Q.If ∣A∣=2 and ∣B∣=4, then match the relations in column I with the angle θ between A and B in column II Column I | Column II
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🔒 Start your 14-day free trial to unlock the full solution →Concept understanding — Triangle Inequality
Triangle Inequality (Bounding the Resultant of Two Vectors)
When you combine two displacements, two velocities, or two forces in "Motion in a Plane," you add them as vectors using the triangle law: place the tail of the second vector at the head of the first, and the resultant runs from the start to the finish. The triangle inequality is simply the statement that this resultant can never be longer than the two vectors laid end to end, and can never be shorter than their difference.
The Intuition
Suppose you walk 3 m in one direction, then 4 m in some other direction. Could you end up 8 m from where you started? No — the farthest you can possibly get is 3+4=7 m, and that only happens if both walks point the same way, so there's no bend at all (a "flat" triangle). The moment the second walk points in a different direction, a real corner appears, and cutting across that corner (the direct path) is always shorter than going via the corner. That's the geometric heart of every triangle: any one side is shorter than the sum of the other two, unless the triangle collapses onto a straight line.
The Precise Statement
For two vectors A and B added by the triangle law, the magnitude of the resultant R=A+B is bounded on both sides:
∣A∣−∣B∣≤∣A+B∣≤∣A∣+∣B∣
- The upper bound ∣A+B∣≤∣A∣+∣B∣ is reached only when A and B point in exactly the same direction (the angle between them is 0∘) — the triangle flattens out.
- The lower bound ∣A+B∣≥∣A∣−∣B∣ is reached only when A and B point in exactly opposite directions (the angle is 180∘).
- For any angle in between, the resultant magnitude lies strictly between these two limits.
This is the vector form of the ordinary triangle inequality ∣x+y∣≤∣x∣+∣y∣ you may already know for numbers: here x and y become vectors, and "the sides of a triangle" become "a vector, another vector, and their sum."
Why It Matters in Kinematics
This bound is genuinely useful when combining physical quantities in the plane:
- Relative velocity: if a boat has speed 5 m/s relative to water and the river flows at 3 m/s, the boat's speed relative to the ground must lie between ∣5−3∣=2 m/s and 5+3=8 m/s, depending on the angle the boat is steered — it can never be less than 2 or more than 8.
- Combining forces or displacements: if you know only the magnitudes of two vectors, not the angle between them, this inequality instantly tells you the range of possible resultant magnitudes without doing any trigonometry. …
The key idea is the magnitude of the cross product of two vectors.
The magnitude of the cross product of two vectors A and B is given by:
∣A×B∣=∣A∣∣B∣sinθ
where θ is the angle between A and B.
Given ∣A∣=2 and ∣B∣=4, the formula becomes ∣A×B∣=(2)(4)sinθ=8sinθ.
We match the relations from Column I with the corresponding angles from Column II:
- If ∣A×B∣=0, then 8sinθ=0⟹sinθ=0⟹θ=0∘. This matches (iv).
- If ∣A×B∣=8, then 8sinθ=8⟹sinθ=1⟹θ=90∘. This matches (iii). …
This problem uses the formula for the magnitude of the cross product, ∣A×B∣=∣A∣∣B∣sinθ, to find the angle θ between vectors A and B for different given magnitudes of their cross product. The final matching is (a)-(iv), (b)-(iii), (c)-(i), (d)-(ii).
The cross product of two vectors, A and B, results in a new vector that is perpendicular to both A and B. The magnitude of this resultant vector, ∣A×B∣, has a specific geometric meaning: it represents the area of the parallelogram formed by the two vectors A and B when placed tail-to-tail.
The magnitude of the cross product is directly related to the magnitudes of the individual vectors and the sine of the angle between them. This relationship is crucial for solving problems like this one, where we need to determine the angle based on the cross product's magnitude.
The magnitude of the cross product of two vectors A and B is given by:
∣A×B∣=∣A∣∣B∣sinθ
where ∣A∣ is the magnitude of vector A, ∣B∣ is the magnitude of vector B, and θ is the angle between A and B (0∘≤θ≤180∘).
We are given the magnitudes of the two vectors:
∣A∣=2
∣B∣=4
Substituting these values into the formula, we get:
∣A×B∣=(2)(4)sinθ=8sinθ
Now, let's use this expression to match each relation in Column I with the corresponding angle in Column II.
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For (a) ∣A×B∣=0:
We set the magnitude of the cross product to 0:
8sinθ=0
sinθ=0
This implies that θ=0∘ or θ=180∘. In the context of the given options, θ=0∘ is the relevant choice. This means the vectors are parallel.
Therefore, (a) matches with (iv) θ=0∘.
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For (b) ∣A×B∣=8:
We set the magnitude of the cross product to 8:
8sinθ=8
sinθ=1 …
Concept: Compute the Cross Product's z-Component From Coordinates Instead of the Magnitude Formula
Method: Place A Along the x-Axis and Use the 2D Determinant
Just as with the dot product, this method fixes A along the x-axis (WLOG) and resolves B at angle θ, then computes the cross product's out-of-plane component directly from the determinant AxBy−AyBx, instead of quoting ∣A×B∣=∣A∣∣B∣sinθ from memory.
Steps
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Coordinates (as before): A=(2,0,0), B=(4cosθ,4sinθ,0).
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Compute the cross product's z-component using the determinant rule:
(A×B)z=AxBy−AyBx=(2)(4sinθ)−(0)(4cosθ)=8sinθ.
Since both vectors lie in the plane, this z-component is the entire cross product, so ∣A×B∣=8sinθ (for θ between 0∘ and 180∘, where sinθ≥0).
- Match each given magnitude against 8sinθ by recognizing the value against the standard sine table, rather than solving sinθ=k as a fresh equation each time:
- ∣A×B∣=0⇒sinθ=0, the table entry for θ=0∘ (iv).
- ∣A×B∣=8⇒sinθ=1, the table entry for θ=90∘ (iii). …
- TG EAPCET 2026Set eng-2026-05-09-AN1 markMCQQ.A bomb of mass ‘m’ at rest explodes into three parts. If these three parts move horizontally with equal speeds in different directions, then the masses of the three parts can be (A) 113m, 3m, 3313m (B) 6m, 3m, 2m (C) 194m, 195m, 1910m (D) 296m, 298m, 2915m
›Reveal solutionSolution
When a bomb at rest explodes into three parts, the total momentum must remain zero. Since the parts move with equal speeds, their masses must satisfy the triangle inequality. Option (A) is the only set of masses that satisfies this condition for non-collinear motion.
Concept and Intuition
This problem is a classic application of the principle of conservation of linear momentum.
- Initial State: The bomb is initially at rest. This means its initial velocity is zero, and therefore, its total initial linear momentum is also zero.
- Explosion: An explosion is an internal process. No external forces act on the bomb system during the explosion (neglecting gravity for the brief moment of explosion, which is standard for such problems).
- Conservation: According to the principle of conservation of linear momentum, if no external forces act on a system, the total linear momentum of the system remains constant. Since the initial momentum was zero, the total final momentum of the three parts must also be zero.
Let the three parts have masses m1,m2,m3 and velocities v1,v2,v3 respectively. The problem states they move with equal speeds, so ∣v1∣=∣v2∣=∣v3∣=v.
The conservation of momentum equation is:
pinitial=pfinal
0=m1v1+m2v2+m3v3
This means the vector sum of the final momenta of the three parts must be zero. For three vectors to sum to zero, they must be able to form a closed triangle when placed head-to-tail.
Let P1=m1v1, P2=m2v2, and P3=m3v3.
Then P1+P2+P3=0.
This implies that the magnitudes of these momentum vectors must satisfy the triangle inequality:
∣P1∣≤∣P2∣+∣P3∣
∣P2∣≤∣P1∣+∣P3∣
∣P3∣≤∣P1∣+∣P2∣
Since the speeds are equal (v), we have ∣P1∣=m1v, ∣P2∣=m2v, and ∣P3∣=m3v. Substituting these into the triangle inequality and dividing by v (since v=0), we get:
m1≤m2+m3
m2≤m1+m3
m3≤m1+m2
Additionally, the sum of the masses of the three parts must equal the original mass of the bomb:
m1+m2+m3=m
The phrase "in different directions" usually implies that the three momentum vectors are not collinear, meaning they form a non-degenerate triangle. For a non-degenerate triangle, the strict inequality (<) must hold for all three conditions. If equality holds for any condition (e.g., m3=m1+m2), it means the vectors are collinear, forming a degenerate triangle.
Step-by-step Derivation
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Total Mass Condition: The sum of the masses of the three parts must be equal to the original mass of the bomb, m. We will check this for each option first.
- (A) 113m+3m+3313m=339m+3311m+3313m=3333m=m. (Satisfied)
- (B) 6m+3m+2m=6m+62m+63m=66m=m. (Satisfied)
- (C) 194m+195m+1910m=1919m=m. (Satisfied)
- (D) 296m+298m+2915m=2929m=m. (Satisfied)
All options satisfy the total mass condition. We must now use the triangle inequality.
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Triangle Inequality Condition: The magnitudes of the masses (m1,m2,m3) must satisfy the triangle inequality. For the parts to move in "different directions" (implying non-collinear motion), the strict inequality must hold:
m1<m2+m3
m2<m1+m3
m3<m1+m2
Let's check each option:
- (A) Masses: 113m,3m,3313m
To compare easily, let's use a common denominator, 33:
m1=339m, m2=3311m, m3=3313m.
- Is m1<m2+m3? 339m<3311m+3313m⟹339m<3324m. (True) …
- (A) Masses: 113m,3m,3313m
To compare easily, let's use a common denominator, 33:
m1=339m, m2=3311m, m3=3313m.
- TG EAPCET 2023Set ap-2023-05-10-FN1 markMCQQ.The correct statement among the following is (A) The resistance of an ideal ammeter is infinity. (B) The resistance of an ideal ammeter is zero. (C) The resistance of an ideal voltmeter is zero. (D) The resistance of a super conductor is infinity.
›Reveal solutionSolution
An ideal ammeter must not disturb the current it measures, so its resistance should be zero. An ideal voltmeter must not draw current from the circuit, so its resistance should be infinite. A superconductor has exactly zero resistance.
The question tests your understanding of how ammeters and voltmeters are meant to behave in circuits — and what "ideal" means in each case. The key is to remember what each device does: an ammeter measures current through it, and a voltmeter measures voltage across it. To avoid affecting the circuit, each must have a specific resistance.
- Ideal ammeter: An ammeter is placed in series with the component whose current you want to measure. If it had any resistance, it would add to the circuit's total resistance and reduce the current — giving a wrong reading. So an ideal ammeter should offer zero resistance — it should act like a perfect wire. That eliminates option (A) and confirms (B). …
- TG EAPCET 2022Set ap-2022-07-30-FN1 markMCQQ.480 m3 of air is being removed from a room in 20 mins via duct. If air is moving outside with speed of 2.5 m/s. The shape and dimension of the duct should be A) a square with each side length 0.4 m B) a rectangle with dimensions 25 cm × 64 cm C) a square with each side length 1.26 m D) a rectangle with dimensions 90 cm × 177 cm (A) (A) only is correct (B) (C) only is correct (C) (A) and (B) only are correct (D) (C) and (D) only are correct
›Reveal solutionSolution
The duct must carry 0.4 m3s−1 at 2.5 ms−1, which fixes the area at 0.16 m2 — the shape is free. Both the 0.4 m square and the 25 cm×64 cm rectangle have that area, so statements (A) and (B) are correct — option (C).
The concept first
For an incompressible fluid in steady flow, the equation of continuity says the volume passing any cross-section per second is the same everywhere:
Q=Av=constant
where Q is the volume flow rate (m3s−1), A the cross-sectional area and v the flow speed. Notice what this equation does not contain: the shape of the cross-section. Only the area matters. That is the whole trick of this question — it offers you two shapes with the correct area and two with the wrong one.
Step-by-step
- Convert the time to SI.
t=20 min=20×60=1200 s
- Compute the required volume flow rate.
Q=tV=1200 s480 m3=0.4 m3s−1
- Apply continuity to get the area.
A=vQ=2.5 ms−10.4 m3s−1=0.16 m2
So any duct whose cross-section measures 0.16 m2 will do.
4. Test each proposed cross-section.
- (A) Square, side 0.4 m: A=(0.4)2=0.16 m2 ✓ Correct.
- (B) Rectangle, 25 cm×64 cm: convert first — 0.25 m×0.64 m, so
A=0.25×0.64=0.16 m2 ✓ Correct.
- (C) Square, side 1.26 m: A=(1.26)2=1.5876≈1.59 m2 ✗ — nearly ten times too big. …
- TG EAPCET 2022Set eng-2022-07-19-AN1 markMCQQ.Statement I : A uniform electric field and a uniform magnetic field are pointed in the same direction. If an electron is projected in the same direction the electron velocity will decrease in magnitude. Statement II : Two infinite long parallel wires are carrying current in the same direction. The magnetic field at a point midway between the wires is zero. Statement III : No net force acts on a rectangular coil carrying a steady current when suspended in a uniform magnetic field. Which of the following is correct? (A) Statements I, II and III are true (B) Statements I and II are true, but statement III is false (C) Statements II and III are true, but statement I is false (D) Statements I, III are true, but Statement II is false
›Reveal solutionSolution
The key idea is to check each statement against the physics of charged particles in fields, magnetic fields from currents, and forces on current loops. Only statements II and III are true, so the correct option is (C).
Concept and Intuition
We need to evaluate three independent physics statements. Statement I involves the Lorentz force on a moving electron in parallel electric and magnetic fields. Statement II is about the superposition of magnetic fields from two parallel currents. Statement III concerns the net force on a current-carrying loop in a uniform field. Each must be judged true or false based on fundamental principles.
Step-by-step reasoning
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Statement I: Electron velocity in parallel E and B fields
- An electron has charge q=−e. The Lorentz force is F=q(E+v×B).
- Here E and B are in the same direction, say along +x. The electron is projected in that same direction, so v is also along +x.
- The cross product v×B=0 because the vectors are parallel. So the magnetic force is zero.
- The electric force is FE=−eE, which is opposite to E (since electron charge is negative). Thus the electric force opposes the motion, slowing the electron.
- Conclusion: The velocity magnitude does decrease. Statement I is true.
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Statement II: Magnetic field midway between two parallel wires
- Two infinitely long parallel wires carry current in the same direction.
- By the right-hand rule, the magnetic field due to each wire at a point midway between them is perpendicular to the line joining the wires.
- For currents in the same direction, the fields from each wire at the midpoint are equal in magnitude but opposite in direction (one points into the page, the other out).
- They cancel exactly, giving zero net field.
- Conclusion: Statement II is true.
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Statement III: Net force on a rectangular coil in a uniform magnetic field
- A current-carrying loop in a uniform magnetic field experiences forces on each side. …
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- TG EAPCET 2022Set eng-2022-07-20-FN1 markMCQQ.Statement (I) : An object subjected to velocities v1 and v2 has a resultant velocity with magnitude ∣v∣=∣v1∣+∣v2∣. Statement (II) : The magnitude of displacement is either less or equal to the path length of an object between two points. Statement (III) : The instantaneous acceleration is the limiting value of the average acceleration as the time interval approaches zero. Which of the following is correct? (A) Statements I, II, III are true (B) Statements I, II are true, but statement III is false (C) Statements II, III are true, but statement I is false (D) Statements I, II, III are false
›Reveal solutionSolution
The key idea is that vector addition depends on direction, not just magnitude. Statement I is false because resultant velocity magnitude equals the sum only when the vectors are parallel and in the same direction. Statement II is true — displacement ≤ path length. Statement III is true — that’s the definition of instantaneous acceleration. So the correct option is (C).
Let’s unpack each statement one by one, starting with the concept that matters most: vectors add according to direction, not just size.
- Statement I: “An object subjected to velocities v1 and v2 has a resultant velocity with magnitude ∣v∣=∣v1∣+∣v2∣.” This is only true when v1 and v2 are in the exact same direction. In general, the magnitude of the resultant vector is given by the law of cosines:
∣v∣=∣v1∣2+∣v2∣2+2∣v1∣∣v2∣cosθ
where θ is the angle between them. The sum ∣v1∣+∣v2∣ is the maximum possible resultant magnitude (when θ=0∘). For any other angle, the resultant is smaller. Since the statement presents this as a universal truth, it is false.
Watch outA common mistake is to treat velocity magnitudes like scalar speeds and just add them. But velocity is a vector — direction always matters. The statement would be correct only if it specified “in the same direction.”
- Statement II: “The magnitude of displacement is either less or equal to the path length of an object between two points.”
Displacement is the straight-line distance between the initial and final positions — a vector quantity. Path length is the total distance actually travelled along the trajectory — a scalar.
- If the object moves in a straight line without turning back, displacement equals path length. …
- TG EAPCET 2022Set eng-2022-07-20-FN1 markMCQQ.Assertion (A): The zeroth law of thermodynamics leads to the concept of temperature. Reason (R): The zeroth law states that two systems in thermal equilibrium with a third system are in thermal equilibrium with each other. The correct option among the following is (A) is true, (R) is true and (R) is the correct explanation for (A) (B) (A) is true, (R) is true but (R) is not the correct explanation for (A) (C) (A) is true but (R) is false (D) (A) is false but (R) is true
›Reveal solutionSolution
The zeroth law of thermodynamics defines thermal equilibrium and provides the logical basis for measuring temperature. Both Assertion and Reason are true, and the Reason correctly explains the Assertion.
The zeroth law is the foundation of thermometry. Without it, you could never be sure that a thermometer reading in one system means the same thing for another system. The law creates an equivalence relation: if A is in equilibrium with C, and B is in equilibrium with C, then A and B are in equilibrium with each other. This transitivity is what allows us to assign a single number — temperature — to the state of thermal equilibrium.
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What the zeroth law actually says.
The Reason (R) states it precisely: two systems separately in thermal equilibrium with a third are in thermal equilibrium with each other. This is not a trivial statement — it is an empirical fact about the world that cannot be derived from other laws. It establishes that "being in thermal equilibrium" is a transitive relation.
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How this leads to temperature.
Because of transitivity, all systems that are in thermal equilibrium with each other share a common property. We call that property temperature. If system A and system B are both in equilibrium with a thermometer (system C), then A and B must have the same temperature — even if they never touch each other. This is why a single thermometer can measure the temperature of any object: the reading on the thermometer tells you the temperature of the object it is in equilibrium with.
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Why the Assertion is true. …
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- TG EAPCET 2021Set ap-2021-08-09-AN1 markMCQQ.The energy emitted versus wavelength plot of a black body depends on the material of the black body Emissivity of completely reflecting surface is zero Emissivity is equal to (absorptive power)2 Black-body radiates more than normal body Which of the following is correct? (A) A & B are true (B) B & C are true (C) C & D are true (D) B & D are true
›Reveal solutionSolution
The key idea is that black‑body radiation is independent of material, a perfect reflector has zero emissivity, emissivity equals absorptivity (not its square), and a black body radiates more than any other body at the same temperature. Only statements B and D are true, so the correct option is (D).
Concept and intuition:
This question tests the fundamental laws of thermal radiation. A black body is an ideal emitter and absorber — its radiation spectrum depends only on temperature, not on the material. Emissivity (ε) and absorptivity (α) are equal for any surface in thermal equilibrium (Kirchhoff’s law). A perfect reflector absorbs nothing, so its emissivity is zero. And because a black body absorbs all incident radiation, it must also emit the maximum possible radiation at a given temperature — any real body emits less.
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Statement A: “The energy emitted versus wavelength plot of a black body depends on the material of the black body.”
This is false. Planck’s law gives the spectral radiance of a black body as a function only of temperature and wavelength — no material property appears. The shape and peak of the curve are universal (Wien’s displacement law). So A is incorrect.
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Statement B: “Emissivity of a completely reflecting surface is zero.”
True. A perfectly reflecting surface absorbs no radiation (absorptivity = 0). By Kirchhoff’s law, emissivity equals absorptivity for a body in thermal equilibrium, so emissivity = 0. It emits no thermal radiation.
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Statement C: “Emissivity is equal to (absorptive power)².” …
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- TG EAPCET 2021Set eng-2021-08-05-AN1 markMCQQ.Given the fact that A) Magnetic field exerts force only on a moving charge B) Electric field exerts force on both stationary and moving charge C) Magnetic field exerts force on charge moving parallel to the direction of the field. Which of the following is true? (A) A and C (B) A and B (C) B and C (D) A, B and C
›Reveal solutionSolution
Magnetic fields only exert force on moving charges, while electric fields exert force on both stationary and moving charges. A charge moving parallel to a magnetic field experiences no magnetic force. Therefore, statements A and B are true.
Concept and Intuition
The fundamental difference in how electric and magnetic fields interact with charges lies in their dependence on the charge's motion.
An electric field is created by charges and exerts a force on any charge, regardless of whether that charge is moving or stationary. The force is directly proportional to the charge and the electric field strength.
A magnetic field, on the other hand, is created by moving charges (currents) and exerts a force only on other moving charges. A stationary charge will not experience a magnetic force. Furthermore, the direction of the magnetic force depends on the direction of the charge's velocity relative to the magnetic field.
Let's evaluate each statement based on these principles.
Step-by-step Evaluation
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Evaluate Statement A: "Magnetic field exerts force only on a moving charge"
The force experienced by a charge q moving with velocity v in a magnetic field B is given by the Lorentz force formula for the magnetic component:
Fm=q(v×B)
From this formula, if the charge is stationary, its velocity v=0. Substituting this into the formula, we get Fm=q(0×B)=0. This means a stationary charge experiences no magnetic force. Only when v=0 can there be a magnetic force (provided v is not parallel to B).
Therefore, statement A is true.
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Evaluate Statement B: "Electric field exerts force on both stationary and moving charge"
The force experienced by a charge q in an electric field E is given by:
Fe=qE
This formula shows that the electric force depends only on the magnitude of the charge q and the electric field E. It does not depend on the velocity v of the charge. Thus, whether the charge is stationary (v=0) or moving (v=0), it will experience an electric force if it is in an electric field.
Therefore, statement B is true. …
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