Work done on a falling elevator by a spring + gravity

AI Thread Summary
The discussion centers on solving a physics problem involving work done by gravity and a spring in a falling elevator scenario. The user initially struggles with the calculations, particularly in part (c), and realizes that they misapplied the work done by gravity. They later deduce that the correct approach involves using the conservation of energy equation, which simplifies the problem significantly. Other participants confirm that using the conservation of energy is indeed a faster method to derive the quadratic equation needed for the solution. The conversation concludes with the user expressing gratitude for the clarification on the approach.
simphys
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Homework Statement
An elevator cable breaks when a 925-kg elevator
is 22.5 m above the top of a huge spring
at the bottom of the shaft. Calculate
(a) the work done by gravity on the elevator before it hits the
spring; (b) the speed of the elevator just before striking the
spring; (c) the amount the spring compresses (note that here
work is done by both the spring and gravity).
Relevant Equations
WE principle + w=Fd
I don't understand what I have done wrong in part (c) I have the initial velocity for the second part of the motion and have the final velocity zero and then the net work done is W_mg + W_Fs and the actual answer for x is 2.37m
Could I get some help/tips please, thanks in advance.

Here is my solution
1657539487834.png

1657539504042.png

small edit: After getting V_2 = 21m/s, x = 3.19m and the ,(comma) is a decimal point here! .(dot) is multiplication
 
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I think I see the problem.. I used the work done by gravity from the free fall motion part.
for the second part it actually is W_mg = mgx (if I am not mistaking)
 
I got the following: ##-4E4*x^2 + 9.074E3*x + 2.04E5=0 ##
which I solved with the calculator directly, but using the quadractic formula would be a lot of writing, is there another way I can look at this problem or something?
 
simphys said:
I got the following: ##-4E4*x^2 + 9.074E3*x + 2.04E5=0 ##
which I solved with the calculator directly, but using the quadractic formula would be a lot of writing, is there another way I can look at this problem or something?
I cannot read the value of k in the posted image, but I deduce it must be 8E4 N/m.
I agree with your quadratic. There's no simpler approach.
 
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haruspex said:
I cannot read the value of k in the posted image, but I deduce it must be 8E4 N/m.
I agree with your quadratic. There's no simpler approach.
Thank you! I thought that there may be a way to just use the initial and final position instead of breaking it up into part, but I didn't try. I supposed that it might've work that is why I asked.
 
simphys said:
Thank you! I thought that there may be a way to just use the initial and final position instead of breaking it up into part, but I didn't try. I supposed that it might've work that is why I asked.
Sorry, I thought you were just asking about solving the quadratic at the end.
Yes, if you only had to solve part c you could just write the conservation of energy equation straight off:
##(h+x)mg=\frac 12kx^2##
and get the quadratic from that.
 
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haruspex said:
Sorry, I thought you were just asking about solving the quadratic at the end.
Yes, if you only had to solve part c you could just write the conservation of energy equation straight off:
##(h+x)mg=\frac 12kx^2##
and get the quadratic from that.
Oh right, way faster thus. Thank you!
 
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