- #1
Ballin27
- 5
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Hello, first post. I hope that someone can help me with this. I will roughly summarize the problem, and what I've done thus far.
A lady was in an elevator that free-fell 6 feet and abruptly came to a halt. We had to determine a reasonable best case and a reasonable worst case value for the impact force that she experienced. The weight of the elevator is unknown and we are to find a value which I have not done yet. The woman weighs 140lbs.
x = x_0 + v_0*t + (1/2)a*t^2
F= ma
For the worst case I made the assumptions that all the weight was acting on one ankle and that the elevator hit the ground and came to a direct stop instantaneously.
The forces I had acting were her body weight, the weight of the elevator, and the ground reaction force.
I ended up with this:
F = 0 = -623.63 (N, bodyweight acting in the neg. direction) -Welevator(Weight of the elevator) + GRF (Ground reaction force)
Thus:
GRF = Welevator +623.23N
This was rather simple and once I find a weight of the elevator I should be fine.
My issue comes in the Best case scenario, where I assumed that the body weight was equally distributed through both ankles and that the elevator decelerates.
I had 1/2BodyWeight going through each ankle, the Welevator going through the middle, 1/2 of the GRF acting on each ankle.
I used:
x = xo + vo*t +1/2a*t^2
0 = 1.8288m + 0t + 1/2(-9.8m/s^2)*t^2
t = .61sec
Not sure if I can use this here, and my problem is deciding how I would get the force from there. I'm assuming I would need a vf and a vi as well as the mass of the elevator which I mentioned before. Would Impulse=momentum work here?
Any and all help would be greatly appreciated. Thanks in advance.
Homework Statement
A lady was in an elevator that free-fell 6 feet and abruptly came to a halt. We had to determine a reasonable best case and a reasonable worst case value for the impact force that she experienced. The weight of the elevator is unknown and we are to find a value which I have not done yet. The woman weighs 140lbs.
Homework Equations
x = x_0 + v_0*t + (1/2)a*t^2
F= ma
The Attempt at a Solution
For the worst case I made the assumptions that all the weight was acting on one ankle and that the elevator hit the ground and came to a direct stop instantaneously.
The forces I had acting were her body weight, the weight of the elevator, and the ground reaction force.
I ended up with this:
F = 0 = -623.63 (N, bodyweight acting in the neg. direction) -Welevator(Weight of the elevator) + GRF (Ground reaction force)
Thus:
GRF = Welevator +623.23N
This was rather simple and once I find a weight of the elevator I should be fine.
My issue comes in the Best case scenario, where I assumed that the body weight was equally distributed through both ankles and that the elevator decelerates.
I had 1/2BodyWeight going through each ankle, the Welevator going through the middle, 1/2 of the GRF acting on each ankle.
I used:
x = xo + vo*t +1/2a*t^2
0 = 1.8288m + 0t + 1/2(-9.8m/s^2)*t^2
t = .61sec
Not sure if I can use this here, and my problem is deciding how I would get the force from there. I'm assuming I would need a vf and a vi as well as the mass of the elevator which I mentioned before. Would Impulse=momentum work here?
Any and all help would be greatly appreciated. Thanks in advance.