Key Concepts

15 Practice MCQs

1. A 2 kg stone & 10 kg bench are both at rest. Which needs more force to start motion? A. Stone
   B. Bench
   C. Same
   D. Depends on surface
   Answer: B
   Solution: F = ma; same a needs F ∝ m.
   Shortcut: Heavier ⇒ more inertia ⇒ more force.
   Tag: Newton-I

2. A 0.05 kg bullet leaves a 5 kg rifle at 400 m/s. Recoil speed of rifle is nearly A. 4 m/s
   B. 0.4 m/s
   C. 40 m/s
   D. 0.04 m/s
   Answer: A
   Solution: 0 = 5v + 0.05×400 ⇒ v = –4 m/s (magnitude 4).
   Shortcut: vrecoil = (mbullet/mgun) vbullet.
   Tag: Conservation of momentum

3. A 500 g ball changes velocity from 10 m/s East to 15 m/s West in 0.01 s. Average force is A. 250 N
   B. 500 N
   C. 1250 N
   D. 2500 N
   Answer: C
   Solution: Δv = –15 –10 = –25 m/s; Δp = 0.5×(–25)= –12.5 kg·m/s; F = Δp/Δt = –12.5/0.01 = –1250 N (magnitude 1250 N).
   Shortcut: F = 2mv/Δt when direction reverses.
   Tag: Impulse

4. When a fast-moving car stops suddenly, passengers lurch forward due to A. Inertia of motion
   B. Inertia of rest
   C. Friction
   D. Gravity
   Answer: A
   Solution: Body tends to keep its state of motion.
   Shortcut: “Lurch forward” ⇒ motion inertia.
   Tag: Newton-I

5. A cricket ball is pushed 2 m by a恒定 20 N force. Work done is A. 10 J
   B. 20 J
   C. 40 J
   D. 400 J
   Answer: C
   Solution: W = Fs = 20×2 = 40 J.
   Shortcut: W = F × distance (when force ∥ displacement).
   Tag: Work-energy

6. Action-reaction forces A. Cancel each other on same body
   B. Act on different bodies
   C. Are always perpendicular
   D. Can be unequal
   Answer: B
   Solution: Newton-III pairs act on two interacting bodies.
   Shortcut: “Different bodies” ⇒ never cancel.
   Tag: Newton-III

7. A 4 kg object accelerates at 3 m/s². Net force is A. 7 N
   B. 12 N
   C. 1.33 N
   D. 0.75 N
   Answer: B
   Solution: F = ma = 4×3 = 12 N.
   Shortcut: F = ma (direct multiplication).
   Tag: Newton-II

8. A 20 g bullet at 300 m/s stops in 0.1 s on hitting wall. Average resistance force is A. 30 N
   B. 60 N
   C. 300 N
   D. 600 N
   Answer: B
   Solution: Δp = 0.02×300 = 6 kg·m/s; F = 6/0.1 = 60 N.
   Shortcut: F = mv/t (grams→kg!).
   Tag: Impulse

9. Two ice skaters push each other; 60 kg moves 2 m/s left, 40 kg moves right at A. 2 m/s
   B. 3 m/s
   C. 4 m/s
   D. 5 m/s
   Answer: B
   Solution: 0 = 60×2 – 40v ⇒ v = 3 m/s.
   Shortcut: v2 = (m1/m2)v1.
   Tag: Conservation of momentum

10. A horse pulls a cart; the cart pulls horse with equal force. Why does system move? A. Forces cancel
    B. Forces act on different bodies
    C. Frictionless ground
    D. Horse heavier
    Answer: B
    Solution: Forward force on cart > ground friction on cart.
    Shortcut: Motion because pair acts on different bodies.
    Tag: Newton-III

11. A 1000 kg car decelerates from 20 m/s to rest in 4 s. Braking force is A. 2000 N
    B. 4000 N
    C. 5000 N
    D. 8000 N
    Answer: C
    Solution: a = (0–20)/4 = –5 m/s²; F = 1000×5 = 5000 N.
    Shortcut: F = m(Δv/t).
    Tag: Newton-II

12. A ball thrown upward has at top point A. Zero velocity, zero acceleration
    B. Zero velocity, acceleration g downward
    C. Non-zero velocity, zero acceleration
    D. Non-zero velocity, non-zero acceleration
    Answer: B
    Solution: v = 0, a = g = 9.8 m/s² downward.
    Shortcut: At peak, v = 0 but gravity still pulls.
    Tag: Free-fall

13. A 2 kg & 3 kg masses collide & stick. Initial momenta 6 kg·m/s & 4 kg·m/s same direction. Final speed is A. 1 m/s
    B. 2 m/s
    C. 3 m/s
    D. 4 m/s
    Answer: B
    Solution: ptotal = 6 + 4 = 10 kg·m/s; mtotal = 5 kg; v = 10/5 = 2 m/s.
    Shortcut: v = (p1+p2)/(m1+m2).
    Tag: Inelastic collision

14. Sudden removal of table-cloth without disturbing dishes demonstrates A. Inertia of rest
    B. Inertia of motion
    C. Gravity
    D. Friction
    Answer: A
    Solution: Dishes tend to stay at rest.
    Shortcut: “Dishes don’t move” ⇒ rest inertia.
    Tag: Newton-I

15. A 50 g bullet at 200 m/s embeds in 950 g stationary block. Velocity of combined mass is A. 5 m/s
    B. 10 m/s
    C. 20 m/s
    D. 40 m/s
    Answer: B
    Solution: 0.05×200 = (0.95+0.05)v ⇒ v = 10/1 = 10 m/s.
    Shortcut: v = (mbullet/mtotal) vbullet.
    Tag: Inelastic collision

Speed Tricks

Quick Revision