<< Graphics`Shapes`

m = 1;
k = 1;
dt = 1;
dx = 1;
restLength = 1;
v = 0;

a = k*(dx - restLength)/m;
vnew = v + a*dt;
vave = (vnew + v)/2;
dx = dx + vave*dt;

s1 = Sphere[1, 4, 4];
s2 = Sphere[1, 4, 4];

Show[TranslateShape[Graphics3D[s1], {dx, 0, 0}], 
        TranslateShape[Graphics3D[s2], {-dx, 0, 0}], 
        RotateShape[Graphics3D[Cylinder[0.2, 4, 2*dx]], 0, Pi/2, 
          Pi/2]];

dx