SOS: Solving the Spring Constant of an Elastic Cord

[Sean1082]

Homework Statement

An elastic cord is 56 cm long when a weight of 73 N hangs from it but is 81cm long when a weight of 160 N hangs from it.

What is the spring constant "k" of this elastic cord?

Homework Equations

mg=kx

The Attempt at a Solution

No idea... my teacher doesn't know how to teach physics and the whole class is completely lost.

 

[Doc Al]

Express what Hooke's law says for each situation:
F₁ = kx₁
F₂ = kx₂

Combine those equations and see what you can deduce.

 

[Sean1082]

I have no idea what you mean by combine...

 

[Doc Al]

Another way to solve this is to call the unknown unstretched length of the spring L. Now write Hooke's law for each of the given conditions. You'll have two equations which you can solve simultaneously.