two extreme positions (A or B), it rises through a height above the mean level P.
2. Kinetic energy completely changes to potential energy.
3. Kinetic energy becomes 0, and now bob only possess potential energy.
4. As it moves towards the point P, its potential energy decreases progressively. Accordingly kinetic energy increases.
5. As the bob reaches the point P, its potential energy becomes 0 and the bob possess only kinetic energy.
6. This process keeps on repeating as long as pendulum oscillates.
7. Because of air resistance bob comes to rest.
8. The pendulum loses its kinetic energy to overcome this fiction and stops after some time.
9. The law of conservation of energy is not violated because energy lost by the pendulum is used to overcome the friction is gained by the surroundings.
10. Hence, the total energy of the pendulum and the surrounding system remain conserved.

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?

Mass of the object = m, initial velocity =v, final velocity =0.

Kinetic energy of an object = (1/2) mv2

To bring the object to rest, the amount of work done should be equal to its kinetic energy = (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?

Given:-

Mass of the car = 1500kg

Initial velocity u= 60 km/h = 60 x (1000/3600) m/s =16.67 m/s

Final velocity v=0

Work done = change in kinetic energy = Final Kinetic energy – Initial kinetic energy.

= (1/2mv22 – 1/2mv12)

Where m = mass of the object, v1 = Initial velocity, v2 = Final velocity.

So, work done = (1/2) x 1500({0}2 – {16.67}2)

= (1/2) x 1500 x {0 + 277.89}

= (1/2) x1500 + 277.89

=208417.5J

Work done =208.41KJ

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.

Case 1:-

Force F is acting in the upward direction and the displacement in the horizontal direction .They both are perpendicular to each other.

Therefore Work W = Fscos900 = 0

=> No work is done as force is 0.

Case 2:-

Force F is in the same direction as the direction of the displacement. So they both are in same direction.

Therefore Work W = Fscos00 = Fs.

=> The work done is taken as (+) ive.

Case 3:-

The force F is acting in the opposite direction to the direction of the displacement i.e. the angle between them is 1800.

Therefore Work W = Fscos1800 = -Fs

=> Work done is taken as (-) ive.

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?

Yes, it is possible. It happens when all the forces acting on the object are such that they cancel each other.

As a result the net force acting on the object becomes 0.So there is no acceleration, which means acceleration is 0.

We know that, F = ma,

In the net force is zero, F = 0, then ma = 0

⇒ a = 0 [as m ≠ 0]

Question 20.

Find the energy in kW h consumed in 10 hours by four devices of power 500 W each.

Power consumed by 1 device = 500W.

Therefore the power consumed by four devices = 4 × 500 W = 2000 W=20KW

Time = 10 hours

Energy consumed = power × time

=2kW x 10h

=20kWh or 20 units (because 1unit = 1KWh)

Question 21.

A freely falling object eventually stops on reaching the ground. What happens to its kinetic energy?

When an object falls freely towards ground, its kinetic energy transform into different forms of energy:-

1. Heat energy is produced when an object hits the ground and collides.
2. Sound or vibration energy is produced due to the collision of the object with the ground.
3. In deforming the body or the ground.