# 7 A proup of students arrange two level tracks side-by-side so they can have a race between two carts. They mount identical fan units (each with two real batteries) on two identical carts and run two experiments. Experiment 1 When the two carts, with fans turned on, are released simultaneously from the end of the tracks, they speed up at the same rate, traveling side-by-side, and so the race ends in a tie. Experiment 2 . The students then add extra mass to one of the carts and repeat the experiment (using the same fan units), and record speed-time data for both carts. The next week when they look at the speed-time data they graphed (shown below), there are two lines (labeled A and B), and they are not sure which is which. Below the graph, identify which line represents the motion of the cart with the added mass? Justify your choice. 60 150 Speed (cm/s) O 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 8.0 Time (3)