# College Physics 2.65 – Velocity graph of a world-class sprinter

## Solution:

### Part A

To find for the average velocity over the straight line graph of the velocity vs time shown, we just need to locate the midpoint of the line. In this case, the average speed for the first 4 seconds is

$\text{v}_{\text{ave}}=6\:\text{m/s}$

### Part B

Looking at the graph, the velocity at exactly 5 seconds is 12 m/s.

### Part C

If we are given the velocity-time graph, we can solve for the acceleration by solving for the slope of the line.

Consider the line from time zero to time, t=4 seconds. The slope, or acceleration, is

$\text{a}=\text{slope}=\frac{12\:\text{m/s}-0\:\text{m/s}}{4\:\text{s}}=3\:\text{m/s}^2$

### Part D

For the first 4 seconds, the distance traveled is equal to the area under the curve.

$\text{distance}=\frac{1}{2}\left(4\:\sec \right)\left(12\:\text{m/s}\right)=24\:\text{m}$

So, the sprinter traveled a total of 24 meters in the first 4 seconds. He still needs to travel a distance of 76 meters to cover the total racing distance. At the constant rate of 12 m/s, he can run the remaining distance by

$\text{t}=\frac{\text{distance}}{\text{velocity}}=\frac{76\:\text{m}}{12\:\text{m/s}}=6.3\:\sec$

Therefore, the total time of the sprint is

$\text{t}_{\text{total}}=4\:\sec +6.3\:\sec =10.3\:\sec$