An object is said to be in motion if its position changes with time. The position of the object can be specified with reference to a conveniently chosen origin. For motion in a straight line, position to the right of the origin is taken as positive and to the left as negative.
-- NCERT Class 11 Physics, Ch. 2, p. 21Frame of Reference
8 MCQs1 revision card9-step worked example
Source: NCERT KinematicsOfficial key: NTA-verifiedLast updated: 7 Oct 2026
Try this first
According to NCERT, with respect to what is the position of an object specified?
- A.The ground, always.
- B.The Sun, always.
- C.The heaviest object nearby.
- D.A conveniently chosen origin.
Tap to see the answer
Answer: D. D is correct: the position of an object can be specified with reference to a conveniently chosen origin (NCERT Class 11 Physics, Chapter 2, page 21).
A is wrong: A is wrong because nothing makes the ground a required origin; the origin is chosen to suit the problem.
B is wrong: B is wrong because the Sun is not a required origin either; the choice is free.
C is wrong: C is wrong because the mass of nearby objects plays no part in choosing an origin.
Frame of Reference, explained for NEET
The trap is treating a sign as a fact about the motion. Whether a velocity or an acceleration is positive or negative depends on the origin and the positive direction you chose, not on whether the body is speeding up or slowing down. A negative acceleration does not mean the body is slowing.
The 2026 NCERT books do not define a frame of reference under that name. What they give is the working rule. The position of an object is specified with reference to a conveniently chosen origin; for motion in a straight line, position to the right of the origin is positive and to the left negative (NCERT Class 11 Physics, Chapter 2, page 21). The origin and the positive direction of an axis are a matter of choice, and you should specify this choice before you assign signs to displacement, velocity and acceleration (NCERT Class 11 Physics, Chapter 2, page 23).
Two consequences from the same page. If a particle is speeding up, its acceleration is in the direction of its velocity; if its speed is decreasing, the acceleration is opposite to the velocity, and this is independent of the choice of origin and axis. But the sign of the acceleration does depend on the choice: with upward taken as positive, the acceleration due to gravity is negative, and for a falling particle that negative acceleration increases the speed, while for a thrown-up particle the same acceleration decreases it (NCERT Class 11 Physics, Chapter 2, page 23).
In a plane the same idea applies. Taking the initial position as the origin of the reference frame gives xo = 0 and yo = 0 (NCERT Class 11 Physics, Chapter 3, page 39). When two velocities are compared, both must be referred to a common reference frame (NCERT Class 11 Physics, Chapter 3, page 46).
Watch-out: write down the origin and the positive direction before the first equation, and keep them for every quantity in the problem.
How do you solve a Frame of Reference question? A worked example
- 1
Given
A stone is thrown vertically upward at 20 m/s from the top of a building 25 m high. Take g = 10 m/s².
- 2
Required
The time to reach the ground and the velocity on arrival, first with the origin at the ground (upward positive) and then with the origin at the top of the building (upward positive).
- 3
Concept
The origin and the positive direction are a matter of choice, and the choice is stated before signs are assigned (NCERT Class 11 Physics, Chapter 2, page 23). Moving the origin changes the positions y0 and y, not the time or the velocity. The same example is worked with the origin at the ground on page 19.
- 4
Formula
y = y0 + v0 t + (1/2) a t² and v = v0 + a t, with a = -10 m/s² when upward is positive.
- 5
Substitution
Origin at the ground: y0 = 25 m, y = 0, giving 0 = 25 + 20t - 5t². Origin at the top: y0 = 0, y = -25 m, giving -25 = 20t - 5t².
- 6
Calculation
Both equations reduce to t² - 4t - 5 = 0, so t = 5.0 s (the root t = -1 s is rejected). Then v = 20 - 10 x 5.0 = -30 m/s in both frames. The 1/2 and the 10 are exact or given values and do not affect the significant figures.
- 7
Final answer
The stone reaches the ground after 5.0 s with velocity -30 m/s, that is 30 m/s downward. Both choices of origin give the same answer.
- 8
Common trap
Reading the negative sign of the velocity or of the acceleration as slowing down. The acceleration is -10 m/s² throughout; the stone slows while rising and speeds up while falling, and the sign of the acceleration does not tell which.
- 9
Similar NEET-style question
For the same stone, take the origin at the top of the building with downward positive. Find the time and the velocity on reaching the ground. (Answer: y0 = 0, y = +25 m, v0 = -20 m/s, a = +10 m/s², so 25 = -20t + 5t², which is t² - 4t - 5 = 0 and t = 5.0 s; v = -20 + 10 × 5.0 = +30 m/s, downward, the same motion as before.)
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Can you answer these Frame of Reference MCQs?
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
MCQ 1Easy RecallPractice
According to NCERT, with respect to what is the position of an object specified?
Show answer and why every option is right or wrong
Answer: D. D is correct: the position of an object can be specified with reference to a conveniently chosen origin (NCERT Class 11 Physics, Chapter 2, page 21).
Why A is wrong: A is wrong because nothing makes the ground a required origin; the origin is chosen to suit the problem.
Why B is wrong: B is wrong because the Sun is not a required origin either; the choice is free.
Why C is wrong: C is wrong because the mass of nearby objects plays no part in choosing an origin.
MCQ 2Easy RecallPractice
For motion in a straight line, how is position on the right of the chosen origin treated?
Show answer and why every option is right or wrong
Answer: A. A is correct: position to the right of the origin is taken as positive and to the left as negative (NCERT Class 11 Physics, Chapter 2, page 21).
Why B is wrong: B is wrong because it reverses the NCERT convention for a horizontal line.
Why C is wrong: C is wrong because only the origin itself has zero position; points to the right are positive.
Why D is wrong: D is wrong because the sign of a position depends on which side of the origin it is, not on the direction of travel.
MCQ 3Easy RecallPractice
What should you do first, before assigning signs to displacement, velocity and acceleration?
Show answer and why every option is right or wrong
Answer: C. C is correct: the origin and the positive direction are a matter of choice, and you should specify this choice before assigning signs (NCERT Class 11 Physics, Chapter 2, page 23).
Why A is wrong: A is wrong because speed has no sign and does not fix the signs of the other quantities.
Why B is wrong: B is wrong because the sign of the acceleration itself depends on the axis, so the axis comes first.
Why D is wrong: D is wrong because unit conversion is useful but does not settle which direction is positive.
MCQ 4Direct ApplicationPractice
A ball is thrown vertically upward. Taking the upward direction as positive, what is the acceleration of the ball (due to gravity) at every instant of its flight?
Show answer and why every option is right or wrong
Answer: B. B is correct: with the vertically upward direction positive, the acceleration due to gravity is negative, and it stays the same throughout the flight (NCERT Class 11 Physics, Chapter 2, page 23).
Why A is wrong: A is wrong because +9.8 m/s² would point upward, but gravity points downward.
Why C is wrong: C is wrong because a particle thrown up has zero velocity at its highest point but its acceleration there is still the acceleration due to gravity.
Why D is wrong: D is wrong because the acceleration does not change sign when the velocity does; it is -9.8 m/s² throughout.
MCQ 5Direct ApplicationPractice
The same ball is described again, now with the downward direction taken as positive. What is the acceleration due to gravity?
Show answer and why every option is right or wrong
Answer: B. B is correct: gravity points downward, and downward is now the positive direction, so the acceleration is +9.8 m/s²; its sign depends on the choice of axis, its direction does not (NCERT Class 11 Physics, Chapter 2, page 23).
Why A is wrong: A is wrong because -9.8 m/s² belongs to the upward-positive choice; the axis has been reversed.
Why C is wrong: C is wrong because gravity acts at every instant; changing the axis does not remove it.
Why D is wrong: D is wrong because 4.9 is half of 9.8 and nothing in the problem halves it.
MCQ 6Direct ApplicationPractice
A truck moves at +20 m/s and a car at +30 m/s, both velocities measured in the road's frame with east positive. What is the velocity of the car relative to the truck?
Show answer and why every option is right or wrong
Answer: D. D is correct: both velocities are in a common reference frame, so v12 = v1 - v2 = 30 - 20 = +10 m/s (NCERT Class 11 Physics, Chapter 3, page 46).
Why A is wrong: A is wrong because +50 m/s adds the velocities, which gives a resultant, not a relative velocity.
Why B is wrong: B is wrong because -10 m/s is the velocity of the truck relative to the car; the order is reversed.
Why C is wrong: C is wrong because +30 m/s is the car's velocity in the road's frame, not as seen from the truck.
MCQ 7CalculationPractice
A stone is thrown vertically upward at 20 m/s from the top of a building 25 m high. Take the origin at the ground, upward positive, and g = 10 m/s². After how long does it reach the ground?
Show answer and why every option is right or wrong
Answer: A. A is correct: with y0 = +25 m, v0 = +20 m/s and a = -10 m/s², the ground is at y = 0, so 0 = 25 + 20t - 5t², which is t² - 4t - 5 = 0 and gives t = 5.0 s (NCERT Class 11 Physics, Chapter 2, pages 19 and 23).
Why B is wrong: B is wrong because 1.0 s is the magnitude of the negative root, t = -1 s, which is rejected because it lies before the throw.
Why C is wrong: C is wrong because 4.0 s is the time for the stone to return to the height of the building, 2v0 / g, not to reach the ground 25 m lower.
Why D is wrong: D is wrong because 2.0 s is the time to reach the highest point, v0 / g.
MCQ 8CalculationPractice
For the same stone, now take the origin at the top of the building with upward positive. What is its velocity when it reaches the ground?
Show answer and why every option is right or wrong
Answer: C. C is correct: the ground is at y = -25 m, the time of fall is 5.0 s (the quadratic is unchanged by moving the origin), and v = v0 + at = 20 + (-10)(5.0) = -30 m/s, meaning 30 m/s directed downward (NCERT Class 11 Physics, Chapter 2, pages 19 and 23).
Why A is wrong: A is wrong because +30 m/s has the right magnitude but points upward in an upward-positive frame; the stone is moving down.
Why B is wrong: B is wrong because -20 m/s is the velocity when the stone passes back through the height of the building, not at the ground.
Why D is wrong: D is wrong because +20 m/s is the launch velocity.
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Frame of Reference: quick recall before you leave
1 card
What to remember before solving Frame of Reference questions
4 NCERT lines
The origin and the positive direction of an axis are a matter of choice. You should first specify this choice before you assign signs to quantities like displacement, velocity and acceleration.
-- NCERT Class 11 Physics, Ch. 2, p. 23If we take the initial position to be the origin of the reference frame as shown in Fig. 3.16, we have : xo = 0, yo = 0
-- NCERT Class 11 Physics, Ch. 3, p. 39An object subjected to two velocities v1 and v2 has a resultant velocity v = v1 + v2. Take care to distinguish it from velocity of object 1 relative to velocity of object 2 : v12= v1 − v2. Here v1 and v2 are velocities with reference to some common reference frame.
-- NCERT Class 11 Physics, Ch. 3, p. 46Frame of Reference: NEET previous year questions (PYQs) with answers
12 questions in Kinematics
No question in our NEET 2020–2025 set targets this topic directly.
More in Kinematics: 16 exam traps and mistakes · 7 formulas · 10 question patterns from its other lessons.
Sources
NCERT refs: Class 11 Physics Chapter 2, p.21 | Class 11 Physics Chapter 2, p.23 | Class 11 Physics Chapter 3, p.39 | Class 11 Physics Chapter 3, p.46
Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.
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