Laws of Motion — Previous Year Questions (Class 11 Physics)
Newton's laws of motion form the foundation of classical mechanics. Understand inertia, force, acceleration, and applications to various scenarios.
TL;DR: Newton's laws of motion form the foundation of classical mechanics. Understand inertia, force, acceleration, and applications to various scenarios.
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Newton's laws of motion form the foundation of classical mechanics. Understand inertia, force, acceleration, and applications to various scenarios.
Laws of Motion — Previous Year Questions with Solutions
Q (2023, 3 marks): State Newton's three laws of motion and explain the first law with an example.
Answer: Newton's First Law (Law of Inertia): An object at rest stays at rest, and an object in motion stays in motion unless acted upon by external force.
Newton's Second Law: F = ma (Force equals mass times acceleration)
Newton's Third Law: For every action, there is equal and opposite reaction.
Example of First Law:
- A passenger in a car thrown forward when brakes are applied (car stops but passenger continues motion)
- A book rests on a table without falling (balanced forces: weight = normal force)
- A moving hockey puck slides on ice for long distance (minimal friction acts as external force)
Q (2022, 3 marks): A force of 20 N acts on a mass of 4 kg for 3 seconds starting from rest. Find acceleration, final velocity, and displacement.
Answer: Using Newton's Second Law: F = ma
20 = 4 × a
a = 5 m/s²
Initial velocity u = 0 (starts from rest)
Time t = 3 s
Final velocity using v = u + at:
v = 0 + 5(3) = 15 m/s
Displacement using s = ut + (1/2)at²:
s = 0 + (1/2)(5)(3)² = 2.5 × 9 = 22.5 m
Q (2021, 5 marks): A 50 kg person stands on a weighing machine in an elevator. What is the normal force when elevator is: (a) at rest, (b) accelerating upward at 2 m/s², (c) accelerating downward at 2 m/s²? (g = 10 m/s²)
Answer: (a) Elevator at rest:
Weight = mg = 50 × 10 = 500 N
Normal force N = 500 N (reading on machine)
(b) Accelerating upward at 2 m/s²:
Net upward force = ma
N - mg = ma
N = m(g + a) = 50(10 + 2) = 50 × 12 = 600 N
Machine reads 600 N (apparent weight increases)
(c) Accelerating downward at 2 m/s²:
Net downward force = ma
mg - N = ma
N = m(g - a) = 50(10 - 2) = 50 × 8 = 400 N
Machine reads 400 N (apparent weight decreases)
Q (2023, 5 marks): Two blocks of masses m1 = 3 kg and m2 = 5 kg are in contact on a frictionless surface. A force F = 16 N is applied on m1. Find acceleration and contact force between blocks.
Answer: Total mass = m1 + m2 = 3 + 5 = 8 kg
Using F = ma for system:
16 = 8 × a
a = 2 m/s²
For contact force between blocks:
Consider m2 alone:
Contact force on m2 = m2 × a = 5 × 2 = 10 N
Or using m1: F - Contact force = m1 × a
16 - Contact force = 3 × 2
Contact force = 10 N
The blocks push each other with force 10 N
Q (2022, 3 marks): A block of mass 2 kg lies on a rough horizontal surface. A horizontal force of 5 N is applied but block doesn't move. Find coefficient of static friction. (g = 10 m/s²)
Answer: Since block is at rest, static friction balances applied force:
Applied force F = Static friction f_s
f_s = 5 N
Normal force N = mg = 2 × 10 = 20 N
Coefficient of static friction:
μ_s = f_s / N = 5 / 20 = 0.25
Note: The minimum force to overcome static friction is 5 N
Q (2023, 3 marks): Explain Newton's third law with three different examples and show how the forces don't cancel despite being equal and opposite.
Answer: Newton's Third Law: Action and reaction forces act on different objects.
Example 1: Walking on ground
- You push ground backward (action)
- Ground pushes you forward (reaction)
- Forces don't cancel because they act on different objects (ground and you)
Example 2: Swimming
- Swimmer pushes water backward (action)
- Water pushes swimmer forward (reaction)
Example 3: Jumping
- Your legs push ground downward (action)
- Ground pushes you upward (reaction)
Why don't they cancel?
- They act on different masses
- They act for the same time
- They produce different accelerations depending on mass
- Example: Person (60 kg) vs Earth (6×10^24 kg) have equal forces but Earth's acceleration is negligible
Frequently Asked Questions
What is inertia and how does it relate to mass?
Inertia is the resistance of an object to change in its state of motion. Mass is the measure of inertia. Objects with larger mass have greater inertia and require larger forces to accelerate them. More massive objects are 'harder to move' because of their greater inertia.
Can Newton's laws apply in all reference frames? Explain.
Newton's laws apply in inertial (non-accelerating) reference frames. In non-inertial (accelerating) frames, pseudo-forces must be introduced. For most practical problems on Earth, we treat it as inertial, though strictly it's non-inertial due to rotation.
More Class 11 Physics PYQs
- Current Electricity
- Ray Optics
- Electrostatics
- Moving Charges and Magnetism
- Electromagnetic Induction
- Alternating Current
🤖 Stuck on any of these? Ask Syllab's free AI Tutor to explain step by step →