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A safe held by a string breaks and lands on a spring

  • Thread starter talaroue
  • Start date
  • #1
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Homework Statement


A 1209 kg safe is 1.54 m above a heavy-duty spring when the rope holding the safe breaks. Thr safe hits the spring and compress it 53 cm. What is he spring constant of the spring?


Homework Equations


K=mV^2/2
U=mgh
Ki+Ui=Kf+Uf


The Attempt at a Solution



I thought I understood momentuem and energy but not anymore. Here is what I did....

Ui=mgh= 2*9.8*1.54
Kf=mV^2/2

Then solved for V and got 5.494 m/s

Then used that velocity for the spring constant by using......

U=k(delta x)^2/2
K=mV^2/2

then solved for k and got 129912 N/m...... but that is coming up wrong what am i doing wrong.
 

Answers and Replies

  • #2
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any ideas?
 
  • #3
PhanthomJay
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Well for one thing you are using m = 2 kg when it is given that m = 1209 kg. Then you neglect the gravitational potential energy in the 2nd set of equations. It is better to do this all in one step: Initial energy = Final energy, where v_initial = v_final = 0.
 
  • #4
tiny-tim
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Hi talaroue! :smile:

(try using the X2 and X2 tags just above the Reply box :wink:)
A 1209 kg safe is 1.54 m above a heavy-duty spring when the rope holding the safe breaks. Thr safe hits the spring and compress it 53 cm.

Ui=mgh= 2*9.8*1.54
No, the safe stops moving 53cm lower. :wink:
 
  • #5
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JAY:

so your saying that i should just use U=mgh and U=k(delta x)^2/2....solve for k and get....

k=2mgh/delta x^2

TINY TIM:

so I didn't go about the problem wrong I just have to add the .53 m to the hieght?
 
Last edited:
  • #6
Astronuc
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The safe starts 1.54 m (154 cm) above the spring and then travels another 0.53 m after contact. That must be considered with respect to gravitational potential energy.
 
  • #7
PhanthomJay
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JAMES:

so your saying that i should just use U=mgh and U=k(delta x)^2/2....solve for k and get....

k=2mgh/delta x^2
Yes, but 'h' is as noted by others, and don't forget to correct your value of 'm'.
 
  • #8
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ahhh I see at least I had the right idea just need to be more careful. If you guys don't mind I am having problems with another problem as well.... it might be a same problem i am going to go back and look at it but I made a thread about it as well....

https://www.physicsforums.com/showthread.php?t=316282"
 
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  • #9
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Yes, but 'h' is as noted by others, and don't forget to correct your value of 'm'.
I wasn't using 2 as my m, that was just noting the second equation is U=k(delta x^2)/2 I didn't combined them when i posted that, i see what you are saying though.
 

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