Question

Spiderman uses his spider webs to save a runaway train moving about 80 km/h , (Figure 1). His web stretches a few city blocks (600 m ) before the 104-kg train comes to a stop

Answer 1

$$0.5mv^2=0.5kx^2$$

$$(104)\left(\frac{80}{3.6}\right)^2=(600)^2k$$

$$k=0.143\frac{N}{m}$$