When Josh is benching he can, at a maximum, lift a barbell of weight W if he is

Question

When Josh is benching he can, at a maximum, lift a barbell of weight W if he is lifting the barbell at a constant velocity. a. If josh is accelerating the barbell upward, will he be able to lift more or less weight? b. How much weight W' can he bench if he is required to accelerate the barbell with vertical acceleration ay relative to his chest? Give a reasonable estimate of this weight using your answer and reasonable estimates for W and a_y. c. Josh transports a bench press apparatus into the PAB elevators and starts benching in there. He makes sure that he's lifting the barbell at a constant velocity relative to the elevator. If elevator has vertical acceleration A, relative to the ground, what is the maximum weight he can lift? Give a reasonable estimate for this maximum weight. d. Repeat part c. in the case that Josh is lifting the barbell with acceleration ay relative to the elevator.

Explanation / Answer

If josh lift the barbell of weight W with constant speed then the maximum force applied by the Josh will be W

a) If josh can lift the barbell of weight W with upward acceleration then the maximum force applied by the Josh will be greater than W

b) If he can lift the barbell of weight W with upward acceleration ay then the maximum force applied by the Josh will be

F - W = may where m = W/g

=> F = W + (W/g)ay

Now the maximum weight W' lift by Josh = F = W + (W/g)ay

so W' = W + (W/g)ay

c) If the elevator will go up with acceleration Ay then the effective weight of the barbell will be

m(g+Ay) = mg+mAy = W+(W/g)Ay

So the maximum weight lift by Josh in the lift, Wm = W + (W/g)Ay

d) If the elevator will go up with acceleration Ay and he can lift the barbell of weight W with upward acceleration ay then the maximum force applied by the Josh will be F and it is given by

F - Wm = may where m = W/g

=> F = Wm + (W/g)ay

Now the maximum weight W' lift by Josh = F = Wm + (W/g)ay

so W' = Wm + (W/g)ay

= [W + (W/g)Ay]+ (W/g)ay

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