### 1.4 Units & Conversions

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• Math Help

You can roughly convert between feet per second and miles per hour as follows.

1. To approximately convert from mph to ft/sec, multiply by 1.5.

2. To approximately convert from ft/sec to mph, multiply by

For instance, the speed of sound is roughly 1100 ft/sec and a normal commercial jet travels at about 500 mph. Is a normal commercial jet faster than the speed of sound?

500 mph is roughly 750 ft/sec. So, a normal commercial jet is not faster than the speed of sound.

• Consumer Suggestion

Have you ever wanted to go skydiving? It is quite easy to train for a skydiving jump. Use a search engine to search for the phrase "skydiving in <enter your city name here>" to learn more about skydiving in your area. Also, visit the USPA or United States Parachute Association for more information about skydiving and becoming a skydiver.

• Checkpoint Solution
1. From the graph, the parachutist jumps from the airplane at a time of 0 seconds. He accelerates for about 20 seconds. Then for another 20 seconds, his speed is constant at about 110 miles per hour. So, he is in free fall for a total of about 40 seconds.
2. At the end of 40 seconds, the parachutist's speed abruptly drops to about 10 miles per hour. This must have been the time when he opened his parachute. When the time is 70 seconds, his speed drops to zero. So, he must have spent about 30 seconds with the parachute open.

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