Q.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’.
Rule (Science): Work is done only when force causes displacement in the direction of force.
(a) Suma is swimming in a pond. ✅ Work is done
Answer: Yes. She applies force on water and her body moves forward (displacement).
Answer: Yes. She applies force on water and her body moves forward (displacement).
(b) A donkey is carrying a load on its back. ❌ Work is NOT done (on the load)
Answer: The force by donkey on load is vertical, but displacement is horizontal. No work on load (in ideal case).
Answer: The force by donkey on load is vertical, but displacement is horizontal. No work on load (in ideal case).
(c) A wind-mill is lifting water from a well. ✅ Work is done
Answer: Water is lifted upward against gravity (displacement upward).
Answer: Water is lifted upward against gravity (displacement upward).
(d) A green plant is carrying out photosynthesis. ❌ No mechanical work (in physics sense)
Answer: It is a chemical process; not described as mechanical work (force–displacement).
Answer: It is a chemical process; not described as mechanical work (force–displacement).
(e) An engine is pulling a train. ✅ Work is done
Answer: Engine applies force and train moves (displacement).
Answer: Engine applies force and train moves (displacement).
(f) Food grains are getting dried in the sun. ❌ No mechanical work
Answer: Drying is due to heat; not a force causing displacement.
Answer: Drying is due to heat; not a force causing displacement.
(g) A sailboat is moving due to wind energy. ✅ Work is done
Answer: Wind applies force on sail and boat moves.
Answer: Wind applies force on sail and boat moves.
Answer
(a) Suma is swimming in a pond. ✅ Work is done
Answer: Yes. She applies force on water and her body moves forward (displacement).
(b) A donkey is carrying a load on its back. ❌ Work is NOT done (on the load)
Answer: The force by donkey on load is vertical, but displacement is horizontal. No work on load (in ideal case).
(c) A wind-mill is lifting water from a well. ✅ Work is done
Answer: Water is lifted upward against gravity (displacement upward).
(d) A green plant is carrying out photosynthesis. ❌ No mechanical work (in physics sense)
Answer: It is a chemical process; not described as mechanical work (force–displacement).
(e) An engine is pulling a train. ✅ Work is done
Answer: Engine applies force and train moves (displacement).
(f) Food grains are getting dried in the sun. ❌ No mechanical work
Answer: Drying is due to heat; not a force causing displacement.
(g) A sailboat is moving due to wind energy. ✅ Work is done
Answer: Wind applies force on sail and boat moves.
Answer: Yes. She applies force on water and her body moves forward (displacement).
(b) A donkey is carrying a load on its back. ❌ Work is NOT done (on the load)
Answer: The force by donkey on load is vertical, but displacement is horizontal. No work on load (in ideal case).
(c) A wind-mill is lifting water from a well. ✅ Work is done
Answer: Water is lifted upward against gravity (displacement upward).
(d) A green plant is carrying out photosynthesis. ❌ No mechanical work (in physics sense)
Answer: It is a chemical process; not described as mechanical work (force–displacement).
(e) An engine is pulling a train. ✅ Work is done
Answer: Engine applies force and train moves (displacement).
(f) Food grains are getting dried in the sun. ❌ No mechanical work
Answer: Drying is due to heat; not a force causing displacement.
(g) A sailboat is moving due to wind energy. ✅ Work is done
Answer: Wind applies force on sail and boat moves.
Did you know 🤯
In physics, even “very tiring” tasks may count as zero work if there is no displacement (like holding weight).
Q.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 gravition the object?
Answer
Answer:
Gravity is a conservative force. Work done depends only on change in height.
Initial and final points are on the same horizontal line ⇒ same height ⇒ \( \Delta h = 0 \)
Gravity is a conservative force. Work done depends only on change in height.
Initial and final points are on the same horizontal line ⇒ same height ⇒ \( \Delta h = 0 \)
\[
W_g = mg(h_i - h_f) = mg(0) = 0
\]
Finally: Work done by gravity = 0 J
Did you know 🤯
Gravity may do positive work while going up? Actually it does negative on ascent and positive on descent, total becomes zero if it returns to same height.
Q.3: A battery lights a bulb. Describe the energy changes involved in the process.
Answer
Answer:
Battery has chemical energy → converts to electrical energy → bulb converts it to light + heat energy.
Finally : Chemical → Electrical → Light + Heat
Battery has chemical energy → converts to electrical energy → bulb converts it to light + heat energy.
Finally : Chemical → Electrical → Light + Heat
Did you know 🤯
An ordinary bulb wastes more energy as heat than as light.
Q.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
Answer:
Use work-energy theorem:
Use work-energy theorem:
\[
W = \Delta K = \frac{1}{2}m(v^2 - u^2)
\]
Given: m = 20 kg, u = 5 m/s, v = 2 m/s
\[
W = \frac{1}{2}\times 20(2^2 - 5^2) = 10(4 - 25) = 10(-21) = -210\, \text{J}
\]
Easy meaning: Negative work means the force slows down the object.
Did you know 🤯
Brakes in vehicles do negative work to reduce speed.
Q.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.
Answer
Answer:
Gravity acts vertically downward, displacement is horizontal.
Angle between force and displacement = \(90^\circ\)
Gravity acts vertically downward, displacement is horizontal.
Angle between force and displacement = \(90^\circ\)
\[
W = F s \cos 90^\circ = 0
\]
Did you know 🤯
That’s why carrying a bag while walking straight doesn’t count as work done by your force on the bag (in physics).
Q.6: The potential energy of a freely falling object decreases progressively. Does this violate the law of conservation of energy? Why?
Answer
Answer:
No. As the object falls, potential energy decreases but kinetic energy increases by the same amount (ignoring air resistance).
PE+KE="constant"
No. As the object falls, potential energy decreases but kinetic energy increases by the same amount (ignoring air resistance).
PE+KE="constant"
Finally : No violation. Energy changes form: PE→KE.
Did you know 🤯
With air resistance, some mechanical energy converts into heat and sound, still total energy remains conserved.
Q.7: What are the various energy transformations that occur when you are riding a bicycle?
Answer
Answer:
Food energy (chemical) → muscle energy
Muscle energy → mechanical energy (pedalling)
Mechanical → kinetic energy (cycle moves)
Some energy → heat due to friction (tyres, chain, brakes) + sound
Finally: Chemical → Mechanical → Kinetic (+ Heat + Sound)
Food energy (chemical) → muscle energy
Muscle energy → mechanical energy (pedalling)
Mechanical → kinetic energy (cycle moves)
Some energy → heat due to friction (tyres, chain, brakes) + sound
Finally: Chemical → Mechanical → Kinetic (+ Heat + Sound)
Did you know 🤯
Most energy loss in cycling on flat roads is due to air resistance, not just friction.
Q.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?
Answer
Answer:
Displacement s=0⇒ work on rock:
Displacement s=0⇒ work on rock:
\[
W = F \cdot s = 0
\]
So no energy is transferred to the rock as mechanical work.
But your body still uses energy, which converts into heat in muscles, internal energy, and sweating.
Finally: No work on rock; energy goes into your body (heat, fatigue).
But your body still uses energy, which converts into heat in muscles, internal energy, and sweating.
Finally: No work on rock; energy goes into your body (heat, fatigue).
Did you know 🤯
Even when external work is zero, your muscles consume energy due to internal processes.
Q.9: A certain household has consumed 250 units of energy during a month. How much energy is this in joules?
Answer
Answer:
1 unit = 1 kWh
1 unit = 1 kWh
\[
1\, \text{kWh} = 1000\, \text{W} \times 3600\, \text{s} = 3.6 \times 10^6\, \text{J}
\]
\[
250\, \text{units} = 250 \times 3.6 \times 10^6 = 9.0 \times 10^8\, \text{J}
\]
Did you know 🤯
Electricity bills are in kWh because joule is too small for daily use.
Q.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.
Answer
Answer:
(a) Potential energy at 5 m
(a) Potential energy at 5 m
\[
PE = mgh = 40 \times 9.8 \times 5 = 1960\, \text{J}
\]
(b) KE when halfway down
Halfway down means height = 2.5 m
Remaining PE at that point:
Halfway down means height = 2.5 m
Remaining PE at that point:
\[
PE_{2.5} = 40 \times 9.8 \times 2.5 = 980\, \text{J}
\]
Total mechanical energy = 1960 J (constant, ignoring air resistance)
\[
KE = 1960 - 980 = 980\, \text{J}
\]
Did you know 🤯
At halfway height, PE and KE become equal (if dropped from rest).
Q.11: What is the work done by the force of gravity on a satellite moving round the earth? Justify your answer.
Answer
Answer:
Gravity acts towards the centre, while satellite displacement is tangential.
So force is perpendicular to displacement at every instant.
Gravity acts towards the centre, while satellite displacement is tangential.
So force is perpendicular to displacement at every instant.
\[
W = F s \cos 90^\circ = 0
\]
Finally: ▭(0 J)
Did you know 🤯
Even with zero work by gravity, gravity still provides the centripetal force needed for circular motion.
Q.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
Answer:
Yes. If net force is zero, an object can still move with constant velocity (Newton’s first law).
Example: In space, a moving object keeps moving without force.
Finally: Yes, displacement can happen without any net force.
Yes. If net force is zero, an object can still move with constant velocity (Newton’s first law).
Example: In space, a moving object keeps moving without force.
Finally: Yes, displacement can happen without any net force.
Did you know 🤯
Astronauts throw a tool in space and it keeps moving until another force acts.
Q.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
Answer:
Displacement of bundle is zero.
Displacement of bundle is zero.
\[
W = F \cdot s = 0
\]
Finally: In physics, no work is done on the bundle.
But he gets tired because body energy converts into heat internally.
But he gets tired because body energy converts into heat internally.
Did you know 🤯
Your muscles consume energy even when you “hold still” due to continuous muscle tension.
Q.14: An electric heater is rated 1500 W. How much energy does it use in 10 hours?
Answer
Answer:
E = Pt
E = Pt
Convert time:
10 h = 10 × 3600 = 36000 s
10 h = 10 × 3600 = 36000 s
\[
E = 1500 \times 36000 = 5.4 \times 10^7\, \text{J}
\]
▭(5.4×10^7 J)
(Also in kWh: 1.5 kW × 10h = 15 kWh)
(Also in kWh: 1.5 kW × 10h = 15 kWh)
Did you know 🤯
Heaters convert almost all electrical energy into heat, so they’re near 100% efficient for heating.
Q.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?
Answer
Answer:
When bob is pulled to one side:
At extreme position: PE maximum, KE = 0
At mean position: KE maximum, PE minimum
It keeps converting: PE↔KE
When bob is pulled to one side:
At extreme position: PE maximum, KE = 0
At mean position: KE maximum, PE minimum
It keeps converting: PE↔KE
It eventually stops due to air resistance and friction at pivot, which convert mechanical energy into heat and sound.
No violation because total energy (including heat) is conserved.
No violation because total energy (including heat) is conserved.
Finally: Mechanical energy decreases, but total energy is conserved (becomes heat/sound).
Did you know 🤯
Clocks use pendulums; they need small energy input to keep oscillating due to losses.
Q.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
Answer:
Initial KE:
Initial KE:
\[
K_i = \frac{1}{2}mv^2
\]
Final KE = 0
Work required (by external retarding force):
Work required (by external retarding force):
\[
W = \Delta K = 0 - \frac{1}{2}mv^2 = -\frac{1}{2}mv^2
\]
▭(W = -1/2 mv^2 )
Easy meaning: Negative sign shows work is done against motion to stop it.
Easy meaning: Negative sign shows work is done against motion to stop it.
Did you know 🤯
Stopping distance increases a lot with speed because KE depends on v^2.
Q.17: Calculate the work required to be done to stop a car of 1500 kg moving at a velocity of 60 km/h?
Answer
Answer:
Convert speed:
Convert speed:
\[
60\, \text{km/h} = \frac{60 \times 1000}{3600} = 16.67\, \text{m/s}
\]
Work to stop:
\[
W = -\frac{1}{2}mv^2 = -\frac{1}{2} \times 1500 \times (16.67)^2
\]
(16.67)^2ⓜ≈277.78┤
\[
W \approx -750 \times 277.78 \approx -208335\, \text{J}
\]
▭(≈-2.08×10^5 J)
Did you know 🤯
Doubling car speed needs about 4 times braking work to stop it.
Q.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.
Fig: Direction of force and displacement (NCERT Class 9 Science – Work and Energy)
Answer
General rule:
- Force and displacement same direction → Positive
- Opposite direction → Negative
- Perpendicular → Zero
Did you know 🤯
In circular motion, centripetal force is perpendicular to motion, so its work is zero.
Q.19: Riya 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
Answer: Yes, if the net (resultant) force is zero.
Forces can cancel each other.
Forces can cancel each other.
\[
F_{\text{net}} = 0 \Rightarrow a = 0
\]
Yes, I agree. Balanced forces give zero acceleration.
Did you know 🤯
A book lying on a table has multiple forces (weight and normal) but acceleration is zero.
Q.20: Find the energy in joules consumed in 10 hours by four devices of power 500 W each.
Answer
Answer:
Total power:P=4×500=2000 W
Total power:P=4×500=2000 W
Time:t=10h=36000s
\[
E = Pt = 2000 \times 36000 = 7.2 \times 10^7\, \text{J}
\]
▭(7.2×10^7 J)
Did you know 🤯
In units: 2 kW×10h=20 kWh
Q.21: A freely falling object eventually stops on reaching the ground. What happenes to its kinetic energy?
Answer
Answer: When it hits the ground, its kinetic energy converts into:
- Heat 🔥
- Sound 🔊
- Deformation energy (dent, bounce) 🧱
Did you know 🤯
A rubber ball bounces because part of KE temporarily stores as elastic potential energy and returns back.
❓ Frequently Asked Questions – Work & Energy (Class 9)
Quick answers for NCERT Solutions, concepts, and numericals.
Q What is work according to science in Class 9?
In science, work is done when a force causes displacement in the direction of the force.
Q When is work said to be done in physics?
Work happens only if force acts and the object moves in the same direction.
Q What is the SI unit of work and energy?
The SI unit of both work and energy is joule (J).
Q What is kinetic energy? Write its formula.
Kinetic energy is energy due to motion. Formula: KE = ½mv².
Q What is potential energy?
Potential energy is energy due to position or configuration.
Q What is power in physics?
Power is the rate of doing work. SI unit: watt (W).
Q Are NCERT solutions enough for Class 9 Work and Energy?
Yes. NCERT is usually enough for school exams because questions follow the textbook + CBSE pattern.
Q Are these solutions based on the latest CBSE syllabus?
Yes, these solutions are written as per the latest CBSE Class 9 syllabus.