## College Physics (4th Edition)

We can find the time required to run at an average speed of 4.00 m/s: $v_{ave} = \frac{d}{t}$ $t = \frac{d}{v_{ave}} = \frac{1000~m}{4.00~m/s} = 250~s$ The run must be completed in 250 seconds. We can find the time it takes to run 500 m at an average speed of 4.20 m/s: $t = \frac{d}{v_{ave}} = \frac{500~m}{4.20~m/s} = 119~s$ The time remaining is $250~s-119~s = 131~s$ We can find the average speed over the last 500 m: $v_{ave} = \frac{d}{t} = \frac{500~m}{131~s} = 3.82~m/s$ Over the last 500 m, Marcella should run at an average speed of 3.82 m/s