Laws of Motion - Previous Year Questions Solved Examples (Class 11 Physics)
Newton's Laws of Motion form the foundation of classical mechanics. These questions test understanding of forces, acceleration, and motion applications.
TL;DR: Newton's Laws of Motion form the foundation of classical mechanics. These questions test understanding of forces, acceleration, and motion application…
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Newton's Laws of Motion form the foundation of classical mechanics. These questions test understanding of forces, acceleration, and motion applications.
Laws of Motion - Previous Year Questions — Solved Numerical Examples (Step by Step)
Example 1: State Newton's three laws of motion and explain the first law using an example. [3 marks]
Solution: First Law: An object at rest remains at rest, and an object in motion continues moving with constant velocity unless acted upon by an external force. This describes inertia. Example: Passengers lurch forward when a bus suddenly brakes because they tend to continue moving forward by inertia while the bus stops due to friction. Second Law: F = ma. Force is proportional to mass and acceleration. Third Law: For every action, there is an equal and opposite reaction.
Example 2: A force of 20 N acts on a mass of 4 kg for 5 seconds. If the object starts from rest, find the acceleration and final velocity. [3 marks]
Solution: Using F = ma: 20 = 4 × a. a = 5 m/s². Using v = u + at: v = 0 + 5 × 5 = 25 m/s. Distance covered: s = ut + (1/2)at² = 0 + (1/2)(5)(25) = 62.5 m.
Example 3: A block of mass 5 kg is on a horizontal surface. Coefficient of friction is 0.2. Find the friction force when a horizontal force of 10 N is applied. [2 marks]
Solution: Normal force N = mg = 5 × 10 = 50 N. Maximum static friction = μs × N = 0.2 × 50 = 10 N. Since applied force (10 N) equals maximum friction (10 N), the block is on the verge of moving or just starts moving. If block moves, kinetic friction = 0.2 × 50 = 10 N.
Example 4: Two blocks connected by a string are pulled on a horizontal surface. Block A (2 kg) is pulled with force 30 N, Block B (3 kg) follows. Neglect friction. Find tension in string and acceleration. [3 marks]
Solution: Total mass = 2 + 3 = 5 kg. Acceleration a = F/m_total = 30/5 = 6 m/s². For block B alone: T = m_B × a = 3 × 6 = 18 N. Verify: For block A: 30 - T = m_A × a = 2 × 6 = 12. T = 30 - 12 = 18 N.
Example 5: Explain the concept of friction. What factors affect friction? [3 marks]
Solution: Friction is the force opposing relative motion between surfaces in contact. Factors affecting friction: (1) Nature of surfaces: roughness/texture affects friction, (2) Normal force: friction = μ × N where μ is coefficient, (3) Area of contact: surprisingly, area does not affect friction significantly for solid surfaces (for liquids and gases it differs), (4) Relative velocity: kinetic friction is roughly constant, static friction can vary up to maximum, (5) Absence of lubricant: lubrication reduces friction. Static friction > kinetic friction for same surfaces.
Example 6: A car of mass 1000 kg is moving at 20 m/s. It comes to rest after traveling 100 m. Find friction force and coefficient of kinetic friction. [3 marks]
Solution: Using v² = u² + 2as: 0 = (20)² + 2 × a × 100. a = -400/(200) = -2 m/s². Using F = ma: F = 1000 × (-2) = -2000 N. Friction force = 2000 N. Normal force N = mg = 1000 × 10 = 10000 N. μk = F/N = 2000/10000 = 0.2.
Example 7: Explain what happens to a person standing in an accelerating bus from the perspective of inertial and non-inertial frames. [3 marks]
Solution: Inertial frame (ground): As bus accelerates forward, person must experience forward force to accelerate with bus. If no force acts (person not holding support), person remains at rest or continues original velocity, moving backward relative to bus. Non-inertial frame (bus): In bus frame, person experiences a pseudo-force backward (opposite to acceleration direction) causing them to fall backward. This apparent force is due to the frame's acceleration, not any real force.
Tips
- Free body diagrams are essential; draw all forces acting on the object clearly.
- Distinguish between static and kinetic friction: static ≤ μs × N, kinetic = μk × N.
- In connected body problems, treat as system first to find acceleration, then analyze individual bodies.
- Remember that friction acts opposite to direction of motion, not along motion.
Frequently Asked Questions
Why does friction act opposite to motion?
Friction resists relative motion between surfaces. It always opposes the tendency of motion to prevent energy dissipation.
Can friction do positive work?
Friction does negative work on the sliding object (opposes motion). However, if considering the object friction acts on, it can sometimes increase kinetic energy.
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