1
The scientific work of pushing a wall hard is zero because —
💡 Hint:
Being tired is a different matter.
Explanation:
There is a force, but the wall does not move, so the work is zero.
Class 9 · Science · Chapter 11 · बिहार बोर्ड (BSEB)CBSE · NCERT 2026-27
Work and Energy
Work and Energy
How to use this page:
1. Read — 11.1 daily work · 11.2 list · 11.3 displacement · 11.4 positive-negative · 11.5 sources · 11.6 sand · 11.7 trolley · 11.8 rubber · 11.9 slinky · 11.10 toy · 11.11 height · 11.12 bow · 11.13 nature · 11.14 gadgets · 11.15 table · 11.16 rope · 11.17 meter, diagram, worked example, board tip
2. Check — each lesson has its own questions; the number follows the lesson
3. Mastery ★ — all of that lesson correct. Redo the wrong ones
4. The picture to remember is a height bar, where the potential energy of a falling object falls as the kinetic energy rises by the same amount.
In the Bihar book this is chapter 11, while in the Exploration book work and energy is chapter 7. Progress stays in this browser.
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- 1 The scientific meaning of work — Activities 11.1, 11.2, 11.3
- 2 W = Fs and the sign — Activity 11.4
- 3 Kinetic energy — Activities 11.5, 11.6, 11.7
- 4 Potential energy — Activities 11.8 to 11.12
- 5 Forms of energy and conservation — Activities 11.13, 11.14, 11.15
- 6 Power and the kilowatt hour — Activities 11.16, 11.17
- Chapter winner — every lesson at mastery ★
🎬 Shorts
This chapter has no Short on the channel yet, so ten empty cards are not shown. The notes, diagrams and checks are complete.
📄 PDF download — notes + answer key
The scientific meaning of work — Activities 11.1, 11.2, 11.3
कार्य का वैज्ञानिक अर्थ — क्रियाकलाप 11.1, 11.2, 11.3 · Section 11.1 · Activities 11.1 to 11.3
Being tired and doing work are not the same thingNotes
In everyday speech, reading, thinking and pushing a wall are also called work. In science, work is done only when a force moves the object along its own direction. Kamali studies for a long time, but no force on the book displaces it, so the scientific work there is zero.
Push a wall hard. You get tired and the wall does not move. There is a force, the displacement is zero, so the work is zero. Stand still with a bag on your head and there is still no displacement and no work.
Activities 11.1, 11.2 and 11.3 — when work was doneActivity
In Activity 11.1 ask three things about everyday jobs: who is doing the work, which object it is, and what happened to the object. In Activity 11.2 make your own list and name the force and the object in each case.
Activity 11.3 asks for two opposite situations. First: a force acted but the object did not move, such as a wall. Second: the object moved but there was no force along that motion, such as a ball already rolling on a smooth table. In both, the scientific work stays zero.
Worked exampleExample
Question: A girl walks along a level road with a bag. How much work does gravity do?
Answer: Gravity is downward and the displacement is horizontal. The two are perpendicular, so the work by gravity is 0 J. Work against gravity is done only while the bag is being lifted.
10-second revision
- Work = force × displacement along that force
- If displacement is zero, work is zero
- A perpendicular force also does zero work
Board tip · BSEBBoard tip
A wall and holding a bag while standing both have no displacement. Write that in the answer.
Board tip · CBSEBoard tip
While walking on the level, gravity is perpendicular, so write its work as zero.
Check your understandingall correct = mastery ★
2
Work has only magnitude and no direction.
Explanation:
True — work is a scalar.
3
If a force acts and the object does not move, the work done is ______.
💡 Hint:
The first situation in Activity 11.3.
Explanation:
Zero. No displacement means no work.
4
While walking on a level road with a bag, the work by gravity is —
💡 Hint:
The force and the walk are perpendicular.
Explanation:
Gravity is downward and the walk is horizontal, so the work is zero.
5
Define work in science. Add one sentence about pushing a wall.
Model answer (board format)
Work is done when a force displaces an object along its direction. A wall does not move, so the work there is zero.
Check your answer:
W = Fs and the sign — Activity 11.4
W = Fs और चिह्न — क्रियाकलाप 11.4 · Sections 11.1.2–11.1.3 · Activity 11.4
The formula and the unitNotes
If a force F acts all along a displacement s in the same direction, the work done is W = F × s. If F = 1 N and s = 1 m, the work is 1 N m. That is called 1 joule. 1 J = 1 N m.
The book numbers: a 5 N force moves an object 2 m along itself, so W = 5 N × 2 m = 10 J. For 7 N and 8 m, W = 7 × 8 = 56 J.
Activity 11.4 — which force is positiveActivity
Lift an object. Your force is along the displacement, so it does positive work. Gravity is downward, opposite the displacement, so gravity does negative work.
Friction on a stopping vehicle also acts opposite the motion, so the work by friction is negative. The sign is about direction, while work itself stays a scalar.
Worked exampleExample
Question: A porter puts a 15 kg load on his head, 1.5 m above the ground. Take g = 10 m/s². Find the work he does.
Formula: W = F × s = mg × s.
Substitution: W = 15 kg × 10 m/s² × 1.5 m = 150 N × 1.5 m = 225 J.
10-second revision
- W = F × s and 1 J = 1 N m
- 5 N × 2 m = 10 J
- While lifting, your work is positive and gravity is negative
Board tip · BSEBBoard tip
In the porter answer find mg first, then multiply by the height. Write the unit J.
Board tip · CBSEBoard tip
The 56 J question is 7 N and 8 m. The 5 N and 2 m question is 10 J. Do not swap the numbers.
Check your understandingall correct = mastery ★
1
A 5 N force moves an object 2 m along itself. The work is —
💡 Hint:
W = F × s.
Explanation:
5 N × 2 m = 10 J.
2
Work is negative when the force is opposite the displacement.
Explanation:
True — gravity does negative work on an object being lifted.
3
1 joule = 1 ______ metre.
💡 Hint:
The unit of force.
Explanation:
1 J = 1 N m.
4
A force of 7 N slides an object 8 m along itself. Find the work. Show the step.
Model answer (board format)
W = F × s = 7 N × 8 m = 56 J.
Check your answer:
5
While a load is lifted, positive work is done by —
💡 Hint:
Activity 11.4.
Explanation:
The lifting force is along the displacement, so the work is positive.
6
Work is a vector, because force is a vector.
Explanation:
False — work has only magnitude. It is a scalar.
Kinetic energy — Activities 11.5, 11.6, 11.7
गतिज ऊर्जा — क्रियाकलाप 11.5, 11.6, 11.7 · Sections 11.2–11.2.2 · Activities 11.5 to 11.7
The capacity to do work because of motionNotes
Energy is the capacity to do work. A fast ball knocks a wicket over. This capacity due to motion is kinetic energy. The formula is KE = ½ mv². With m in kilograms and v in metres per second, the energy comes out in joules.
If the speed doubles, the energy becomes four times, because of the square. If the mass doubles, the energy doubles.
Activities 11.5, 11.6 and 11.7Activity
In Activity 11.5 list sources of energy and see which ones are tied to the Sun. Nuclear energy is the example a group discusses as not coming from present sunlight.
In Activity 11.6 drop a heavy ball on wet sand from 25 cm, 50 cm, 1 m and 1.5 m. The deepest dent is made by the ball that hits with the most kinetic energy, because the height was greater.
In Activity 11.7 put a mass on the pan so the trolley moves and hits a wooden block. A larger mass or a larger speed shifts the block more. Joule tested the conservation of energy. The unit of energy and work, the joule, is named after him.
Worked exampleExample
Question: An object of 15 kg moves at a uniform speed of 4 m/s. Find the kinetic energy.
Formula: KE = ½ mv².
Substitution: KE = ½ × 15 kg × (4 m/s)² = ½ × 15 × 16 = 120 J.
10-second revision
- KE = ½ mv², unit joule
- The square of the speed is used
- A ball dropped from a greater height makes a deeper dent in sand
Board tip · BSEBBoard tip
15 kg and 4 m/s give 120 J. Do not forget the half.
Board tip · CBSEBoard tip
In the sand answer join the depth to kinetic energy, not only to weight.
Check your understandingall correct = mastery ★
1
A 15 kg object has a speed of 4 m/s. The kinetic energy is —
💡 Hint:
Half × mass × speed squared.
Explanation:
½ × 15 × 16 = 120 J.
2
If the speed doubles, the kinetic energy also only doubles.
Explanation:
False — speed is squared, so the energy becomes four times.
3
The formula of kinetic energy is ½ m ______.
💡 Hint:
The square of the speed.
Explanation:
½ mv².
4
A ball is dropped on sand from 25 cm and from 1.5 m. Which dent is deeper, and why?
Model answer (board format)
The dent from 1.5 m is deeper. From a greater height the ball hits with more kinetic energy.
Check your answer:
Potential energy — Activities 11.8 to 11.12
स्थितिज ऊर्जा — क्रियाकलाप 11.8 से 11.12 · Sections 11.2.3–11.2.4 · Activities 11.8–11.12
Energy stored in shape or in heightNotes
Energy that is not yet changing the speed is stored as potential energy. A stretched rubber band, a compressed slinky and a toy car wound with a key hold energy in this way. More windings send the car farther.
The gravitational potential energy of a mass m at a height h is PE = mgh. It is the work used to take the object slowly up to h against gravity.
Activities 11.8 to 11.12Activity
In 11.8 stretch a rubber band and release it. It springs back, because energy was stored in the stretch. In 11.9 stretch or compress a slinky. In both cases energy comes from the change of shape.
In 11.10 wind a toy car and see whether the distance changes with the number of windings. In 11.11 raise an object. On release it can fall and do work. You gave the energy by doing work against gravity.
In 11.12 make a bamboo bow and a light arrow. Pulling the string changes the shape of the bow, and the potential energy becomes the kinetic energy of the arrow.
Worked exampleExample
Question: A 10 kg object is 6 m above the ground. Take g = 9.8 m/s². Find the potential energy.
Formula: PE = mgh.
Substitution: PE = 10 kg × 9.8 m/s² × 6 m = 98 × 6 = 588 J.
A second set: a 12 kg object has 480 J of potential energy and g = 10 m/s². h = 480 / (12 × 10) = 4 m.
10-second revision
- PE = mgh
- 10 kg, 6 m and g = 9.8 give 588 J
- A rubber band, a slinky, a key and a bow store potential energy of shape
Board tip · BSEBBoard tip
The 588 J uses g = 9.8. Do not take 10 and write 600 J.
Board tip · CBSEBoard tip
In the bow the form of energy changes: potential to kinetic. The total belongs to the next lesson.
Check your understandingall correct = mastery ★
1
A 10 kg object is at a height of 6 m. g = 9.8 m/s². The potential energy is —
💡 Hint:
mgh, and g here is 9.8.
Explanation:
10 × 9.8 × 6 = 588 J.
2
A stretched rubber band has potential energy.
Explanation:
True — on release it tries to regain its length.
3
The formula of gravitational potential energy is m g ______.
💡 Hint:
Height.
Explanation:
mgh.
4
A 12 kg object has a potential energy of 480 J. g = 10 m/s². Find the height.
Model answer (board format)
PE = mgh, so 480 = 12 × 10 × h. h = 480 / 120 = 4 m.
Check your answer:
5
The arrow flies when the bowstring is released because —
💡 Hint:
Activity 11.12.
Explanation:
Potential energy stored by the change of shape becomes the kinetic energy of the arrow.
Forms of energy and conservation — Activities 11.13, 11.14, 11.15
ऊर्जा का रूप और संरक्षण — क्रियाकलाप 11.13, 11.14, 11.15 · Sections 11.2.5–11.2.6 · Activities 11.13–11.15
The form changes, the total does notNotes
Mechanical energy is the sum of potential and kinetic energy. Heat, chemical, electrical and light energy are other forms. Green plants make food from sunlight. Coal and petrol are fuels from old living things. Air moves because of a pressure difference, and the water cycle also runs on the energy of the Sun.
When energy changes form, the total energy stays unchanged. That is the law of conservation of energy. If there is friction, some part becomes heat and sound, but that too is energy and is not destroyed.
Activities 11.13, 11.14 and 11.15Activity
In 11.13 ask in a group: how plants make food, why air moves, how coal formed, and which conversions run the water cycle. In 11.14 list household gadgets and name the change of form in each, such as a fan turning electrical energy into motion.
In 11.15 a 20 kg object falls from 4 m. Take g = 10 m/s². At the top Ep = 800 J and Ek = 0. For each 1 m downward, Ep falls by 200 J and Ek rises by 200 J. Just above the ground Ep = 0 and Ek = 800 J. The sum is 800 J in every row.
Worked exampleExample
Question: A 20 kg body is dropped from 4 m. g = 10 m/s². Find Ep and Ek at a height of 2 m.
Formula: Ep = mgh and Ek = total − Ep, because at the start the total = mg × 4.
Substitution: total = 20 × 10 × 4 = 800 J. At 2 m, Ep = 20 × 10 × 2 = 400 J. Ek = 800 − 400 = 400 J.
10-second revision
- When the form changes, the total energy stays the same
- 20 kg, 4 m and g = 10 give a total of 800 J
- At 2 m both Ep and Ek are 400 J
Board tip · BSEBBoard tip
In the table write the same sum in every row. Do not leave only Ep in one row.
Board tip · CBSEBoard tip
A fan and a bulb change to different forms: motion, and light plus heat.
Check your understandingall correct = mastery ★
1
A 20 kg body falls from 4 m. g = 10 m/s². At 2 m the kinetic energy is —
💡 Hint:
At half the height, half the potential energy remains.
Explanation:
The total is 800 J. Ep = 400 J, so Ek = 400 J.
2
Energy is destroyed while it changes form.
Explanation:
False — the total energy stays unchanged. Friction can turn it into heat.
3
In Activity 11.15, with g = 10, the total energy of 20 kg at 4 m is ______ J.
💡 Hint:
mgh = 20 × 10 × 4.
Explanation:
800 J.
4
In a bulb the main conversion is —
💡 Hint:
A gadget from Activity 11.14.
Explanation:
A bulb turns electrical energy into light and heat.
5
A 20 kg body falls from 4 m. g = 10 m/s². What is Ek just above the ground? Write the steps.
Model answer (board format)
At the start Ep = 20 × 10 × 4 = 800 J and Ek = 0. At the ground Ep = 0. By conservation Ek = 800 J.
Check your answer:
6
Green plants take their food energy only from the soil, not from the Sun.
Explanation:
False — in Activity 11.13 plants make food by taking energy from the Sun.
Power and the kilowatt hour — Activities 11.16, 11.17
शक्ति और किलोवाट घंटा — क्रियाकलाप 11.16, 11.17 · Sections 11.3–11.3.1 · Activities 11.16, 11.17
The rate of doing workNotes
Power is the rate of doing work. P = W / t. 1 watt is the power that does 1 joule of work per second. 1 W = 1 J/s. 1 kW = 1000 W.
The same work done in less time means more power. Average power = total energy used / total time. Large amounts are awkward in joules, so the commercial unit is the kilowatt hour. 1 kWh = 1000 × 3600 = 3.6 × 10^6 J. At home one unit means 1 kWh.
Activities 11.16 and 11.17Activity
In 11.16 two children of the same weight each climb 8 m up a rope. A takes 15 s and B takes 20 s. The work of both is the same, because the force and the height are the same. A, who takes less time, does more work in one second, so the power of A is greater.
In 11.17 read the household electric meter at 6:30 in the morning and at 6:30 in the evening, for about a week. Find the daytime and nighttime units separately. Do not open the meter cover. Write only the reading printed behind the glass.
Only look at the meterCaution
Touching the wires or the cover of the meter is dangerous. The activity is about the reading, not about repairs.
Worked exampleExample
Question: A boy of 50 kg climbs 45 steps in 9 s. Each step is 15 cm. g = 10 m/s². Find the power.
Formula: P = W / t = mgh / t.
Substitution: h = 45 × 0.15 m = 6.75 m. Weight = 50 × 10 = 500 N. W = 500 × 6.75 = 3375 J. P = 3375 / 9 = 375 W.
A lamp uses 1000 J in 10 s. P = 1000 / 10 = 100 W.
10-second revision
- P = W / t and 1 W = 1 J/s
- Same work, less time, more power
- 1 kWh = 3.6 × 10^6 J
Board tip · BSEBBoard tip
The step height is in centimetres. Turn 15 cm into 0.15 m before adding.
Board tip · CBSEBoard tip
A unit and a joule are different. The meter counts kWh, not joules.
Check your understandingall correct = mastery ★
1
A lamp uses 1000 J of electrical energy in 10 s. The power is —
💡 Hint:
P = W / t.
Explanation:
1000 J / 10 s = 100 W.
2
The person who does the same work in less time has more power.
Explanation:
True — power is the rate of doing work. In Activity 11.16 the power of A is greater.
3
1 kWh = ______ J.
💡 Hint:
1000 × 3600.
Explanation:
3600000 J, that is 3.6 × 10^6 J.
4
Two girls each weigh 400 N. Both climb 8 m. A takes 20 s and B takes 50 s. Find the power of each.
Model answer (board format)
W = 400 × 8 = 3200 J, the same for both. P of A = 3200 / 20 = 160 W. P of B = 3200 / 50 = 64 W.
Check your answer:
5
A household electric meter measures energy in —
💡 Hint:
One unit = 1 kWh.
Explanation:
The meter counts kilowatt hours. Activity 11.17 asks for this reading.
❓ Full question bank — with answers and explanations — 66 questions
Pick a type. The 31 lesson checks are separate — each lesson has as many as its topic needs. All correct earns mastery ★.
BSEB modelpractice · not annual60 tried · 0 correctMultiple choiceMCQ130 tried · 0 correctTrue / falseTrue / false80 tried · 0 correctFill in the blanksFill80 tried · 0 correctMatchMatch20 tried · 0 correctAssertion–reasonAssertion–reason50 tried · 0 correctBalancingBalancing40 tried · 0 correctIdentifyIdentify20 tried · 0 correctShort and longVSA, SA, LA120 tried · 0 correctCBSE-stylenot a PYQ60 tried · 0 correct
No question is marked as a verified past paper. The BSEB set is a model for practice. CBSE items are CBSE-style, not a copy of any year’s paper.
Multiple choice
0/13Pick one option.
1
Scientific work requires —
💡 Hint:
A wall tires you but the work is zero.
Explanation:
Both a force and a displacement along it are needed.
2
1 joule equals —
💡 Hint:
Force times distance.
Explanation:
1 J = 1 N m.
3
The work of 7 N × 8 m is —
💡 Hint:
A direct product.
Explanation:
56 J.
4
An example of negative work is —
💡 Hint:
The opposite direction.
Explanation:
Gravity does negative work while the object goes up.
5
In KE = ½ mv², if speed doubles the energy —
💡 Hint:
The square.
Explanation:
The square of the speed becomes four times.
6
In PE = mgh, 12 kg, 480 J and g = 10 give a height of —
💡 Hint:
h = PE / (mg).
Explanation:
480 / 120 = 4 m.
7
A 20 kg body at 4 m, g = 10. The potential energy is —
💡 Hint:
m g h.
Explanation:
20 × 10 × 4 = 800 J.
8
By conservation of energy, in a falling body —
💡 Hint:
Look at the bar in the figure.
Explanation:
If friction is left out, the sum stays constant.
9
Weight 400 N, 8 m, 20 s. The power is —
💡 Hint:
Work first, then divide by time.
Explanation:
W = 3200 J. P = 3200 / 20 = 160 W.
10
1 watt equals —
💡 Hint:
A rate.
Explanation:
1 W = 1 J/s.
11
1 kWh in joules is —
💡 Hint:
1000 × 3600.
Explanation:
3.6 × 10^6 J.
12
The energy in the key of a toy car is —
💡 Hint:
Activity 11.10.
Explanation:
Winding the key stores potential energy.
13
Work by gravity is zero in level walking because —
💡 Hint:
The directions are at 90°.
Explanation:
Gravity is vertical and the walk is horizontal.
True or false
0/81
Pushing a wall is scientific work, because it causes tiredness.
Explanation:
False — there is no displacement.
2
Work is a scalar.
Explanation:
True — it has only magnitude.
3
At 15 kg and 4 m/s the kinetic energy is 120 J.
Explanation:
True — ½ × 15 × 16 = 120.
4
For 10 kg at 6 m with g = 9.8, the potential energy is 600 J.
Explanation:
False — 10 × 9.8 × 6 = 588 J.
5
The potential energy of a bow can become the kinetic energy of the arrow.
Explanation:
True — Activity 11.12.
6
1 kWh equals 1 joule.
Explanation:
False — 1 kWh = 3.6 × 10^6 J.
7
Average power is total energy divided by total time.
Explanation:
True — that is the average of the rate.
8
The meter cover should be opened to take the reading.
Explanation:
False — read only the outside reading.
Fill in the blanks
0/81
The formula of work is W = F × ______.
💡 Hint:
s, the displacement.
Explanation:
Distance along the force.
2
Kinetic energy = ½ ______ v².
💡 Hint:
m.
Explanation:
In kilograms.
3
Potential energy = mg ______.
💡 Hint:
h.
Explanation:
In metres.
4
Power = work / ______.
💡 Hint:
Time.
Explanation:
A rate.
5
1 W = 1 J / ______.
💡 Hint:
A second.
Explanation:
Per second.
6
1 kW = ______ W.
💡 Hint:
1000 W.
Explanation:
The meaning of kilo.
7
At home 1 unit = 1 ______.
💡 Hint:
kWh.
Explanation:
The commercial unit.
8
The joule is named after ______.
💡 Hint:
Joule.
Explanation:
The name beside Activity 11.7.
Match
0/21
Match the quantity with its formula.
Column B: A. ½ mv² · B. mgh · C. W / t · D. F × s
1. Work
2. Kinetic energy
3. Potential energy
4. Power
Explanation:
Work is F s, kinetic is ½ mv², potential is mgh, power is W/t.
2
Match the situation with the kind of work.
Column B: A. Positive · B. Negative · C. Zero, no displacement · D. Zero, perpendicular
1. Pushing a wall
2. Lifting a load
3. Gravity while going up
4. Level walk, gravity
Explanation:
The wall is zero, lifting is positive, gravity is negative, and a level walk is perpendicular zero.
Assertion–reason
0/50 both true and the reason fits, 1 both true but the reason does not fit, 2 only the assertion, 3 only the reason.
1
Assertion (A): The scientific work of pushing a wall is zero.
Reason (R): The displacement is zero.
Explanation:
Both are true and R is the reason.
2
Assertion (A): 1 kWh equals 3.6 × 10^6 J.
Reason (R): Work is a vector.
Explanation:
A is true. R is false, because work is a scalar.
3
Assertion (A): The same work in less time means more power.
Reason (R): Power is the direction of displacement.
Explanation:
A is true. R is false — power is a rate, not a direction.
4
Assertion (A): If speed doubles, kinetic energy only doubles.
Reason (R): Kinetic energy uses the square of the speed.
Explanation:
A is false. R is true — so the energy becomes four times.
5
Assertion (A): In a falling body, kinetic energy rises as height falls.
Reason (R): Potential energy changes into kinetic energy and the total can stay constant.
Explanation:
Both are true and R explains A.
Coefficient practice
0/4These equations are not results of this chapter. They are only practice in filling coefficients for H2 + O2 → H2O.
1
This is coefficient practice. This equation is not a result of this chapter. Balance it by filling coefficients (blank = 1).
H2 + O2 → H2O
Explanation: 2H2 + O2 → 2H2O
2
This is coefficient practice. This equation is not a result of this chapter. Balance it by filling coefficients (blank = 1).
H2 + O2 → H2O
Explanation: 2H2 + O2 → 2H2O
3
This is coefficient practice. This equation is not a result of this chapter. Balance it by filling coefficients (blank = 1).
H2 + O2 → H2O
Explanation: 2H2 + O2 → 2H2O
4
This is coefficient practice. This equation is not a result of this chapter. Balance it by filling coefficients (blank = 1).
H2 + O2 → H2O
Explanation: 2H2 + O2 → 2H2O
Classify
0/21
Is the work in each situation zero or not?
Pushing a wall
Sliding 2 m by 5 N
Gravity on the level
Lifting 15 kg by 1.5 m
Explanation:
The wall and perpendicular gravity are zero. Sliding and lifting are not zero.
2
Place each example in a form of energy.
A moving ball
A stretched rubber band
Water at a height
The supply before a bulb glows
Explanation:
The ball is kinetic, the rubber band and the height are potential, and the supply is electrical.
Very short answer
0/51
Define 1 joule.
Model answer (board format)
1 joule is the work done when a force of 1 N displaces an object by 1 m along itself.
Check your answer:
2
Define power and write 1 watt.
Model answer (board format)
Power is the rate of doing work, P = W/t. 1 watt = 1 joule per second.
Check your answer:
3
Write the law of conservation of energy in one sentence.
Model answer (board format)
Energy is neither created nor destroyed, it only changes from one form to another and the total stays the same.
Check your answer:
4
Name the commercial unit of energy.
Model answer (board format)
The kilowatt hour, kWh. One unit equals this.
Check your answer:
5
Give one example each of positive and negative work.
Model answer (board format)
Lifting a load is positive. The work of gravity while going up is negative.
Check your answer:
Short answer
0/41
A porter puts a 15 kg load 1.5 m up. g = 10 m/s². Find the work.
Model answer (board format)
W = mg × s = 15 × 10 × 1.5 = 225 J.
Check your answer:
2
A 50 kg boy climbs 45 steps of 15 cm each in 9 s. g = 10 m/s². Find the power.
Model answer (board format)
h = 45 × 0.15 = 6.75 m. W = 500 × 6.75 = 3375 J. P = 3375 / 9 = 375 W.
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3
The speed of a car is to rise from 30 km/h to 60 km/h. Mass 1500 kg. How would you find the work done?
Model answer (board format)
Change the speeds to m/s: about 8.33 and 16.67. Work = final KE − initial KE = ½ m (v² − u²).
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4
Write the steps that turn 1 kWh into joules.
Model answer (board format)
1 kWh = 1000 W × 3600 s = 3600000 J = 3.6 × 10^6 J.
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Long answer
0/31
A 20 kg object falls from 4 m. g = 10 m/s². Make a table of Ep and Ek at 4 m, 2 m and the ground, and state conservation.
Model answer (board format)
At 4 m, Ep is 800 J and Ek is 0. At 2 m both are 400 J. At the ground Ep is 0 and Ek is 800 J. The sum is 800 J everywhere, which is conservation when no loss is assumed.
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2
Write the definitions of work, energy and power. Give one numerical example of each in one line.
Model answer (board format)
Work is force times displacement, 5 N × 2 m = 10 J. Energy is the capacity to do work, ½ × 15 × 16 = 120 J. Power is the rate of doing work, 1000 J / 10 s = 100 W.
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3
What do Activities 11.16 and 11.17 ask? Join power and the unit in your own words.
Model answer (board format)
In 11.16, for the same weight and the same height, the climb that takes less time has more power. In 11.17 the meter is read morning and evening to find daytime and nighttime units. One unit is 1 kWh, which is 3.6 × 10^6 J.
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BSEB model paper · practice
0/6This model set is for practice. It is not a question from any year’s annual examination.
1
The work of a porter lifting 15 kg by 1.5 m, g = 10, is —
💡 Hint:
mgh.
Explanation:
15 × 10 × 1.5 = 225 J.
2
Two children do the same work but A takes less time. Who has more power?
💡 Hint:
The rate.
Explanation:
A has more power.
3
The power of a 50 kg boy on 45 steps is ______ W when the time is 9 s and g = 10.
💡 Hint:
375 W.
Explanation:
h = 6.75 m.
4
Why is the work zero for a person standing with a bag on the head?
Model answer (board format)
There is a force but the displacement is zero, so W = F × 0 = 0.
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5
This is coefficient practice. This equation is not a result of this chapter. Balance H2 + O2 with H2O. A blank means 1.
H2 + O2 → H2O
Explanation: 2H2 + O2 → 2H2O
6
Write the formulas of potential and kinetic energy. Using the height bar, say how the energy is shared when a 20 kg body falls from 4 m. g = 10 m/s².
Model answer (board format)
PE = mgh and KE = ½ mv². At the start 800 J is potential. At half the height, 400 J is potential and 400 J is kinetic. At the ground 800 J is kinetic. The sum stays 800 J.
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CBSE-style questions
0/6These are competency-based practice questions. They are not copies of a CBSE paper.
1
A student says that a lot of work was done while reading a book. The best comment in science is —
💡 Hint:
The Kamali example.
Explanation:
Without displacement the work is zero.
2
A car has mass 1500 kg. Its speed changes from 0 to 4 m/s. The work done equals —
💡 Hint:
Work equals the kinetic energy gained.
Explanation:
½ × 1500 × 16 = 12000 J.
3
Assertion (A): One unit on the meter is 1 kilowatt hour.
Reason (R): 1 kWh = 3.6 × 10^6 J.
Explanation:
Both are true. R supports A by giving the value of the unit.
4
Assertion (A): Friction does negative work on a car that is stopping.
Reason (R): Negative work means that the displacement is zero.
Explanation:
A is true. R is false — in negative work the displacement is opposite the force, not zero.
5
Two friends of equal mass climb the same stairs with the same bag. One runs, the other walks. Compare the work and the power.
Model answer (board format)
The height and the weight are the same, so the work is about the same. The one who runs takes less time, so that power is greater.
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6
How do a fan and a drawn bow change the form of energy? Where does conservation show?
Model answer (board format)
A fan turns electrical energy into kinetic energy and some heat. A bow turns potential energy into the kinetic energy of the arrow. In both, the total energy remains and only the form changes.
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🏛️ Board exam corner — Bihar Board (BSEB)— CBSE
Switch board with BSEB | CBSE above. The lessons follow the same NCERT chapter.
BSEB model · practiceBoard tip
This page has no verified annual-exam question, because no source page has been added. The model set below is practice in the board pattern.
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CBSE-style · competency-based
These are case and assertion-reason practice items. Do not treat them as past CBSE questions.
🔁 Spaced review — today’s questions
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🧠 What you learned + equation sheet
What you learned
- Scientific work needs both a force and a displacement along that force.
- W = F × s, and 1 joule = 1 newton metre. Work is a scalar.
- Work is positive along the displacement and negative when the force is opposite.
- Kinetic energy is KE = ½ mv² and it rises quickly as speed rises.
- Gravitational potential energy is PE = mgh, and the total energy stays when the form changes.
- Power is P = W / t. 1 W = 1 J/s, and 1 kWh = 3.6 × 10^6 J.
| What | Keep this |
|---|---|
| Work | W = F × s |
| 1 joule | 1 N × 1 m |
| Kinetic energy | ½ mv² |
| Potential energy | mgh |
| Power | P = W / t |
| 1 kWh | 3.6 × 10^6 J |
The notes are original writing. The textbook was used only for activity order and numbers. “Verified” will be used only on a question that has a source page.