Читать книгу Physics I For Dummies - Steven Holzner - Страница 75

Here’s one more example. Say you’re in your rocket ship, happily speeding along at some 3.25 kilometers per second (about 7,280 miles per hour) when you see a sign: Speed Zone 215 km Ahead — New Speed Limit: 3.0 km/s.

You jam on the brakes (which are a retro rocket in the front of the rocket ship). The retro rocket is capable of accelerating your ship at –10.0 meters/second².

It’s a tense moment. Will you get your speed down to below 3.0 kilometers per second in 215 kilometers of acceleration? Find out, using your old friend:

In this case, you want to solve for the final speed, which is

where , , and . Plugging in the data and solving for v_f gives you the following:

Whew, you think — 2.50 kilometers per second is well under the speed limit of 3.0 kilometers per second. You’re safe.

You can now consider yourself a motion master.