A football player is moving southward and suddenly turns eastward with the same speed to avoid an opponent. Th

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:

Accepted Answer

1. **Identify the Principle:** According to Newton's Second Law of Motion, the net force ($\mathbf{F}$) acting on an object is proportional to the rate of change of momentum ($d\mathbf{p}/dt$). Since mass is constant, the direction of force is the same as the direction of the change in velocity ($\Delta \mathbf{v}$). 
   $$ \mathbf{F} \propto \Delta \mathbf{p} = m(\mathbf{v}_f - \mathbf{v}_i) $$
2. **Analyze Vectors:**
   - Initial Velocity ($\mathbf{v}_i$): Southward.
   - Final Velocity ($\mathbf{v}_f$): Eastward.
   - Change in Velocity ($\Delta \mathbf{v}$): $\mathbf{v}_f - \mathbf{v}_i = \mathbf{v}_f + (-\mathbf{v}_i)$.
3. **Determine Direction:**
   - The vector $-\mathbf{v}_i$ points **North** (opposite to South).
   - The vector $\mathbf{v}_f$ points **East**.
   - The resultant of a Northward vector and an Eastward vector acts along the **North-East** direction.
4. **Conclusion:** The force responsible for the turn (friction) acts along the North-East direction.
   (Reference: NCERT Class 11, Physics Part I, Chapter 4: Motion in a Plane, Section 4.7-4.9 discusses vector subtraction; Chapter 5: Laws of Motion, Section 5.5 discusses force and momentum change [1]).

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