📑 Topics on this page (Jump to Topic):
📌 1. Basic Terms & Equations of Motion
| Term | Meaning | Type |
|---|---|---|
| Distance | Total length of path travelled | Scalar |
| Displacement | Straight-line distance from start to end, with direction | Vector |
| Speed | Distance/time | Scalar |
| Velocity | Displacement/time | Vector |
| Acceleration | Rate of change of velocity (m/s²) | Vector |
- Equations of motion: v = u + at, s = ut + ½at², v² = u² + 2as.
- Displacement can be zero but distance cannot (e.g. one full round of a circle). Distance ≥ |displacement|.
- In free fall, all bodies (without air resistance) fall with the same acceleration g ≈ 9.8 m/s² — Galileo.
- In uniform circular motion speed is constant but velocity changes, so there is (centripetal) acceleration.
📌 2. Newton's Laws of Motion
| Law | Statement | Example |
|---|---|---|
| First (law of inertia) | A body keeps its state unless an external force acts | Passengers fall back when a bus starts suddenly; dust leaves a beaten carpet |
| Second | F = ma = rate of change of momentum | A fielder pulls hands back while catching (more time, less force) |
| Third | Every action has an equal and opposite reaction | Rocket, recoil of a gun, swimming, walking |
- Momentum p = mv (kg·m/s). With no external force total momentum is conserved.
- Impulse = force × time = change in momentum.
- Inertia depends on mass — a heavier body has more inertia.
- 1 N is the force that gives 1 kg an acceleration of 1 m/s².
📌 3. Friction & Circular Motion
- Order of friction: static > sliding (kinetic) > rolling. That is why ball bearings reduce friction.
- Ways to reduce friction: lubricants (oil/grease), polishing, ball bearings, streamlined shape.
- Centripetal force = mv²/r, towards the centre. Centrifugal force is a pseudo force — cream separator, washing-machine dryer.
- Raising the outer edge of a road at a curve = banking; a cyclist leans inward on a turn.
⚠️ Exam Traps:
- Walking is impossible without friction — friction is not always "bad".
- Action and reaction act on different bodies, so they do not cancel each other.
- Moving in a circle at constant speed is still accelerated motion.
📌 4. Practice MCQs
📝 Practice MCQs (prepared by SF Prep — not PYQs)
Q1. Newton's first law of motion is also called the: Practice
Correct answer: (A)
The first law says a body keeps its state of rest or motion — this is inertia.
The first law says a body keeps its state of rest or motion — this is inertia.
Q2. The launching of a rocket is based on Newton's: Practice
Correct answer: (A)
Gases are pushed down (action) and the rocket moves up (reaction).
Gases are pushed down (action) and the rocket moves up (reaction).
Q3. Passengers fall backward when a bus starts suddenly. This is due to: Practice
Correct answer: (C)
The lower body moves with the bus while the upper body tends to stay at rest.
The lower body moves with the bus while the upper body tends to stay at rest.
Q4. The SI unit of momentum is: Practice
Correct answer: (A)
p = mv, so kg·m/s (= N·s).
p = mv, so kg·m/s (= N·s).
Q5. For a body moving in a circle at uniform speed, which is true? Practice
Correct answer: (B)
Direction changes constantly, so velocity changes; centripetal acceleration points to the centre.
Direction changes constantly, so velocity changes; centripetal acceleration points to the centre.
Q6. The correct decreasing order of friction is: Practice
Correct answer: (D)
Starting to slide is hardest; rolling is easiest.
Starting to slide is hardest; rolling is easiest.
Q7. A car starts from rest with acceleration 2 m/s². Its velocity after 5 s is: Practice
Correct answer: (B)
v = u + at = 0 + 2×5 = 10 m/s.
v = u + at = 0 + 2×5 = 10 m/s.
Q8. Why does a fielder pull his hands back while catching a ball? Practice
Correct answer: (C)
F = Δp/t; more time means less force and less hurt.
F = Δp/t; more time means less force and less hurt.
Q9. The recoil of a gun is explained by: Practice
Correct answer: (D)
The bullet gains forward momentum, so the gun gains equal backward momentum.
The bullet gains forward momentum, so the gun gains equal backward momentum.
Q10. A cream separator (centrifuge) works on: Practice
Correct answer: (B)
On spinning, heavier particles move out and lighter cream stays near the centre.
On spinning, heavier particles move out and lighter cream stays near the centre.
Q11. Which of these is a vector? Practice
Correct answer: (D)
Displacement has both magnitude and direction.
Displacement has both magnitude and direction.
Q12. After one full round of a circle of radius r, the displacement is: Practice
Correct answer: (B)
Start and end points are the same, so displacement is zero; distance is 2πr.
Start and end points are the same, so displacement is zero; distance is 2πr.
Q13. Why does a cyclist lean inward while turning? Practice
Correct answer: (C)
Leaning lets a component of the normal reaction supply force towards the centre.
Leaning lets a component of the normal reaction supply force towards the centre.
Q14. One newton equals: Practice
Correct answer: (D)
F = ma: 1 kg × 1 m/s² = 1 N. (g·cm/s² is a dyne)
F = ma: 1 kg × 1 m/s² = 1 N. (g·cm/s² is a dyne)
Q15. If a feather and a coin are dropped together in a vacuum: Practice
Correct answer: (A)
In a vacuum g is the same for all bodies.
In a vacuum g is the same for all bodies.
Q16. Impulse is equal to: Practice
Correct answer: (B)
Impulse = F × t = Δp.
Impulse = F × t = Δp.
Q17. Inertia of a body is measured by its: Practice
Correct answer: (A)
More mass = more inertia.
More mass = more inertia.
Q18. Why are ball bearings used in machines? Practice
Correct answer: (D)
Rolling friction is the least.
Rolling friction is the least.
Q19. A 5 kg body moves at 4 m/s. Its momentum is: Practice
Correct answer: (C)
p = mv = 5 × 4 = 20 kg·m/s.
p = mv = 5 × 4 = 20 kg·m/s.
Q20. A body is thrown up at 20 m/s (g = 10 m/s²). Its maximum height is: Practice
Correct answer: (C)
v² = u² − 2gh → 0 = 400 − 20h → h = 20 m.
v² = u² − 2gh → 0 = 400 − 20h → h = 20 m.