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A lift carrying a man on a bathroom scale goes into free fall, accelerating downward at the acceleration due to gravity. What does the scale read?
AHis full weight, since gravity is unchanged
BZero, so he appears to be weightless
CHalf his weight, since the lift is falling
DTwice his weight, since the fall adds force
Answer & Solution
Correct answer: B. Zero, so he appears to be weightless
1. The scale reading is the mass times the acceleration plus the mass times the acceleration due to gravity.
2. In free fall the acceleration is downward and equal in size to the acceleration due to gravity.
3. Substituting a downward acceleration makes the two terms cancel exactly.
4. The scale therefore reads zero and the person appears to be weightless.
5. Gravity still acts on him, so his weight has not actually vanished.
_Source: OpenStax College Physics (CC BY 4.0), Ch 4 "Dynamics: Force and Newton's Laws of Motion", section 4.7 Further Applications of Newton's Laws of Motion_
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