I have a rather short question for part d. As we know that the change in energy= work done, I do not understand why, to solve part d, the equation is 0.5*4.3*(4.56)^2=44.7 ? Why is the change in energy 44.7 joules and not 0.5*3.5*(5.6)^2 – 44.7? Clearly the change in energy is 10 joules.