These AP 9th Class Physics Important Questions 10th Lesson Work and Energy will help students prepare well for the exams.

## AP 9th Class Physics 10th Lesson Work and Energy Important Questions

### Class 9 Physical Science Chapter 10 Important Questions – 2 Marks

Question 1.

What is Mechanical Energy ?

Answer:

The sum of potential energy and kinetic energy of an object is called mechanical energy. Mechanical Energy = Potential Energy + Kinetic Energy

Question 2.

Define Law of donservation of energy. Write the expression for it.

Answer:

Energy can neither be created nor destroyed, it can only be transferred from one form to another.

Potential Energy + Kinetic Energy = Constant

Question 3.

Write the two conditions need to be satisfied for work to be done.

Answer:

The two conditions need to be satisfied for work to be done:

- a force should act on an object and
- the object must be displaced.

Question 4.

A bullock is pulling a cart. The cart moves. There is a force on the cart and the cart has moved. Do you think that work is done in this situation ?

Answer:

- Yes, work is done in this situation.
- Work is defined as the product of the force applied on an object and the displacement of the object in the direction of the force.
- In this case, the force applied by the bullock causes a displacement of the cart, which means that work is done on the cart.

Question 5.

What is work done ? Write its formula.

Answer:

- Work done is equal to the product of the force and displacement.
- Work done = force × displacement
- W = F × s

Question 6.

What is the work done when the force on the object is zero ?

Answer:

- W = F.s
- Here, F = 0, then W = 0 × s = 0
- So, there no work is done.

Question 7.

What would be the work done when the displacement of the object is zero ?

Answer:

- W = F.s
- Here, s = 0, then W = F × 0 = 0
- So, there no work is done.

Question 8.

What is the condition for the work done by a force to be positive or negative ?

Answer:

- The work done by a force can be either positive or negative depending on the angle between the force and the displacement.
- If the force and displacement are in the same direction, the work done is positive, whereas if the force and displacement are in opposite directions, the work done is negative.

Question 9.

When do we say work negative or positive ?

Answer:

- Work done is negative when the force acts opposite to the direction of displacement.
- Work done is positive when the force is in the direction of displacement.

Question 10.

Where do we get energy ?

Answer:

The Sun is the biggest natural source of energy for us. Many of our energy sources are derived from the Sun. We can also get energy from the nuclei of atoms, the interior of the earth and the tides.

Question 11.

How is energy measured ?

Answer:

Energy is measured in terms of its capacity of doing work and the unit of energy is the same as that of work, which is joule (J). 1 J is the energy required to do 1 joule of work. Sometimes, a larger unit of energy called kilojoule (kJ) is used, where 1 kJ equals 1000J.

Question 12.

How does an object with energy do work ?

Answer:

An object that possesses energy can exert a force on another object. When this happens, energy is transferred from the former to the latter. The second object may move as it receives energy and therefore does some work.

Question 13.

Why the units of energy are same as that of work done ?

Answer:

The energy possessed by an object is thus measured in terms of its capacity of doing work. The unit of energy is, therefore, the sMie as that of work, that is, joule (J).

Question 14.

Define energy. Give examples of energy.

Answer:

Energy is a property of matter and can be defined as the ability to do work.

Example of energy : Mechanical energy (potential energy + kinetic energy), heat energy, chemical energy, electrical energy and light energy.

Question 15.

Can a moving object do work ? Give examples.

Answer:

- A moving object can do work.
- An object moving faster can do more work than an identical object moving relatively slow.
- Examples : A moving bullet, blowing wind, a rotating wheel, a speeding stone can do work.

Question 16.

What is kinetic energy ? Give examples to objects which have kinetic energy.

Answer:

- Kinetic energy is the energy possessed by an object due to its motion.
- A falling coconut, a speeding car, a rolling stone, a flying aircraft, flowing water blowing wind, a running athlete etc. possess kinetic energy.

Question 17.

How does a stretched rubber band possess potential energy ?

Answer:

- We transfer energy when we stretch a rubber band.
- The energy transferred to the band is its potential energy.

Question 18.

How does a toy car possess potential energy ?

Answer:

- We do work while winding the key of a toy car.
- The energy transferred to the spring inside is stored as potential energy.
- The potential energy possessed by the object is the energy present in it by virtue of its position or configuration.

Question 19.

What is the gravitational potential energy of aii object ?

Answer:

The gravitational potential energy of an object at a point above the ground is defined as the work done in raising it from the ground to that point against gravity.

Question 20.

What is the law of conservation of energy ?

Answer:

The law of conservation of energy states that energy can neither be created nor destroyed, but it can only be converted from one form to another. The total amount of energy in a system remains constant before and after any transformation.

Question 21.

What is the total energy of an object at the start of free fall ?

Answer:

At the start of free fall, the total energy of an object is its potential energy, which is given by mgh.

Question 22.

What is the relationship between the potential energy and kinetic energy of an object during free fell ?

Answer:

During free fall, the potential energy of an object decreases while the kinetic energy increases. The sum of the potential and kinetic energy remains constant at all points during the fall.

Question 23.

What is the mechanical energy of an object ?

Answer:

The sum of the potential energy and kinetic energy of an object is called its total mechanical energy.

Question 24.

What happens to the potential energy of an object during free fall ?

Answer:

During free fall, the potential energy of an object decreases as it falls lower to the ground. This decrease in potential energy is equal to the increase in kinetic energy of the object.

Question 25.

Define power in terms of work and time.

Answer:

Power is defined as the rate of doing work or the rate of transfer of energy. If an agent does a work W in time t, then power is given by: Power = work/time (or) P = W/t.

Question 26.

What is the relationship between power and energy transfer rate ?

Answer:

Power measures the rate of transfer of energy. Therefore, the higher the power of an agent, the faster it can transfer energy.

Question 27.

What is 1 watt ?

Answer:

1 watt is the power of an agent, Which does work at the rate of 1 joule per second.

We can also say that power is 1 W when the rate of consumption of energy is 1 J s^{-1}.

Question 28.

What is average power ?

Answer:

The power of an agent may vary with time. Therefore, the concept of average power is useful.

We obtain average power by dividing the total energy consumed by the total time taken.

Question 29.

What is work and how is it calculated ?

Answer:

Work is defined as the magnitude of the force multiplied by the distance moved by the object in the direction of the applied force. It is calculated by multiplying the force applied to an object by the distance the object moves in the direction of the force.

Question 30.

What is energy and how is it related to work ?

Answer:

An object having the capability to do work is said to possess energy. Energy has the same unit as that of work. Work is the transfer Of energy from one object to another.

Question 31.

What is kinetic energy and how is it calculated ?

Answer:

Kinetic energy is the energy possessed by an object in motion. An object of mass, m moving with velocity, v has a kinetic energy of \(\frac{1}{2}\)mv^{2}.

Question 32.

What is potential energy and how is it calculated ?

Answer:

Potential energy is the energy possessed by a body due to its change in position or shape. The gravitational potential energy of an object of mass, m raised through a height, h from the earth’s surface is given by mgh.

Question 33.

What would happen if nature does not allow the transfer of energy ? Discuss with few examples.

Answer:

If nature could not allow the transfer of energy, the normal life is not possible, e.g.:

- If the solar energy is not transferred into chemical energy, in which plants prepare their food, we cannot find any plant on the earth. Life without plants is impossible. We can not get food, cloth, shelter and fresh air for our survival.
- Water cycle is also an example of transfer of energy in nature. If this does not take place, ground water cannot be recharged, we cannot find any water further.

Question 34.

Observe the following examples.

Are all the people mentioned in the above examples doing work ?

Answer:

No.

In example (1) and (2) there is displacement. Hence they are doing work.

In example (3) and (4) there is no displacement. Hence they are not doing any work.

Problems

Question 35.

A ball with a mass of 0.2 kg is dropped from a height of 2 metres. Calculate its potential energy and kinetic energy just before it hits the ground.

Solution:

Potential energy = mgh = 0.2 × 9.8 × 2 = 3.92 J

Kinetic energy = 0.5 mv^{2} = 0.5 × 0.2 × 200 = 20 J

Question 36.

A 500 N force is applied to a block with a mass of 10 kg and it is displaced by 2 metres. Calculate the work done on the block.

Solution:

Work done = force × distance = 500 × 2 = 1000 J

Question 37.

A car with a mass of 1200 kg is moving at a speed of 30 m/s. Calculate its kinetic energy.

Solution:

Kinetic energy = 0.5 mv^{2} = 0.5 × 1200 × (30)^{2} = 540000 J

Question 38.

A 2 kg object is dropped from a height of 5 metres. Calculate its velocity just before it hits the ground.

Solution:

Potential energy at the top = mgh = 2 × 9.8 × 5 = 98 J

Kinetic energy just before hitting the ground = 98 J

Kinetic energy = 0.5 mv^{2}

v^{2} = (2 × 98)/2 = 98 ⇒ v = \(\sqrt{98}\) ≈ 9.9 m/s

### Work and Energy Class 9 Important Questions – 3 Marks

Question 1.

Differentiate potential energy and kinetic energy.

Answer:

Concept | Potential Energy | Kinetic Energy |

Definition | Energy an object has due to its position or state | Energy an object has due to its motion |

Formula | PE = mgh | KE = 1/2mv^{2} |

Units | Joules (J) | Joules (J) |

Example | A ball at the top of a hill has potential energy because of its height | A ball rolling down a hill has kinetic energy because of its motion |

Question 2.

Differentiate work and energy.

Answer:

Concept | Work | Energy |

Definition | The transfer of energy from one object to another by force acting over a distance | The ability to do work |

Formula | W= Fds | E ? W + PE + KE |

Units | Joules (J) | Joules (J) |

Example | Lifting a box from the floor to a shelf requires work | A battery has stored chemical energy that can be converted into other forms of energy, such as electrical energy or heat |

Question 3.

Differentiate energy and power.

Answer:

Concept | Energy | Power |

Definition | The ability to do work | The rate at which work is done |

Formula | E = W + PE + KE | P = W/t |

Units | Joules (J) | Watts (W) |

Example | A rollercoaster has potential and kinetic energy that is converted into work done on the riders | A light bulb has a certain power rating, indicating how quickly it uses electrical energy to produce light |

Question 4.

Write three daily life examples of ‘work done’.

Answer:

- Push a pebble lying on a surface. The pebble moves through a distance. You exerted a force on the pebble and the pebble got displaced. In this situation, work is done.
- A girl pulls a trolley and the trolley moves through a distance. The girl has exerted a force on the trolley and it is displaced. Therefore, work is done.
- Lift a book through a height. To do this you must apply force. The book rises. There is a force applied on the book and the book has moved. Hence, work is done.

Question 5.

1 A force of 5 N is acting on an object. The object is displaced through 2 m in the direction of the force. If the force acts on the object all through the displacement, find the work done.

Answer:

W = Fs

Work done = 5N × 2m = 10Nm (or) 10 J.

Question 6.

Write an activity to show the potential energy in a slinky.

Answer:

- Take a slinky as shown given figure.
- Ask a friend to hold one of its ends.
- You hold the other end and move away from your friend.
- Now you release the slinky.
- It will jump because, potential energy is stored in the slinky when it stretched.

Question 7.

Which energy is stored in a toy car ? Explain with an activity.

Answer:

- Take a toy car.
- Wind it using its key.
- Please the car on the ground.
- It moves.
- It acquires energy from our body while winding the key.
- The energy acquired depend on the number of windings.
- The energy which is possessed by the wounded key is potential energy.

Question 8.

Write an activity to describe potential energy possessed by an object at height.

Answer:

- Lift an object through a certain height.
- The object can now do work.
- It begins to fall when released.
- This implies that it has acquired some energy.
- If raised to a greater Height it can do more work and hence possesses more energy.
- It got the energy while lifting it to height.

### Important Questions on Work and Energy Class 9 – 5 Marks

Question 1.

Write 8 examples of kinetic energy.

Answer:

Examples of kinetic energy include :

- A moving car
- A swinging pendulum
- A roller coaster going downhill
- A spinning top
- A baseball being thrown
- Water flowing down a waterfall
- Wind blowing through the trees
- A bouncing ball

Question 2.

Write 8 examples of potential energy.

Answer:

Examples of potential energy include

- A stretched spring
- A book on a high shelf
- A battery being charged
- A compressed gas cylinder
- Water stored behind a dam
- A weight lifted off the ground
- A stretched rubber band
- An archer’s bow being drawn.

Question 3.

Express the kinetic energy of an object in the form of an equation.

(OR)

Derive E_{k} = mv^{2}

- Let us now express the kinetic energy of an object in the form of an equation.
- Consider an object of mass; m moving with a uniform velocity, u.
- Let it now be displaced through a distance ‘s’ when a constant force, ‘F acts on it in the direction of its displacement.
- The work done, W is Fs.
- The work done on the object will cause a change in its velocity.
- Let its velocity change from u to v. Let ‘a’ be the acceleration produced.
- The relation connecting the initial velocity (u) and final velocity (v) of an object moving with a uniform acceleration (a) and the displacement, s is v
^{2}– u^{2}= 2as. - s = \(\frac{\mathrm{v}^2-\mathrm{u}^2}{2 \mathrm{as}}\)
- We know F = ma ⇒ W = ma \(\frac{\mathrm{v}^2-\mathrm{u}^2}{2 \mathrm{as}}\) (or) W = \(\frac{1}{2}\)m(v
^{2}– u^{2})

If the object is starting from its stationary position, that is, u = 0, then W = \(\frac{1}{2}\) mv^{2}

It is clear that the work done is equal to the change in the kinetic energy of an object.

If u = 0, the work done will be \(\frac{1}{2}\) mv^{2}.

Thus, the kinetic energy possessed by an object of mass, m and moving with a uniform velocity, v is E_{k}= \(\frac{1}{2}\) mv^{2}.

Question 4.

What is the gravitational potential energy ? Derive a expression for it.

(OR)

Derive E_{p} = mgh.

Answer:

- The gravitational potential energy of an object at a point above the ground is defined as the work done in raising it from the ground to that point against gravity.
- Consider an object of mass, m. Let it be raised ht through a height, h from the ground.
- A force is required to do this.
- The minimum force required to raise the object is equal to the weight of the object, mg.
- The object gains energy equal to the work done on it.
- Let the work done on the object against gravity be W.
- That is, work done, W = force × displacement = mg × h = mgh
- Since work done on the object is equal to mgh, an energy equal to mgh units is gained by the object.
- This is the potential energy (E
_{p}) of the object. - E
_{p}= mgh

Question 5.

Explain law of conservation of energy for a free fall object.

Answer:

- Whenever energy gets transformed, the total energy remains unchanged. This is the law of conservation of energy.
- According to this law, energy can only be converted from one form to another; it can neither be created nor destroyed.
- The total energy before and after the transformation remains the same.
- Consider a pimple example. Let an object of mass, m be made to fall freely from a height, h. At the start, the potential energy is mgh and kinetic energy is zero.
- The kinetic energy zero because its velocity is zero.
- The total energy of the object is thus mgh. As it falls, its potential energy will change into kinetic energy.
- If v is the velocity of the object at a given instant, the kinetic energy would be 1/2mv
^{2} - As the fall of the object continues, the potential energy would decrease while the kinetic energy would increase.
- When the object is about to reach the ground, h = 0 and v will be the highest.
- Therefore, the kinetic energy would be the largest and potential energy the least.
- However, the sum of the potential energy and kinetic energy of the object would be the same at all points.
- That is, potential energy + kinetic energy = constant
- mgh + \(\frac{1}{2}\) mv
^{2}= constant - The sum of kinetic energy and potential energy of an object is its total mechanical energy.

Question 6.

ACTIVITY 1 : Kamali is preparing for examinations. She spends lot of time in studies. She reads books, draws diagrams, organises her thoughts etc.,

ACTIVITY 2: You sure working hard to push a huge rock. Let us say the rock does not move despite all the effort.

ACTIVITY 3 : You climb up the steps of a staircase and reach the second floor of a building just to see the landscape from there.

We have discussed in the above paragraphs a number of activities which we normally consider to be work in day-to-day life. For each of these activities, ask the following questions and answer them :

i) What is the work being done on ?

ii) What is happening to the object ?

iii) Who (what) is doing the work ?

Answer:

Activity | What is the work being done on ? | What is happening to the object ? | Who (what) is doing the work ? |

1. | Reading, drawing | No object | Kamali |

2. | Pushing a huge rock | No change | A person |

3. | Climbing steps | No object | A person |

Note : No work is done in the above three activities according to scientific definition of ‘work done’.

Question 7.

Lift an object up. Work is done by the force exerted by you on the object. The object moves upwards. The force you exerted is in the direction of displacement. However, there is the force of gravity acting on the object. Which one of these forces is doing positive work ? Which one is doing negative work ? Give reasons.

Answer:

1) When you lift an object up, the force you exert on the object is in the upward direction, which is the same direction as the displacement of the object. Therefore, the force you exert is doing positive work.

2) On the other hand, the force of gravity acting on the object is in the downward direction, which is opposite to the displacement of the object. Therefore, the force of gravity is doing negative work.

Question 8.

Write an activity to prove that an object at height can do more work.

Answer:

- Take a heavy ball.
- Drop it on a thick bed of sand. A wet bed of sand would be better.
- Drop the ball on the sand bed from a height of about 25 cm.
- The ball creates a depression.
- Repeat this activity from heights of 50 cm, 1 m and 1.5 m.
- Ensure that all the depressions are distinctly visible.
- Mark the depressions to indicate the height from which the ball was dropped.
- Compare their depths.
- The ball dropped from 1.5 m height gives deepest depression. ,
- Potential energy has caused the ball to make a deeper dent.

Question 9.

Prove that a moving object does work and it possesses energy.

(OR)

Show that an object moving faster can do more work than an identical object moving relatively slow.

Answer:

- Set up the apparatus as shown in adjacent figure.
- Place a wooden block of known mass in front of the trolley at a convenient fixed distance.
- Place a known mass on the pan so that the trolley starts moving. The trolley moves forward and hits

the wooden block. - Fix a stop on the table in such a manner that the trolley stops after hitting the block. The block gets displaced.
- Note down the displacement of the block. This means work is done on the block by the trolley as the block has gained energy.
- Repeat this activity by increasing the mass on the pan.
- We can observe that when mass increases the displacement also increases.
- In this activity, the moving trolley does work and hence it possesses energy.

Question 10.

Write an activity to show the potential energy.

Answer:

- Take a rubber band.
- Hold it at one end and pull from the other. ’
- The band stretches.
- Release the band at one of the ends.
- The band will tend to regain its original length.
- Obviously, the band had acquired energy in its stretched position.
- The energy gets stored due to the work done on the object.
- The energy transferred to an object is stored as potential energy.

Question 11.

Many of the human activities and the gadgets we use involve conversion of energy from one form to another. Make a list of such activities and gadgets. Identify in each activity/gadget the kind of energy conversion that takes place.

Answer:

Activity/Gadget | Type of Energy Conversion |

Car | Chemical (fuel) to Mechanical (movement) |

Air conditioner | Electrical to Thermal (cooling) |

Radio | Electrical to Sound |

Lamp | Electrical to Light |

Microwave | Electrical to Thermal (cooking) |

Television | Electrical to Light and Sound |

Blender | Electrical to Mechanical (blending) |

Cellphone | Electrical to Radio waves |

Computer | Electrical to Thermal (processing) and Sound (speakers) |

Hair dryer | Electrical to Thermal (drying) and Mechanical (blowing) |

Solar panels | Solar (light) to Electrical |

Wind turbine | Wind (kinetic) to Electrical |

Hydroelectric dam | Water (kinetic) to Electrical |

Generator | Mechanical to Electrical |

### Extra Questions on Work and Energy Class 9 – 4 Marks

Question 1.

An arrow is placed in the middle of a bow and then the string is stretched and released, the arrow fly.

• Identify the changes in energy in the above situation.

Answer:

Muscular Energy → Potential energy → Kinetic energy

II) Observe the diagrams I and II carefully. An object mass is lifted from A to B to a height ‘h’ along the path I and II.

• What would be the work done on the object in both cases ? Give reason for your answer.

Answer:

Work done on the object = mgh, work done by gravity not depend on path of travel, depends on height.