# Srping Problem

1. Aug 20, 2005

### teknodude

Spring Problem

I need some help or hints on this exercise

When a certain linear spring has a lenth of 180mm, the tension in it is 170N. FOr a length of 160mm, the compressive force in the spring is 130N.

a. What is the stiffness of the spring in SI units? In U.S. Customary units?
b. What is its unstretched length in SI units? In U.S. customary units?

This exercise is from my Engineering statics book in the section talking about units.

a. They are asking for the spring constant k, but for which spring do they want?
I calculated the stiffness of the first spring, 944 N/m and 64.7 lb/ ft just divided the force and the length.

b. the keyword unstretched is confusing me in this part.

Yea the topic is mispelled, couldn't change it

Last edited: Aug 20, 2005
2. Aug 20, 2005

### VietDao29

Nope, there is only one spring.
The problem is:
If the spring has the length of 0.18 m, its tension is 170 N.
If the spring has the length of 0.16 m, its compressive force is 130 N.
Let l0 (measured in m) be its normal length (when the spring is unstretched).
Hooke's law: F = -kx. So you have:
$$\left\{ \begin{array}{l} k(l_0 - 0.16) = 130 \\ k(0.18 - l_0) = 170 \end{array}\right$$ using Hooke's law.
From there can you solve for k and l0?
Viet Dao,

Last edited: Aug 20, 2005
3. Aug 20, 2005

### teknodude

Yea, i can solve it now. Just solve the system. I think what was throwing me off was tension and compressive force. I was thinking the spring was being compressed every time, but in fact it is being pulled for for 0.18m and then compressed to 0.16m.

Thanks man