# Homework Help: Elastic potential question

1. Nov 13, 2013

### physicsdb

question: a grapefruit of mass 289g is attached to an unstretched vertical spring of force constant 18.7 N/m, and is allowed to fall.

a) determine the net force and the acceleration on the grapefruit when it is 10.0 cm below the unstretched position and moving downward.

b)air resistance will cause the grapefruit to come to rest at some equilibrium position. How far will the spring be stretched?

Relevant equations: Fspring=kx Espring=1/2kx^2

My attempt: Fnet=mg-kx im getting 26.452 but the answer for part a) is 0.962 down and 3.33ms/^2

2. Nov 13, 2013

### Staff: Mentor

Hi Physicsdb. Welcome to Physics Forums.

Show us your work for how you arrived at the number 26.452. Also, what are its units?

3. Nov 13, 2013

### physicsdb

i figured to find the net force it would be the force of gravity minus the force of the string,
Fnet=Fg-Fs
Fnet=mg-kx
Fnet=2.89kg(9.8m/s^2)-(18.7N/m)(0.1m)
Fnet=28.322-1.87
Fnet=26.452 N

4. Nov 13, 2013

### Staff: Mentor

That should be 0.289 kg.