Average speed = (distance covered) / (time to cover the distance)
Tissa covered 60 meters in 10 seconds. Her average speed was
(60 m) / (10 sec) = 6 m/s.
That's the slope of the dotted line.
Lilly covered 60 meters in 8 seconds. Her average speed was
(60 m) / (8 sec) = 7.5 m/s .
That's the slope of the solid line.
Lilly covered the same distance in less time, and both girls arrived at the finish line together. Technically, in science talk, we would say that Lilly ran "faster", and her average speed was "greater".
We can detect that by looking at the graph, because Lilly's line has the characteristic of being "steeper", and we know that the slope of the line on a distance/time graph is "speed".
If net external force acting on the system is zero, momentum is conserved. That means, initial and final momentum are same → total momentum of the system is zero.
