91Ó°ÊÓ

Useful work: Useful work includes all the work done by a person, because these are accomplished by exerting forces (nonconservative in nature) on the outside work.

Daily energy need of a person is,

\(E = {P_{sl}} \times {T_{sl}} + {P_w} \times {T_w} + {P_{ac}} \times {T_{ac}} + {P_{cy}} \times {T_{cy}} + {P_s} \times {T_s} + {P_{st}} \times {T_{st}}\)

Here,\({P_{sl}}\)is the power consumed in sleeping\(\left( {{P_{sl}} = 83{\rm{ W}}} \right)\),\({T_{sl}}\)is the time spent in sleeping\(\left( {{T_{sl}} = 7.00{\rm{ h}}} \right)\),\({P_w}\)is the power consumed in walking\(\left( {{P_w} = 280{\rm{ W}}} \right)\),\({T_w}\)is the time spent in walking\(\left( {{T_w} = 2.00{\rm{ h}}} \right)\),\({P_{ac}}\)is the power consumed in attending classes\(\left( {{P_{ac}} = 210{\rm{ W}}} \right)\),\({T_{ac}}\)is the time spent in attending classes\(\left( {{T_{ac}} = 4.00{\rm{ h}}} \right)\),\({P_{cy}}\)is the power consumed in cycling\(\left( {{P_{cy}} = 400{\rm{ W}}} \right)\),\({T_{cy}}\)is the time spent in cycling\(\left( {{T_{cy}} = 2.00{\rm{ h}}} \right)\),\({P_s}\)is the power consumed in sitting relaxed\(\left( {{{\rm{P}}_{\rm{s}}}{\rm{ = 125 W}}} \right)\),\({T_s}\)is the time spent in sitting relaxed\(\left( {{T_s} = 3.00{\rm{ h}}} \right)\),\({P_{st}}\)is the power consumed in studying\(\left( {{P_{st}} = 210{\rm{ W}}} \right)\), and\({T_{st}}\)is the time spent in studying \(\left( {{T_{st}} = 6.00{\rm{ h}}} \right)\).

Putting all known values,

\(\begin{aligned}E &= \left( \begin{aligned}{l}\left( {83{\rm{ W}}} \right) \times \left( {7.00{\rm{ h}}} \right) + \left( {280{\rm{ W}}} \right) \times \left( {2.00{\rm{ h}}} \right) + \left( {{\rm{210 W}}} \right){\rm{ \times }}\left( {{\rm{4}}{\rm{.00 h}}} \right) + \\\left( {{\rm{400 W}}} \right) \times \left( {2.00{\rm{ h}}} \right) + \left( {125{\rm{ W}}} \right) \times \left( {3.00{\rm{ h}}} \right) + \left( {210{\rm{ W}}} \right) \times \left( {6.00{\rm{ h}}} \right)\end{aligned} \right)\\ &= 4416{\rm{ W}} \cdot {\rm{h}} \times \left( {\frac{{1{\rm{ J}}/{\rm{s}}}}{{1{\rm{ W}}}}} \right) \times \left( {\frac{{3600{\rm{ s}}}}{{1{\rm{ hr}}}}} \right)\\ &= 15897600{\rm{ J}} \times \left( {\frac{{{\rm{1 kcal}}}}{{4184{\rm{ J}}}}} \right)\\ &= 3799.62{\rm{ kcal}}\end{aligned}\)

Therefore, the required daily energy needs of a person is \(3799.62{\rm{ kcal}}\).