# Spings Launching Balloons



## Solar_Chris (Mar 30, 2010)

Question 56 in the 6 minute solutions for Machine Design is a problem about launching balloons with springs. I believe the solution to the number of springs required to launch the balloons is wrong. This should be a conservation of energy analysis. The springs do not exert the full 2.51 pound force/per spring the entire duration of the launch. The problem should be set up as (1/2)*mass of balloon / (gc) * (velocity)^2 = (1/2)*(Keq)*(Deflection)^2.

The kinetic energy induced into the balloon is equal to the energy stored in the springs prior to launch. Does anyone else think the solution in the book is wrong.


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## Oughtsix (Apr 3, 2010)

I set it up and solved it several times as an energy balance and finally looked at the answer. I would not think of solving it that way even given acceleration time. I don't think the answer makes sense either. 10 springs launching a 4.5 lb balloon 100 yards while imparting energy for 5" over 1 sec. I can picture it done with several dozen springs in series with a large deflection. I haven't been impressed with the SMS book yet. To many mistakes and odd balls.


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## BrianC (Apr 3, 2010)

I agree as well. I solved this problem yesterday, and was disappointed with the method the manual used to get an answer. An energy balance should solve the problem once the initial velocity is calculated.

I am also getting more and more frustrated with the 6MS machine design book the deeper I get. I plan to push through the last dozen or so problems today, but I am questioning several of the solutions. Before I bought the book I noticed it was rated somewhat low on Amazon. I figured it was an anomaly, because the other 6MS books are recommended so highly. I now understand the low rating.


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## Lily (Sep 19, 2010)

Hi,

I did this problem for the second time this morning and I still think the conservation of energy should be used, so the sms solution is wrong.

However, trying to resolve the problem with the conservation of energy, I am not sure what deflexion to use for the spring, because the max extension that is 5 inch is the lenght difference between the lenght of the free spring and the spring when it is fully extended (just before launching the ball). But to be able to launch a balloon, the spring must be compressed first, so should we use 5 inch X 2, assuming that the extension is equal to the compression ?


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