Students attach a force probe in the middle of string a to measure ta and then use a different force probe to provide the applied force f to the box of mass m1. with the box of mass m3 held in place by one student, a second student pulls to the left on the force probe that provides the applied force f. as the second student pulls with a steady force, and with the boxes remaining at rest, the following pairs of readings from the two force probes are recorded at different times.