anonymous 5 years ago served a ball at 155 miles per hour, the fastest serve ever recorded. What was the speed of the tennis ball, in feet per second, rounded to the nearest foot?

1. anonymous

Just a change of units. $155\frac{miles}{hour} \times \frac{hour}{3600 \sec}\times \frac{5280 feet}{mile}=227\frac{1}{3}\frac{feet}{second }\approx227\frac{ft}{\sec}$

2. anonymous

Note, every time you change the units, for example, by multiplying by$\frac{hour}{3600\sec}$we're really multiplying by 1 and therefore NOT changing the physical measurement. This is because, in this example, there are 3600 seconds in an hour, so we equate $1h=3600s \rightarrow \frac{1h}{3600s}=1$after dividing both sides by 2600 seconds. Hope it helps.

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