Work, Energy and Power

209 out of 320, against a target of 200 and a danger line of 160. She cleared it — on the hardest paper I have built for her.

This was the first paper calibrated to the Chaitanya standard: bands of 16 easy / 43 moderate / 40 hard instead of the usual 25/45/30, eleven per cent of it at five or more linked steps, with calculus, infinite series and vector forms that none of her earlier papers contained.

And she scored higher on the hard band than on the easy one — 75% against 69%. That is not a fluke of sampling; it is the same thing the 1 November analysis found. She is not losing marks to difficulty. She is losing them to five mechanical habits, and this paper names them precisely.

The score

57 right, 19 wrong, 4 unanswered. 57×4 − 19 = 209. Of the 111 marks lost, 95 went to wrong answers and 16 to the four she did not attempt.

Where the 95 marks went

Band	Right	Of	Rate
Easy	9	13	69%
Moderate	24	35	69%
Hard	24	32	75%

Section	R	W	B
Work done by a force	10	2	0
Power	8	2	0
Potential energy and its conservation	8	2	0
Elastic collisions in one dimension	6	5	1
Inelastic collisions	10	1	1
Restitution and repeated impacts	5	1	2
Collisions in two dimensions and explosions	5	3	0
Motion in a vertical circle	5	3	0

Family	Questions	Marks
An operation dropped part-way through	Q7, Q24, Q39, Q77	20
EXCEPT stems — a true statement chosen	Q11, Q56, Q72	15
Elastic-collision limiting cases	Q36, Q38, Q66	15
Constant power — one idea, both questions	Q17, Q18	10
An expression written upside down	Q30, Q70	10
Assertion–Reason — both to option (1)	Q64, Q80	10
Answered the question next door	Q34	5
Reading a graph	Q40	5
A five-step chain	Q76	5

20 marks — the most expensive

1. The method was right and one operation went missing

Q	What it asked	Hers	Right	The one step that vanished
Q7	work done as v goes from (3î+4ĵ) to (6î+8ĵ) m s⁻¹, m = 2 kg	10 J	75 J	She took the speeds, 5 and 10, and did 2×(10−5). The kinetic energy needs the square: ½×2×(100−25) = 75.
Q24	U = (3x² + 4y²) J, force at (2 m, 1 m)	−(6î + 8ĵ)	−(12î + 8ĵ)	−∂U/∂x = −6x, and she then wrote 6 instead of putting x = 2 in. The y half, 8y with y = 1, she did correctly — so she knew the method and substituted into only one of the two terms.
Q39	4 kg at 5 m s⁻¹ hits 6 kg at rest, elastic	1 and 4	−1 and 4	Both magnitudes right. The first trolley rebounds, so its velocity is negative. She dropped the sign, not the physics.
Q77	tension at the lowest point, 0.5 kg on a 1 m string just completing the circle	5 N	30 N	5 N is mg. At the lowest point of a circle that is just completed the tension is 6mg, and she stopped at the weight.

Four questions, four different chapters, and in every one the physics was chosen correctly and then an operation was left out — a square, a substitution, a sign, a factor of six. This is the same family that cost her 25 marks on 1 November, where it showed up as powers of ten.

Before writing the answer down, read the expression back and ask one question of it: have I put every number in, and is the sign right? On Q24 that single reading would have caught the missing x = 2 in under five seconds.

15 marks — and a regression

2. On EXCEPT questions she chose a statement because it was true

Q	The question	She chose	The answer
Q11	Each of these is a unit of energy, EXCEPT one	kilowatt hour	watt
Q56	In a perfectly inelastic collision each of these is conserved, EXCEPT one	the total energy	the total kinetic energy
Q72	For a two-dimensional collision each of these is true, EXCEPT one	the velocity of the centre of mass is unchanged	kinetic energy is conserved whatever the collision

All three times she picked a statement that is true. Kilowatt hour is energy. Total energy is conserved. The centre of mass velocity is unchanged. She was not confused about the physics in any of the three — she answered the question the stem was not asking.

This is a regression. On the four-chapter test of 1 November she went 7 of 8 on negative stems and the family was marked settled in her log. Here it is 2 of 5.

When the stem carries EXCEPT, NOT or wrong, put a mark against every option that is true as you read it. The answer is the one left unmarked. Never choose the first option that reads correctly — on these questions a correct-reading option is the trap.

10 marks — and the cleanest finding on the paper

3. Assertion–Reason: she got both the ones she marked up, and lost both the ones she did not

Q	Did she annotate it?	Result
Q10	Yes — a red X drawn straight through the Assertion sentence, marking it false	right
Q21	Yes — flagged in orange and worked on	right
Q64	No. The printed working line is completely empty.	wrong — chose (1)
Q80	No. The printed working line is completely empty.	wrong — chose (1)

Two marked, two right. Two unmarked, two wrong — and both defaulted to option (1). This is not a knowledge gap. On Q10 she judged an Assertion false by drawing an X on it and answered correctly; four questions later she stopped doing it.

On Q64 the ball bounces lower because energy is lost at each impact, not because momentum is not conserved during it — both statements are true and the second does not cause the first. On Q80 the bob leaves the circle because the tension reaches zero; energy conservation is true but explains only why it slows, not where it lets go. Two true statements side by side are not a cause and its effect.

The line is printed under every one of these questions for exactly this. A: true or false. R: true or false. Then, separately, does R explain A? She has already proved it works — it is 2 for 2 on this paper.

25 marks

4. Two pockets of real physics to revise

Constant power — Q17 and Q18, one idea and she lost both. With P constant, v ∝ t^1/2, s ∝ t^3/2 and v ∝ s^1/3. She gave s^1/2 and t. Worth writing those three relations out once and keeping them.

Elastic-collision limiting cases — Q36, Q38, Q66. A light body on a very heavy one rebounds with its whole speed, not half (Q38). Energy transfer is greatest when the masses are equal, which is why hydrogen moderates better than deuterium (Q36). Two equal masses colliding obliquely, one at rest, separate at 90° (Q66). These three results carry a disproportionate number of NEET questions.

Q34 is the one to look at hardest, because it is not a knowledge gap at all: asked what fraction the neutron retains, she computed ²⁴⁄₄₉, which is the fraction it transfers. The arithmetic was perfect and she answered the other question. She had also flagged this one and come back to it.

What went right — and it is a lot

The working appeared, and it worked

Red annotation on nine questions — Q3, Q6, Q9, Q10, Q15, Q21, Q34, Q46 and Q73 — and seven of the nine are right. On the four-chapter test the working reached seven questions out of 180; here it reached nine out of 80, on a harder paper. It is also more varied than before: a dashed line splitting the force–displacement graph into two blocks with their areas written in (Q3), link lines drawn across a match table (Q9), an X through a false Assertion (Q10), a full free-body diagram on the incline (Q6).

But look at what she wrote on the two incline-and-string diagrams. On Q6 she drew the arrows in the right places and then labelled the along-the-slope arrow mg cos θ and the vertical one mg sin θ. On Q46 she did it again: an arrow up along the string labelled mg cos, a second arrow straight down labelled mg sin. Both times the two labels are the wrong way round — the component along the slope is mg sin θ, the one perpendicular to it is mg cos θ.

This is the most important thing on the paper, and it does not show up in the score at all. Q6 came out right, because the result is a standard one she already knows — the wrong diagram did no damage. Q46 is the one she left blank. She resolved the forces, labelled them the wrong way round, and the numbers stopped making sense, so she moved on. Four marks, and the next time it will be a confident wrong answer instead of a blank.

And one old fault alongside it. Q46 carries the most sustained calculation on the whole script and no option was ticked. That is the third time she has finished work and not answered: Alcohols Q8, full test Q156, now this.

Draw the slope triangle once, label it correctly, and keep that sheet: along the slope mg sin θ, into the slope mg cos θ. The memory hook is that at θ = 0 the slope is flat, nothing slides, and sin 0 = 0. And when the working stops, look back at the options before turning the page.

Match the column, clean again

3 of 3. With the four on the Chaitanya screen test, that is seven consecutive match questions without an adjacent swap — the family that cost her 20 marks on the four-chapter test and seven questions across four papers before that. On this evidence it is fixed.

Flagging, and only four left unanswered

She flagged Q21 and Q34 — this time with an orange underline beneath the question number rather than a ring — and came back to both. Q21 is right; Q34 is the retained-versus-transferred slip above.

Four questions unanswered out of 80. Three of them, Q46, Q58 and Q61, are multi-step numericals, which is the pattern the November analysis predicted. The fourth, Q33, is not: it is an easy conceptual question — equal masses, head-on, one at rest — with no working beside it and no flag on it. That one is worth asking her about directly.

Thirteenth paper running

Two things still not done

The cover blanks are empty again — no name, no date, no start or end time. Thirteen papers now with no timing data at all, which is the single most useful number I still do not have. Two seconds of writing would tell us whether the four blanks are a pacing problem or a confidence one.

The response grid on the last page is untouched, also for the thirteenth time. This matters more than it used to: her NEET in 2027 is a computer-based test, where the answer is committed in a separate place from the question. The grid is the closest thing a printed paper has to that, and it is the habit the real exam will demand.

What to carry into the next paper

57 right, 19 wrong, 4 unanswered: 57×4 − 19 = 209. Every lost mark is accounted for — 19×5 = 95 to wrong answers and 4×4 = 16 to the unanswered, totalling 111 from 320. The script was read by the geometric tick-finder, and every wrong answer and every blank was then re-checked by eye against the page image; the two readings agree on all of them. This paper was built to the Chaitanya difficulty envelope measured on 2 November: bands 16/43/40 against the 25/45/30 of her earlier papers.

Work, Energy and Power — error notes