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Q5.22: NCERT Physics - Part 1 WORK, ENERGY AND POWER 5.22 Solution: A person trying to lose weight (dieter) lifts a 10 kg mass, one …
⚠️ Common Exam Traps & Conceptual Pitfalls
Students often add the energy lost when the mass is lowered, assuming the person does work both up and down; however, the person does work only against gravity during the lift, while the potential energy released on the way down is dissipated, not supplied by her. Multiply $mgh$ by the number of lifts, not by two, to avoid this error.
A person trying to lose weight (dieter) lifts a 10 kg mass, one thousand times, to a height of 0.5 m each time. Assume that the potential energy lost each time she lowers the mass is dissipated. (a) How much work does she do against the gravitational force? (b) Fat supplies 3.8 _ 10_ J of energy per kilogram which is converted to mechanical energy with a 20% efficiency rate. How much fat will the dieter use up?
Step-by-Step Verified Solution
**Answer:** (a) W = 5 _ 10_ J (b) Fat used = 6.58 _ 10__ kg
**Step-by-Step Explanation:**
(a) Work per lift = mgh = 10 _ 10 _ 0.5 = 50 J. Total work = 1000 _ 50 = 5 _ 10_ J.
(b) Energy from fat = 5 _ 10_/0.20 = 2.5 _ 10_ J. Fat used = 2.5 _ 10_/(3.8 _ 10_) = 6.58 _ 10__ kg.
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