physics help???? thought i know how to do it but i don't!!!!?

Question

A 1200-kg car rolling on a horizontal surface has speed v = 65 km/h when it strikes a horizontal coiled spring
and is brought to rest in a distance of 2.2 m.. What is the spring stiffness constant of the spring?

Answer 1

Easiest way to do it is with energy:

First, convert the km/h to m/s: v = 65*1000/3600 = 18.06 m/s
The car has a kinetic energy of 1/2·m·v^2 = 195698.2 Joules.

So the spring will store an elastic potential energy of 195698.2 J. Since elastic energy is 1/2·k·x^2, where k is the stiffness constant, you can get k:

k = 2*195698.2/2.2^2 = 80867 N/m.

Answer 2

Use energy conservation.