# Homework Help: Hooke's Law and force constant

1. Sep 2, 2010

### Ogakor

1. The problem statement, all variables and given/known data
A force of 720 N stretches a certain spring a distance of 15 cm. What is its force constant? If a 60-kg mass is hung on it, how far will it stretch?

Solving for Force Constant
given:
F (Restoring Force) = 720 N
ΔL (spring's elongation) = 15 cm x (1 m / 100 cm) = 0.15 m
k (Force constant) = ?

k = F / ΔL
k = 720 N / 0.15 m
k = 4,800 N/m

Solving for spring's elongation if mass is 60kg
given:
m = 60 kg
g = 9.8 m/s2
F = ?
ΔL = ?
ΔL = F / k
since F = mg,
ΔL = mg / k
Im not really sure what k is because the mass change. If I'll use the k from the first question, this will be the outcome.
ΔL = (60kg)(9.8m/s2) / 4800 N/m

ΔL = 0.1225 m

My problem is ithat, Im not sure what k (Force constant) is (in the 2nd problem). I've solved it with the first problem but since the mass change, maybe force constant is not the same as the first question. And if k is not the same, there will be two unknowns.
What do you think? Am I gonna use the k in the first question or am I gonna use another solution? What solution should I use?
On the other hand, I think force is constant because of the name. Not really sure, Im a noob at physics.

Last edited: Sep 2, 2010
2. Sep 2, 2010

### rock.freak667

The force constant for a spring is well...constant. So you did it correctly.

3. Sep 2, 2010

### Ogakor

thanks rock.freak