PYQ + trends

1. TITLE

NEET 2023 Q6: Finding the exact direction of force during a sharp turn — Full solution + Why they asked this

2. THE ACTUAL NEET QUESTION

NEET 2023 — Question
A football player is moving southward and suddenly turns eastward with the same speed to avoid an opponent. The force that acts on the player while turning is
A) Along northward
B) Along north-east
C) Along south-west
D) Along eastward

3. OFFICIAL ANSWER

NEET Answer Key: (B)

4. COMPLETE SOLUTION

Step-by-step calculation:
* First, assign vector directions to the player's movement. Let North be $+\hat{j}$, South be $-\hat{j}$, East be $+\hat{i}$, and West be $-\hat{i}$.
* The initial momentum of the player moving southward is $\vec{P}{i} = mu(-\hat{j})$. 
* The final momentum of the player after turning eastward is $\vec{P}{f} = mu(\hat{i})$
* Calculate the change in momentum ($\Delta\vec{P}$):

$\Delta\vec{P} = \vec{P}{f} - \vec{P}{i}$
$$\Delta\vec{P} = mu(\hat{i}) - mu(-\hat{j})$$
$$\Delta\vec{P} = mu(\hat{i} + \hat{j})$$

.
* The resulting vector $mu(\hat{i} + \hat{j})$ has positive x (East) and positive y (North) components, placing it firmly in the North-East direction.
* According to Newton's Second Law, the direction of the force is identical to the direction of the change in momentum. Therefore, the force acts along the North-East direction.

Formula used:
* $\Delta\vec{P} = \vec{P}{f} - \vec{P}{i}$ 
* $\vec{F} = \frac{\Delta\vec{P}}{\Delta t}$

Why this approach:
We must use the vector nature of momentum. Force isn't just about the total change in speed (which here is zero, as speed remained constant); it is entirely dependent on the change in the velocity vector. The second law of motion dictates that the applied force takes place exactly in the direction of the rate of change of momentum.

5. WHY NEET ASKED THIS

  • What concept were they testing? The examiner wanted to test the understanding of the vector nature of momentum and Newton's Second Law ($\vec{F} = d\vec{p}/dt$), specifically checking if students know that force aligns with $\Delta\vec{P}$, not just the final velocity.
  • What's the common mistake they expected students to make? Students often ignore the initial velocity vector and assume the force acts in the new direction of motion (Eastward). Another common error is doing the vector subtraction backward  ($\vec{P}{i} - \vec{P}{f}$),  yielding a South-West direction.
  • Which option was the "trap"? Option D (Along eastward) is a trap for those who think force acts in the final direction of motion. Option C (Along south-west) is the trap for those who mess up the vector subtraction signs.

6. TREND ANALYSIS

PATTERN ALERT:

Year How they asked Twist
2024 Block pulled by a cord on a frictionless plane (Tension & acceleration) Focus shifted to 1D force balance using Free Body Diagrams  ($T - Mg\sin30 = Ma$). 
2023 Football player turning 90 degrees Tested 2D vector subtraction to find the exact direction of force without giving numerical values.
2021 Ball dropped, striking the ground, and rebounding  Required calculating the exact magnitude of impulse (change in momentum) in a 1D vertical plane ($v = \sqrt{2gh}$).
2016 Rigid ball striking a wall at an angle and rebounding  Combined 2D vector components with impulse calculation ($F \times t = 2mv \cos\theta$).

Prediction for NEET 2026:
Expect a variation where a particle hits a surface at an angle (like 30° or 60°) and rebounds, asking you to identify not just the magnitude, but the exact vector equation of the impulse imparted by the surface.

7. SIMILAR QUESTIONS TO PRACTICE

"If you understood this, you can solve:"
* NEET 2023 (mpr) Q5: A 1kg object strikes a wall at 60° and reflects at the same angle (calculating force exerted on the wall).
* NEET 2021 Q9: Ball of mass 0.15kg dropped from 10m and rebounding (calculating magnitude of impulse).
* NEET 2016 Phase-II Q18: Rigid ball striking a wall at 60° (calculating impulse imparted by the wall).

8. QUICK REVISION FOR THIS TYPE

For finding the direction of force or impulse questions, always:
* First check: The initial and final velocity vectors and explicitly write them out in $\hat{i}$ and $\hat{j}$ format.
* Then apply: The change in momentum formula  $\Delta\vec{P} = \vec{P}{final} - \vec{P}{initial}$.  Keep the negative signs intact during subtraction!
* Watch out for: Assuming the force is in the direction of final motion. Remember, $\vec{F}$ is always parallel to $\Delta\vec{v}$ or $\Delta\vec{p}$, not necessarily to $\vec{v}_{final}$.

9. NCERT BASE

This question tests: NCERT Class 11, Chapter 4 (Laws of Motion), specifically the sections on Newton's Second Law of Motion and the vector nature of momentum.

10. CTA

All PYQs from this chapter → Laws of Motion Guide
Practice similar questions → Mock exams