# Extension spring force

1. Mar 21, 2013

### Z-inc

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.

2. Mar 21, 2013

### Simon Bridge

Welcome to PF;
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.

3. Mar 21, 2013

### Z-inc

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.

4. Mar 21, 2013

### Simon Bridge

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?