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Q4.13: NCERT Physics - Part 1 LAWS OF MOTION 4.13 Solution: A man of mass 70 kg stands on a weighing scale in a lift which…
⚠️ Common Exam Traps & Conceptual Pitfalls
Students often confuse high velocity with high acceleration, incorrectly adding or subtracting the lift's speed from $g$ in part (a). Remember that uniform speed implies zero acceleration, so the normal force equals the true weight ($mg$) regardless of how fast the lift is moving.
A man of mass 70 kg stands on a weighing scale in a lift which is moving (a) upwards with a uniform speed of 10 m s__, (b) downwards with a uniform acceleration of 5 m s__, (c) upwards with a uniform acceleration of 5 m s__. What would be the readings on the scale in each case? (d) What would be the reading if the lift mechanism failed and it hurtled down freely under gravity?
Step-by-Step Verified Solution
**Answer:** (a) 70 kg (b) 35 kg (c) 105 kg (d) Zero
**Step-by-Step Explanation:**
(a) Uniform speed means acceleration = 0. Reading = mg/g = 70 kg [citation:10].
(b) Downward acceleration: apparent weight = m(g - a) = 70(10 - 5) = 350 N. Reading = 350/10 = 35 kg.
(c) Upward acceleration: apparent weight = m(g + a) = 70(10 + 5) = 1050 N. Reading = 1050/10 = 105 kg.
(d) Free fall: a = g. Apparent weight = m(g - g) = 0. Reading = 0 [citation:10].
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