As we are interested in the forces as a function of time, we don't want the collision to be between two hard surfaces, or the period of time over which the forces act will be too brief to measure, eve...As we are interested in the forces as a function of time, we don't want the collision to be between two hard surfaces, or the period of time over which the forces act will be too brief to measure, even at the impressive sampling rate of our force sensors. The motion sensors are located on opposite ends of the track on which the carts are confined to roll, and measure the velocities of the carts closest to them.