| Publisher | : Oriental Book Company Private Limited |
| Material | : Madhyamik Physical Science Solution |
| Subject | : Physical Science |
| Class | : 9 (Madhyamik) |
| Chapter Name | : Work, Power And Energy |
Very Short Answer Questions
Question 1
Define work.
Answer
Work is said to be done by or against a force when its point of application moves in the direction of the force and is measured by the product of the force and displacement of the point of application in the direction of the force.
Question 2
How much work is done by a man who tries to push the wall of a house but fails to do so?
Answer
If there is no displacement of the point of application of force, then no work is done. Since the man is applying force but the wall does not move, the work done is zero.
Question 3
What is the work done by the earth in moving around the sun?
Answer
The force acting on the earth due to the sun is a centripetal force, which acts perpendicular to the displacement of the earth. Since work done by a force perpendicular to displacement is zero, the work done by the earth in moving around the sun is zero.
Question 4
When is work done by a force negative?
Answer
Work done is negative when the displacement is in a direction opposite to the direction of the applied force, i.e., when the angle θ = 180° between force and displacement.
Question 5
When is work done by a force positive?
Answer
Work done is positive when the displacement of a body is in the direction of the force, i.e., when the angle θ = 0° between force and displacement.
Question 6
When is work done by a force zero?
Answer
Work done by a force is zero when:
- There is no displacement of the point of application of force.
- The displacement is perpendicular to the applied force (θ = 90°).
Question 7
What is the SI unit of work?
Answer
The SI unit of work is joule (J).
Question 8
Define power.
Answer
Power of an agent is the rate at which it does work. It is the work done per unit time.
Power = Work Done / Time
Question 9
What is SI unit of power?
Answer
The SI unit of power is watt (W).
Question 10
Does power depend on total amount of work done?
Answer
No, power does not depend on the total amount of work done, but on the time required to do the work.
Question 11
Define energy.
Answer
Energy is the capacity of a body to do work. It is expressed in joules in the SI system.
Question 12
What is SI unit of energy?
Answer
The SI unit of energy is joule (J).
Question 13
Define kinetic energy?
Answer
Kinetic energy is the energy acquired by a body by virtue of its motion. It is given by the formula:
Ek = 1/2 mv²
Question 14
Define potential energy.
Answer
Potential energy is the energy acquired by a body by virtue of its position or configuration. It is given by the formula:
Ep = mgh
Question 15
Identify the kind of energy possessed by a rolling stone.
Answer
A rolling stone possesses kinetic energy because it is in motion.
Question 16
State the law of conservation of energy.
Answer
The law of conservation of energy states that energy can neither be created nor destroyed; it can only be transformed from one form to another. The total energy of an isolated system remains constant.
Short Answer Questions
Question 1
Distinguish between positive and negative work. Give examples.
Answer
Positive work: When force and displacement are in the same direction (θ = 0°), work done is positive.
Example: Lifting an object upward.
Negative work: When force and displacement are in opposite directions (θ = 180°), work done is negative.
Example: Friction acting against motion.
Question 2
Which would have greater effect on the kinetic energy of an object—doubling the mass or doubling the velocity?
Answer
Kinetic energy is given by Ek = 1/2 mv².
- If mass is doubled, kinetic energy increases 2 times.
- If velocity is doubled, kinetic energy increases 4 times.
Thus, doubling velocity has a greater effect on kinetic energy.
Question 3
An object is dropped from a height h when is its (i) potential energy maximum, (ii) kinetic energy maximum?
Answer
(i) Potential energy is maximum at height h (at the starting point).
(ii) Kinetic energy is maximum when it reaches the ground (h = 0).
Question 4
Two bodies of same mass start from rest and move with velocity v and 2v respectively. Find the ratio of their kinetic energies.
Answer
Kinetic energy (Ek) = 1/2 mv²
For first body: Ek₁ = 1/2 m v²
For second body: Ek₂ = 1/2 m (2v)²
= 1/2 m × 4v²
= 4Ek₁
Ratio of kinetic energies = 1:4
Question 5
What do you mean by work and power? Give their units in SI.
Answer
Work: Work is said to be done when a force causes displacement in the direction of the force.
SI unit: Joule (J).
Power: Power is the rate of doing work. SI unit: Watt (W).
Question 6
What is energy? What is difference between potential and kinetic energies? What is watt? Is it possible to change the kinetic energy of a body without applying force?
Answer
Energy: The capacity of a body to do work.
Difference between potential and kinetic energy:
- Potential energy: Energy due to position (e.g., water in a dam).
- Kinetic energy: Energy due to motion (e.g., moving car).
Watt: SI unit of power (1 watt = 1 joule/second).
Changing kinetic energy without applying force: No, as per Newton’s laws, a force is required to change velocity.
Question 7
Explain the different units of work and power. Is work a vector or a scalar quantity?
Answer
Units of Work:
SI Unit: Joule (J) → Work done when a 1 Newton force moves a body 1 meter in its direction.
CGS Unit: Erg → Work done when a 1 Dyne force moves a body 1 cm in its direction.
- 1 Joule = 10⁷ Ergs
Other Units: Watt-hour, Kilowatt-hour (used in electrical energy calculations).
Units of Power:
SI Unit: Watt (W) → Power when 1 Joule of work is done in 1 second.
Other Units:
- Kilowatt (kW) → 1 kW = 1000 W
- Horsepower (hp) → 1 hp = 746 W
Work is a scalar quantity because it depends only on the magnitude of force and displacement, not direction.
Question 8
Give definition of power. What do you mean by horse power and watt? Obtain relation between them.
Answer
Power is the rate at which work is done or the rate of energy transfer.
Horsepower (hp) is a practical unit of power used for engines and machines.
1 Horsepower (hp) = 746 Watts (W)
Numerical Questions
Question 1
An object of mass 40 kg is raised to a height of 5m above the ground. What is its potential energy? If the object is allowed to fall, find its kinetic energy when it is halfway down.
[Ans. 200 J; 100 J]
Answer
Potential energy is given by the formula:
Potential energy = mass × gravitational acceleration × height
PE = 40 × 10 × 5
PE = 2000 J
At halfway down, the height is 2.5 m. Potential energy at this height:
PE_half = 40 × 10 × 2.5
PE_half = 1000 J
Since total energy remains conserved, kinetic energy at halfway down is:
KE_half = Total energy – PE_half
KE_half = 2000 – 1000
KE_half = 1000 J
Question 2
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.
[Ans. 210 J]
Answer
Work done is equal to the change in kinetic energy:
Initial KE = (1/2) × 20 × (5)²
= 250 J
Final KE = (1/2) × 20 × (2)²
= 40 J
Work done = Final KE – Initial KE
Work done = 250 – 40 = 210 J
Question 3
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?
[Ans. (1/2)mv²]
Answer
Work done = (1/2) m v²
Question 4
Calculate the work to be done to stop a car of 1500 kg moving at a velocity of 60 km/h.
[Ans. 208333.33 J]
Answer
Velocity = 60 km/h
= 60 × (1000/3600)
= 16.67 m/s
Kinetic energy of the car:
KE = (1/2) × 1500 × (16.67)²
KE = 208333.33 J
Since the work done to stop the car is equal to its kinetic energy, the required work is 208333.33 J.
Question 5
A 5 kg ball is thrown upward with a speed 10 ms⁻¹ (Take g = 10 ms⁻²).
(a) Calculate the maximum height attained by it.
(b) Find the potential energy when it reaches the highest point.
(c) Find the total energy when it is at a height of 5 m. What will be its kinetic energy when it just reaches the ground? [g = 9.8 ms⁻²]
[Ans. 5 m; 250 J]
Answer
(a) Calculate the maximum height attained by it.
Using the formula:
h = (v²) / (2g)
h = (10 × 10) / (2 × 10)
h = 5 m
(b) PE = m × g × h
PE = 5 × 10 × 5
PE = 250 J
(c) Total energy remains constant, so total energy at any height is 250 J.
At the ground, kinetic energy is equal to the total energy:
KE = 250 J
Question 6
A ball of mass 200 g falls from a height of 5 m. What will be the ratio of its initial energy to its energy when it reaches the ground?
[Ans. 1:1]
Answer
Initial energy = mgh
= 0.200 × 10 ×5
= 10 J
Final energy = 10 J
Ratio = Initial energy / Final energy = 1:1
Question 7
A body of mass 5 kg is moving with a velocity of 10 ms⁻¹. What will be the ratio of its kinetic energy, if the mass of the body be doubled and its velocity be halved? Calculate kinetic energy.
[Ans. 2:1]
Answer
Initial kinetic energy:
Initial KE = (1/2) × 5 × (10)²
= 250 J
New kinetic energy when mass is doubled and velocity is halved:
New KE = (1/2) × (2 × 5) × (5)²
= 125 J
Ratio of kinetic energy = 250:125 = 2:1
Question 8
Two bodies A and B of equal mass are kept at heights 20 m and 30 m respectively. Find the ratio of their potential energies.
[Ans. 2:3]
Answer
Potential energy is directly proportional to height.
PEA / PEB = hA / hB
= 20 / 30
= 2:3
Question 9
Calculate the height through which a body of mass 0.4 kg should be lifted if the energy spent for doing so is 1.0 J. [g = 10 ms⁻²]
[Ans. 0.2 m]
Answer
Energy (E) = mgh
h = E/mg
= 1/(0.4 × 10)
= 0.2 m
height (h) = 0.2 m
Question 10
The work done by the heart is 1 J per beat. Calculate the power of the heart if it beats 72 times in 1 minute.
[Ans. 1.2 W]
Answer
Power = Work done / Time
Power = (1 × 72) / 60
Power = 1.2 W
Question 11
6.4 KJ of energy causes a displacement of 64 m in a body in the direction of the force in 2.5s. Calculate (i) the force applied and (ii) power in horse power. [1 hp = 746W]
[Ans. (i) 100N (ii) 3-4 hp]
Answer
(i) Force applied:
Work done = Force × Displacement
Force = Work done / Displacement
Force = 6400 / 64
Force = 100 N
(ii) Power in horsepower:
Power = Work done / Time
Power = 6400 / 2.5
Power = 2560 W
Convert to horsepower:
Power in hp = 2560 / 746
Power in hp = 3.43 hp
So, the power is approximately 3.4 hp.
Question 12
Calculate the power of a pump which can lift 400 kg of water to store it in a water tank at a height of 19m in 40s. [Take g = 10 ms⁻²]
[Ans. 1900 W]
Answer
Power = Work done / Time
Work done = m × g × h
= 400 × 10 × 19
= 76000 J
Power = 76000 / 40
Power = 1900 W
Objective Questions
Fill in the blanks:
Question (i)
The ___ work is done, when the displacement is in the direction of the force.
Answer
Positive
Explanation:
Work is said to be positive when the displacement occurs in the direction of the applied force.
Question (ii)
In SI, the absolute unit of work is ___.
Answer
Joule
Explanation:
The SI unit of work is joule (J), which is defined as the work done when a force of one newton moves a body one meter in the direction of the force.
Question (iii)
Work done per unit time is called ___.
Answer
Power
Explanation:
Power is the rate at which work is done or energy is transferred per unit time.
Question (iv)
One horse power is equal to ___ watt.
Answer
746
Explanation:
One horsepower (hp) is equivalent to 746 watts (W) and is commonly used to measure engine power.
Question (v)
The capacity of a body to do work is ___.
Answer
Energy
Explanation:
Energy is defined as the capacity of a body to perform work.
Question (vi)
A stretched spring has potential energy due to its ___.
Answer
Shape
Explanation:
A stretched or compressed spring possesses potential energy due to its deformation, which is elastic potential energy.
Question (vii)
The gravitational potential energy of body when raised from ground to certain height is ___ of the path followed.
Answer
Independent
Explanation:
Gravitational potential energy depends only on the height and not on the path taken.
Question (viii)
Larger the mass of a body ___ is the kinetic energy of the body.
Answer
Greater
Explanation:
Kinetic energy is directly proportional to mass; hence, a larger mass results in greater kinetic energy.
Question (ix)
Total energy of a system is ___, when energy is changed from one form to other.
Answer
Conserved
Explanation:
According to the law of conservation of energy, the total energy in a system remains constant, though it may transform from one form to another.
Question (x)
The mechanical energy is of ___ types.
Answer
Two
Explanation:
Mechanical energy consists of potential energy and kinetic energy.
State whether true or false:
Question (i)
The absolute unit of work in CGS system is joule.
Answer
False
Explanation:
The absolute unit of work in the CGS system is erg, not joule.
Question (ii)
The work is a scalar quantity.
Answer
True
Explanation:
Work is a scalar quantity because it depends on magnitude only and not on direction.
Question (iii)
Power of an agent depends only on the total work done.
Answer
False
Explanation:
Power depends on both work done and the time taken to do the work.
Question (iv)
One of the practical unit of power is horse power.
Answer
True
Explanation:
Horsepower (hp) is a practical unit of power commonly used for engines.
Question (v)
Energy is expressed in the units of work.
Answer
True
Explanation:
Since energy is the capacity to do work, its unit is also joule, which is the same as that of work.
Question (vi)
Kinetic energy of a body is due to the position of the body.
Answer
False
Explanation:
Kinetic energy is due to motion, whereas potential energy is due to position.
Question (vii)
When the displacement is in the direction of the force, then the work done is called negative work.
Answer
False
Explanation:
Work done is positive when displacement occurs in the same direction as the force. It is negative when displacement is in the opposite direction.
Question (viii)
Centripetal force is a no work force.
Answer
True
Explanation:
Centripetal force acts perpendicular to displacement, so no work is done by it.
Question (ix)
The capacity of a body to do mechanical work is its mechanical energy.
Answer
True
Explanation:
Mechanical energy is the sum of kinetic and potential energy, which determines the capacity to do work.
Question (x)
Only mechanical energy is conserved.
Answer
False
Explanation:
The total energy (including mechanical, thermal, electrical, etc.) is conserved in an isolated system.
Match the following:
| Column-I | Column-II |
|---|---|
| (i) absolute unit of work | (a) watt |
| (ii) power | (b) kinetic energy |
| (iii) a stretched spring | (c) conservation of energy |
| (iv) flowing water | (d) joule |
| (v) freely falling body | (e) potential energy |
Answer
| Column-I | Column-II |
|---|---|
| (i) Absolute unit of work | (d) Joule |
| (ii) Power | (a) Watt |
| (iii) A stretched spring | (e) Potential energy |
| (iv) Flowing water | (b) Kinetic energy |
| (v) Freely falling body | (c) Conservation of energy |
Multiple Choice Questions
Question (i)
SI unit of energy is:
(a) joule
(b) erg
(c) dyne
(d) newton
Answer
(a) joule
Explanation: The SI unit of energy is joule (J).
Question (ii)
The potential energy of a boy is maximum when he is:
(a) sitting on the floor
(b) standing on the floor
(c) sleeping on the floor
(d) sitting on a chair
Answer
(d) sitting on a chair
Explanation: Potential energy depends on height. Sitting on a chair gives more height than sitting or sleeping on the floor.
Question (iii)
Water stored in a dam possesses:
(a) KE
(b) PE
(c) electric energy
(d) no energy
Answer
(b) PE
Explanation:
Water at rest in a dam has potential energy due to its height.
Question (iv)
The kinetic energy of an object is K. If its mass is reduced to half, then its kinetic energy will be:
(a) K
(b) 2K
(c) K/2
(d) K/4
Answer
(c) K/2
Explanation:
Kinetic energy is directly proportional to mass. If mass is halved, kinetic energy is also halved.
Question (v)
The work done on an object does not depend upon the:
(a) force applied
(b) initial velocity
(c) displacement
(d) angle between force and displacement
Answer
(b) initial velocity
Explanation:
Work depends on force, displacement, and the angle between them, but not on initial velocity.
Question (vi)
In case of negative work, the angle between the force and displacement is:
(a) 0°
(b) 45°
(c) 90°
(d) 180°
Answer
(d) 180°
Explanation:
Negative work occurs when force and displacement are in opposite directions.
Question (vii)
In a tug of war, work done by a losing team is:
(a) zero
(b) positive
(c) negative
(d) none of these
Answer
(c) negative
Explanation:
The losing team moves in the direction opposite to the applied force.
Question (viii)
A stone is thrown vertically upward. It comes to rest momentarily at the highest point. What happens to its kinetic energy?
(a) It converts into elastic potential energy
(b) It converts into gravitational potential energy
(c) It converts into chemical energy
(d) It is completely destroyed
Answer
(b) It converts into gravitational potential energy
Explanation:
When a stone is thrown upwards, its kinetic energy decreases as it rises and is completely converted into gravitational potential energy at the highest point. The energy is not lost but transformed.
Question (ix)
A force of 100N acting on a body does 1000 J work. The distance through which the body is displaced is:
(a) 5m
(b) 10m
(c) 1cm
(d) 50m
Answer
(b) 10m
Explanation:
Given:
Work = 1000 J
Force = 100 N
Displacement = Work / Force
= 1000 / 100
= 10 meters
Question (x)
When electric current passes through an electric bulb, electric energy is converted into:
(a) heat energy only
(b) light energy only
(c) both heat and light energy
(d) both light and chemical energy
Answer
(c) both heat and light energy
Explanation:
When electric current flows through a bulb, some energy is converted into light energy (visible glow), and the rest is dissipated as heat energy (bulb becomes hot).
