2012 F=ma problem 21

ksrivatsan

Hi,

I'm just training for the 2013 F=ma exam, and I took some old exams. I tried to do the 2012 exam, and could not figure out problem 21. Can anyone help me, please?

1. The problem statement, all variables and given/known data
A spring system is set up as follows: a platform with weight 10 N is on top of two springs, each with a spring constant of 75 N/m. On top of the platform is a third spring with spring constant 75N/m. If a ball with weight of 5.0 N is fastened to the top of the third spring and then slowly lowered, by how much does the height of the spring system change?

(A) 0.033 m
(B) 0.067 m
(C) 0.100 m (correct answer)
(D) 0.133 m
(E) 0.600 m

2. Relevant equations

F=kx

3. The attempt at a solution
The bottom springs act as one spring with K=150 N/m, thus will contract 0.033 m under the weight
The top spring will compress 0.067 m under the weight
so is it just addition...?

Doc Al

Yes, that's all there is to it. You are just combining the two compressions to get the total compression.

ksrivatsan

Thanks Doc Al!