A cart of unknown mass \(M\) with a single flag of length \(L\) mounted on top travels on a horizontal, frictionless track. The cart passes through a photogate with transit time \(\Delta t_1\), collides with a force sensor fixed at the end of the track, and rebounds back through the same photogate with transit time \(\Delta t_2\). For multiple trials with different initial speeds, the force sensor records force over time to determine the magnitude of the collision impulse \(J = \int F \, dt\). To obtain a linear graph whose slope is equal to the cart mass \(M\), which of the following quantities should be plotted on the vertical and horizontal axes?