8. A toy train car rest on a vertical spring as shown if the springs unstretched length is 0.500 m and it has a spring constant of 90.0 N/m what is the mass of the toy train car?

We will have the following:
First, we remember that:
[tex]\begin{cases}F=ma \\ \\ F=-kx \\ \\ ma=-kx\end{cases}[/tex]And we will then have:
[tex]m(9.8m/s^2)=-(90.0N/m)(0.350m-0.500m)\Rightarrow m=-\frac{(90.0N/m)(-0.150m)}{9.8m/s^2}[/tex][tex]\Rightarrow m=\frac{135}{98}kg\Rightarrow m\approx1.38kg[/tex]So, the mass of the toy train car is approximately 1.38 kg.