I have been annoyed by a problem that I can't figure out. The topic is work and kinetic energy (chapter 6 in the sixth edition of physics for scientist and engineers by Paul A. Tipler). Problem 63 pg. 199 A single horizontal force F acts in the +x direction on a mass m. The intial velocity is zero, the velocity is given as a function of x v=Cx where C is a contant. If the starting point x=0 and the final point is x = x' find the force and the work. my solution is w= .5mv^2 = .5m(Cx')^2 Therefore Fx' = .5m(Cx')^2 therefore F=.5mx'C^2 which is the correct answer. However, first I tried to do this f=ma = m (dv/dt) = m[C*dx/dt] = mCx' Why is this wrong???