Extension Spring Force Calc: Diam/Len for 5lb Proj 25-30ft

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Discussion Overview

The discussion focuses on determining the appropriate diameter and length of extension springs for a projectile device designed to propel a 5lb object approximately 25-30 feet. The conversation explores the necessary spring constants and the relationship between spring dimensions and their performance in this application.

Discussion Character

  • Exploratory
  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • One participant seeks to find the diameter and length of four extension springs to propel a 5lb object 25-30 feet, noting the need for the springs to extend approximately 3 feet.
  • Another participant points out that additional information is needed, specifically how the spring constant relates to the diameter and length of the springs and how the springs will be combined.
  • A different participant suggests using two extension springs on either side of a metal bar and expresses uncertainty about the spring constant, indicating a need for clarification on how to achieve the required force.
  • One reply proposes using conservation of momentum and ballistic equations to determine the necessary spring constant, outlining a method to work backwards from the desired range to find the required muzzle velocity and kinetic energy.
  • This participant also questions the assumptions made in the calculations and how the spring constant is related to the dimensions of the springs.

Areas of Agreement / Disagreement

Participants do not reach a consensus, as there are multiple competing views on how to approach the problem, and the discussion remains unresolved regarding the specific calculations and assumptions needed.

Contextual Notes

Limitations include the lack of specific values for the spring constants and the assumptions regarding projectile motion and energy conservation that have not been fully articulated or agreed upon.

Z-inc
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I need to determine the correct diameter and length of a combination of 4 extension springs to use in a projectile device. The springs must be able to extend to approx 3ft, and generate enough force to propel a set of objects that are 3ft in length and have an overall weight of approx 5lbs. The objects must be propelled approx 25-30ft.
 
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Welcome to PF;
There is not enough information to answer your question.
You need to know how the spring constant of each spring depends on the diameter and length of the springs in question and how you are going to combine the four of them.

You can work out a ballpark spring constant requirement from the data using conservation of energy and a model for the projectile motion.
 
Extension Spring

I want to use two extention springs on either side of a metal bar to propell a 5lb weight 25 -30ft. I need to determine the diameter and length of the springs that will provide enough force to propell that 5lb object. Therefore, I don't know the spring constant. I'm not sure if that helps in what I am asking for.

Thanks for the reply.
 
You can work out what the spring constant needs to be using conservation of momentum and ballistic equations.

Work it backwards:
You know the maximum range ... what angle gives you the max range?
What muzzle velocity is needed at that angle to get the needed range?
(What were the assumptions and are they reasonable?)

From the muzzle velocity - what is the kinetic energy?
What is the equation for the amount of energy stored in a spring system with effective spring constant k? ... What is k?

This will be the spring constant for the system of two springs so - knowing k for the combination, what is k for each spring?

Now you know what each spring needs to be able to pull.
How is the spring constant related to the diameter and length of the springs?
 

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