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I am trying to figure out a question in first semester physics... I have a different but similar question that I have the answer for that I am trying to work to figure out the process for the other question.

What is the spring constant k of a spring designed to stop a 1300 kg car travelling 90 km/h so that the occupants experience a maximum acceleration of 5 g? (no friction or other forces mentioned)

I am getting an answer that is about 4 times larger than the given answer of 5 x 10^3 N/m

Here is what I did:

-Changed 90 km/h to 25 m/s

-Calculated 5g as 49 m/s/s

-set 1/2kx^2 = 1/2mv^2 --> KE = Potential Elastic Energy

-found x to be 6.378m because an acceleration of 49m/s/s should stop a car going 25 m/s in 6.378m

- used above numbers in above equation to get 19973.5076???

Does anyone know what I did wrong? Was it a math mistake or should I not be solving for x in that way?

Thanks Very Much

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# Homework Help: Spring Constant and Potential Energy Help Please

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