## Hooke's law with spring

When a 4.00 kg object is hung vertically on a certain light spring that obeys Hooke's law, the spring stretches 2.50 cm. (a)If the object is replaced with one of mass 1.50 kg, how far will the spring stretch?
b) How much work must an external agent do (i.e. a force coming from the 'environment'), to stretch the same spring 4.00 cm from its unstretched position?

For a) I used the equation F= -kx to solve for k.
F=ma so ma=-kx since the object is vertical.
(4.00)(-9.8)= -k(.025m)
k= 1568 N/m

then I plugged it back in to solve for x
ma= -kx
(1.5)(-9.8)= -1568x
x= 0.00938 m

Is there something I'm doing wrong?
b) W= 1/2k(xi)^2 - 1/2k(xf)^2
W= 0-1/2(1568)(.04m)^2
= -1.25 J

The homework says this is wrong but I can't figure out what else to do?
Any help would be appreciated.
 PhysOrg.com science news on PhysOrg.com >> Galaxies fed by funnels of fuel>> The better to see you with: Scientists build record-setting metamaterial flat lens>> Google eyes emerging markets networks

Recognitions:
Homework Help
$$(1.50)/(4.00)*(2.5 cm)=0.938 cm$$