Halliday, Resnick & Walker — Force and Motion–I: Problem 12

Two horizontal forces \(\vec{F}_1\) and \(\vec{F}_2\) act on a \(4.0 \text{ kg}\) disk that slides over frictionless ice, on which an \(xy\) coordinate system is laid out. Force \(\vec{F}_1\) is in the positive direction of the \(x\) axis and has a magnitude of \(7.0 \text{ N}\). Force \(\vec{F}_2\) has a magnitude of \(9.0 \text{ N}\). Figure 5.14 gives the \(x\) component \(v_x\) of the velocity of the disk as a function of time \(t\) during the sliding. What is the angle between the constant directions of forces \(\vec{F}_1\) and \(\vec{F}_2\)?