A block of mass m lies on a horizontal frictionless surface and is attached to one end of a horizontal spring (with spring constant k)whose other end is fixed. The block is initially at rest at the position where the spring is unstretched (x = 0). When a constant horizontal force F→ in the positive direction of the x-axis is applied to it, a plot of the resulting kinetic energy of the block versus its position x is shown in figure. What is the magnitude F→ ?