# Question Video: Calculating Speed given Distance and Time Physics • 9th Grade

A sprinter runs the 100-meter race at a certain average speed. The runner crosses the finish line 9.5 s after the race starts. What is the runner’s average speed? Round your answer to one decimal place.

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### Video Transcript

A sprinter runs the 100-meter race at a certain average speed. The runner crosses the finish line 9.5 seconds after the race starts. What is the runner’s average speed? Round your answer to one decimal place.

Okay, so we’ve been told here that we’ve got a sprinter who’s running a 100-meter race. That’s the distance that the sprinter was running. And we’ve been told that they finish the race 9.5 seconds after it starts. Or in other words, it takes them 9.5 seconds to run 100 meters. We’ve been asked to find their average speed. So we can recall that the average speed of an object, or in this case a person, is defined as the total distance moved divided by the total time taken to move that distance.

So in our scenario here, the total distance moved is 100 meters. And the total time taken to move that distance is 9.5 seconds. And hence, we can say that the average speed of the sprinter, which we’ll call 𝑆 subscript avg, is given by dividing 100 meters by 9.5 seconds. And when we look at the units, we see that it’s going to be in meters per second. So, when we evaluate the right-hand side, we find that the average speed of the runner is 10.526 dot dot dot meters per second.

However, this is not our final answer because, remember, we’ve been asked to give our answer to one decimal place. So here is the first decimal place. We’re gonna work out whether it stays the same or whether it rounds up. To do this, we need to look at the next decimal place. In this case, this value is a two. Now two is less than five. Therefore, this value is going to stay the same. If, instead, this number was greater than or equal to five, then we would have to round up this decimal place. However, in this case, we don’t have to do that.

And hence, we can say that our final answer is that the runner’s average speed is 10.5 meters per second to one decimal place.