Laws Of Motion Common Mistakes Class 11 Exam Prep Revision — Grandmaster Guide
Ayush (Founder)
Exam Strategist
- F = ma
- F = μN
- F = (m₁ + m₂)v/(m₁ + m₂)
- a = Δv/Δt
- v = u + at
- s = ut + (1/2)at²
- v² = u² + 2as
- F₁₂ = -F₂₁
- T = 2π √(l/g)
- W = F × s
- P = Fv
- K = (1/2)mv²
- U = mgh
- Impulse = Δp = FΔt
🪤 The 5 Mistakes That Cost Marks
- Assuming friction is always kinetic, when ∈ fact it can be static or kinetic
- Forgetting to consider the normal force ∈ problems involving inclined planes
- Not using the correct equation for centripetal force: F = (mv²)/r
- Incorrectly applying Newton's third law, forgetting that forces always come ∈ pairs
- Not considering the system as a whole when applying conservation of momentum
✏️ 3 Solved PYQs
- A block of mass 2 kg is moving with a velocity of 4 m/s on a frictionless surface. A force of 6 N is applied to the block ∈ the direction of motion. Calculate the acceleration of the block.
- F = ma, so a = F/m = 6/2 = 3 m/s²
- A car of mass 1500 kg is moving with a velocity of 60 km/h. The driver applies the brakes and the car comes to rest ∈ 10 s. Calculate the force applied by the brakes.
- v = u + at, so a = (v - u)/t = (0 - 60 × (1000/3600))/10 = -1.67 m/s²
- F = ma, so F = 1500 × -1.67 = -2505 N
- A particle of mass 1 kg is attached to a string of length 1 m. The particle is rotating ∈ a circular path with a velocity of 2 m/s. Calculate the tension ∈ the string.
- F = (mv²)/r, so T = (1 × 2²)/1 = 4 N
🧠 The One Thing Most Students Get Wrong
- Most students get the concept of pseudo force wrong. Pseudo force is a fictitious force that appears to act on an object when it is ∈ a non-inertial frame of reference. It is equal ∈ magnitude and opposite ∈ direction to the force that is actually acting on the object.
👁️ Ayush's Note
- To solve problems ∈ laws of motion, always start by drawing a free body diagram. This will help you identify all the forces acting on the object and make it easier to apply Newton's laws.
- Always consider the system as a whole when applying conservation of momentum.
- Make sure to use the correct equation for centripetal force: F = (mv²)/r.
- Pseudo force is a key concept ∈ laws of motion. Make sure you understand it well.
🔁 Last 5 Minutes Box
- Last minute revision: go through all the formulas and make sure you understand the concepts of Newton's laws, friction, centripetal force, and pseudo force.
- Check your notes and make sure you have all the formulas written down.
- Go through the solved PYQs and make sure you understand the steps.
- Take a few deep breaths and try to relax. You have prepared well for this exam.
📝 Practice MCQs
1. A block of mass 1 kg is moving with a velocity of 2 m/s on a frictionless surface. A force of 3 N is applied to the block ∈ the direction of motion. What is the acceleration of the block?
A) 1 m/s²
B) 2 m/s²
C) 3 m/s²
D) 4 m/s²
Answer: C) 3 m/s². Explanation: F = ma, so a = F/m = 3/1 = 3 m/s²
2. A car of mass 1500 kg is moving with a velocity of 60 km/h. The driver applies the brakes and the car comes to rest ∈ 10 s. What is the force applied by the brakes?
A) -1000 N
B) -1500 N
C) -2000 N
D) -2505 N
Answer: D) -2505 N. Explanation: v = u + at, so a = (v - u)/t = (0 - 60 × (1000/3600))/10 = -1.67 m/s², F = ma, so F = 1500 × -1.67 = -2505 N
3. A particle of mass 1 kg is attached to a string of length 1 m. The particle is rotating ∈ a circular path with a velocity of 2 m/s. What is the tension ∈ the string?
A) 2 N
B) 3 N
C) 4 N
D) 5 N
Answer: C) 4 N. Explanation: F = (mv²)/r, so T = (1 × 2²)/1 = 4 N
4. A block of mass 2 kg is moving with a velocity of 4 m/s on a frictionless surface. A force of 6 N is applied to the block ∈ the direction of motion. What is the acceleration of the block?
A) 1 m/s²
B) 2 m/s²
C) 3 m/s²
D) 4 m/s²
Answer: B) 3 m/s². Explanation: F = ma, so a = F/m = 6/2 = 3 m/s²
5. A car of mass 1500 kg is moving with a velocity of 60 km/h. The driver applies the brakes and the car comes to rest ∈ 10 s. What is the distance traveled by the car before it comes to rest?
A) 50 m
B) 100 m
C) 150 m
D) 166.7 m
Answer: D) 166.7 m. Explanation: v = u + at, so a = (v - u)/t = (0 - 60 × (1000/3600))/10 = -1.67 m/s², s = ut + (1/2)at² = 60 × (1000/3600) × 10 + (1/2) × -1.67 × 10² = 166.7 m
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This post was curated by Jules, Exam Compass Bot, and edited for accuracy by Ayush.
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