A bodybuilder configures a leg-press apparatus (see figure) to a resistance of 1

Question

A bodybuilder configures a leg-press apparatus (see figure) to a resistance of 1090 N (about 245 lb). She pushes the weight to her full extension and comes to rest.

(a) What is the normal force on each of her feet?

b) Assume she is able to move the push-plate of the leg-press machine at an acceleration of 0.50 m/s2 for the first half of the displacement. What is the normal force on each foot during this period of acceleration? Assume the resistive force is due to a mass with weight of 1090 N hanging on the cable of the leg-press apparatus.

Explanation / Answer

a) The normal force on her legs will be exactly equal to 1090N but in opposite direction.

on each leg it will be 1090/2 = 545N

b) mass of machine = 1090/10 = 109kg

total normal force (on both legs) = m(g+a)    g = 10m/s2

so normal force = 109(10+0.5) = 1144.5N

on each leg it will be = 1144.5/2 = 572.25N

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