121. We apply a force of 200 Newtons on a wooden Box and push it on the floor at constant velocity. The magnitude of frictional force will be: (A) 100 Newtons (B) 200 Newtons (C) 300 Newtons (D) 400 Newtons
Answer: (B) Constant velocity → Net force = 0 → F_friction = F_applied = 200 N.
122. A truck starts from rest down a hill with a constant acceleration. It achieves 400 meters in 20 seconds. If the weight of the truck is 7 tons then what will be the force acting on it? (A) 11000 Newtons (B) 12000 Newtons (C) 13000 Newtons (D) 14000 Newtons
Answer: (B) s = ut + ½at² → 400 = 0 + ½a(20)² → a = 2 m/s² m = 7 tons = 7000 kg F = ma = 7000 × 2 = 14000 N → Wait, error. Wait: 400 = ½a(400) → a = 2 m/s² → F = 14000 N → (D). But per key: (B). Wait: Weight = mg = 7×10³×10 = 70,000 N, but force down hill. Question: “force acting on it” → net force = ma = 7000×2 = 14000 N → (D).
123. Energy consumed in a home is 250 units then the total energy in Joule will be: (A) 9×10⁸ (B) 8×10⁸ (C) 9×10⁹ (D) 9×10⁸
Answer: (D) 1 unit = 1 kWh = 3.6×10⁶ J → 250 × 3.6×10⁶ = 9×10⁸ J.
124. In which medium, the speed of sound is maximum? (A) Steel (B) Water (C) Air (D) Hydrogen
Answer: (A) Speed: Steel (~5000 m/s) > Water (~1500) > Air (~340) > H₂.
125. The potential energy of a freely falling body continuously decreases. (A) The principle of conservation of energy is violated (B) The principle of conservation of energy is not violated (C) Gravitational force is violated (D) Gravitational force is not violated
Answer: (B) PE → KE; total energy conserved.
126. A pair of Oxen exerts a force of 140 newton while ploughing the field. The field ploughed is 15 meter long. The work done in ploughing the length of the field? (A) 1900 Joules (B) 2000 Joule (C) 2100 Joules (D) 2200 Joule
Answer: (C) W = F × d = 140 × 15 = 2100 J.
127. The focal distance of a plane mirror is (A) +1 cm (B) –1 cm (C) 2 cm (D) Infinity
Answer: (D) Plane mirror: f = ∞.
128. Which term in the following does not denote the electric power in electric circuit? (A) I²R (B) IR² (C) VI (D) V/I R
Answer: (D) Power = VI = I²R = V²/R. V/IR = ρ (resistivity) → not power.
129. In short circuit, the value of electric current in a time circuit: (A) Very low (B) Do not change (C) Increases very high (D) Continuously changing
Answer: (C) Short circuit → R→0 → I→∞.
130. What is the device to protect equipments from the electric shock? (A) Fuse (B) Generator (C) Motor (D) Current controller
Answer: (A) Fuse breaks circuit at high current.