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A person trying to lose weight by burning fat lifts a mass of 10 kg upto a height of 1 m 1000 times. Assume that the potential energy lost each time he lowers the mass is dissipated. How much fat will he use up considering the work done only when the weight is lifted up? Fat supplies 3.8×10⁷ J of energy per kg which is converted to mechanical energy with a 20% efficiency rate. Take g=9.8 m s⁻².

Asked in JEE Main 2016 · Work done by gravity and contact forces

• (1) 2.45×10⁻³ kg
• (2) 6.45×10⁻³ kg
• (3) 9.89×10⁻³ kg
• (4) 12.89×10⁻³ kg

Step-by-step solution

Why the other options are wrong

• (1) No single slip gives this. Sanity check: forgetting the 20% efficiency gives 2.58×10⁻³ kg, not 2.45.
• (2) No single slip gives this; it is exactly half the answer, and nothing in the data halves.
• (3) No single slip gives this. Sanity check: 98 kJ÷7.6 MJ/kg is 12.9×10⁻³ kg.

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