# SMS 2nd ed 2nd print #56 Mechanical



## erniepower (Apr 9, 2010)

Has anyone else looked at this problem? I beleive the solution in the book takes the wrong approach.

I believe this should be a conservation of energy problem. Not a constant acceleration problem. You can calculate the needed takeoff velocity using the projectile motion equations. That seems simple enough. Then the book uses that takeoff velocity, and initial velocity of 0 and and acceleration time of 1 sec to calculate an acceleration and from that a force. Then they use this force to find the number of springs needed. I think they assume that the force calculated would be constant through the release of the spring but as soon as the balloon starts to move, the force will be less because the displacement of the spring will be less.....

I think you will have to find the kinetic energy of the balloon at take off, and set it equal to the potential energy stored in the springs to find the total number of springs.

If I am off my rocker, let me know.....


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## BrianC (Apr 10, 2010)

I think a few of us agree with you. Here is a link to an earlier thread.

http://engineerboards.com/index.php?showtopic=11828


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## Shanks (Apr 10, 2010)

While looking through errata for this problem, I came across this, a slight different approach still not the conservation of energy principle.

p. 53, Solution 65: The text and equations for the equivalent spring constant and the minimum number of springs needed should be removed and substituted with the following: “The force per spring can be calculated using F = kd = (0.5 lbf/in)(0.5 in) = 2.5 lbf. The number of springs required to launch the pumpkin would be N = Ftotal/Fper spring = 23.8 lbf/2.5 lbf = 9.52 springs. Rounding up to the nearest whole number, the number of springs is 10. (Bert Hartman) 9/26/200

PS: This problem is #65 in first print I've...


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