Verified Solution • 2025–26 Session
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Q4.1: LAWS OF MOTION Solutions — NCERT Physics - Part 1
⚠️ Common Exam Traps & Conceptual Pitfalls
Students frequently confuse the presence of individual forces (like friction or gravity) with the net force, incorrectly assuming that motion requires a non-zero resultant force. Remember that any object moving with constant velocity (including zero velocity) has zero acceleration, and by Newton's Second Law ($F_{net} = ma$), the vector sum of all forces must be zero.
Give the magnitude and direction of the net force acting on (a) a drop of rain falling down with a constant speed, (b) a cork of mass 10 g floating on water, (c) a kite skillfully held stationary in the sky, (d) a car moving with a constant velocity of 30 km/h on a rough road, (e) a high-speed electron in space far from all material objects, and free of electric and magnetic fields.
Step-by-Step Verified Solution
**Answer:** (a) Zero (b) Zero (c) Zero (d) Zero (e) Zero
**Step-by-Step Explanation:**
(a) The rain drop falls with constant speed, so its acceleration is zero. By Newton's second law, net force is zero [citation:3][citation:13].
(b) The cork floats on water. Its weight is balanced by the buoyant force. Net force is zero [citation:13][citation:14].
(c) The kite is stationary. By Newton's first law, net force acting on it is zero [citation:3][citation:13].
(d) The car moves with constant velocity, so acceleration is zero. Net force is zero [citation:3][citation:14].
(e) The electron is free from all fields, so no net force acts on it [citation:3][citation:13].
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