Posted by ablin on .
a force of 3.5 N is applied to a spring which obeys Hooke's law and produces and extension of 0.07m.
calculate the spring constant
what is the size of the force that will produce and extension of 4.0 cm
caluclate the unstretched length of the spring if its total length is 17.5 cm when applied force is 5 N.

physic 
drwls,
The spring constant k = Force/extension.
In your case, k = 3.5/0.07 = 50 N/m
Use that k to calculate the force when the extension = 0.04 m. It should be (4/7) times 3.5 = ___ Newtons.
For the final question, calculate the extension when F = 5N,
extension = F/k = 0.1 m = 10 cm
and subtract that from 0.175 m to get the unstretched length.