work and potential energy problem?
1) a weightlifter bench presses a 160 kg barbell from his chest to a height equal the length of his arms 0.800m. calculate work. soo i know that work is = f*d but i dont really get it.
also whats the potential energy of the barbell relative to the weightlifter's chest after he lifts the barbell?
lastly how many food calories does the weightlifter use if he does this lift 35 times? assume that the weightlifter does no work in letting the barbell drop back to his chest
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work is a force acting through a distance
no force → no work (as in coasting through space)
no distance → no work (as in orbiting the Earth - force of gravity acts at right angle to motion)
in this case
F = m g = 160kg * 9.81N/kg = 1570N
W = F d = 1570N * 0.800m = 1256J
the increased gravitational PE is equal to the work which caused the increase
so
delta(GPE) = 1256J
do it 35 times
W = 35 * 1256J = 43960J or 43.96kJ
a dietary Calorie is 4.18kJ
a dietary Calorie is actually a kcal
so
43.96kJ / 4.18kJ/Cal = 10.5Cal