Question

A50kg woman repeatedly lifts a 20kg barbell 0.80m from her chest to an extended position above her head.

A) if her body retains 10J of thermal energy for each joule of work done while lifting, how many times must she lift the barbell to warm her body to 0.50°C?

B) state any assumptions you used

C) list all of the physics ideas you used to solve this problem

Answer 1

Gain in Potential energy while lifting each time = 20Kg X 9.8 m/s2 X 0.8m = 156.8 J

If 10J energy is retained for every Joule of work, enrgy gain per lift= 1568 J

A) if temperature gain is to be 0.50 deg C, she needs to abosorb energy = 50Kg X sp.heat of human body X 0.50 deg C =105000J

no of lift cycles = 105000/15688 = 67 lifts

Since 70% of human body is water ,it is assumed that sp heat of body is same as that of water at 4200J/Kg/deg c

Heat required = 50 Kg X 4200 J/Kg/deg C X 0.50 deg c = 105000 J