A 2.8 kg toy car at rest on a horizontal surface is accelerated by a spring to 4.5 m/s in a distance of 5.3 cm. What is the spring constant of the spring? Hello, I was working on this problem in my homework but my professor hardly went over the spring constant in lecture. I was working on the problem but keep coming up with the incorrect answer. Is starting off with F=(1/2)kx^2 the correct way to start off with this sort of problem?