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billmoakhl · Jan 30, 2011, 08:26 PM

Corvette going steady 200mph, crossing starting gate of dragster quarter mile run. At instant Corvette crosses starting gate, dragster starts and finishes quarter mile in 4.428 seconds. How far in feet will Corvette travel in that time? What is average speed of dragster?

jcaron2 · Jan 30, 2011, 08:35 PM

Distance = average speed x time

That same formula is used to answer both questions. But first you need to convert the Corvette's speed from miles per hour to feet per second (by multiplying by 5280 ft/mile, then dividing by 3600 s/h).

In the first question, you have the average speed and the time for the Corvette, so you can calculate the distance. In the second question, you have the distance and the time for the dragster, so you can calculate average speed.

Can you take it from there?

billmoakhl · Jan 31, 2011, 06:15 PM

I got 1255.4666 feet traveled by the Corvette by the time the dragster hit the finish line.
I got an average speed of 203.252 for the dragster.

My grandson (10th grade) got 1034.4 feet for the Corvette and 298.10298 avg speed of the dragster.

jcaron2 · Jan 31, 2011, 08:56 PM

I got 1255.4666 feet traveled by the Corvette by the time the dragster hit the finish line.
I got an average speed of 203.252 for the dragster.

My grandson (10th grade) got 1034.4 feet for the Corvette and 298.10298 avg speed of the dragster.

The good news is, you're right and your grandson's wrong about the average speed of the dragster! :) Unfortunately, the bad news is you're both wrong about the Corvette.

Corvette's speed:

$200 \text{mph} \cdot \frac{5280 \text{ft/mile}}{3600 \text{sec/h}} = 293.3333 \text{ft/sec}$

So to find the total distance traveled by the Corvette you just use distance=rate*time:

293.333 ft/sec * 4.428 sec = 1298.9 ft

Meanwhile, the dragster went 1/4 mile in 4.428 sec (which is 4.428/3600 = 0.00123 hrs). So if we plug that into the formula:

0.25 mile = average_speed * 0.00123 hr
average_speed = 0.25 mile/0.00123 hr = 203.3 mph

Make sure you rub it in your grandson's face that you're better in Physics than he is. :) You've got to take points whenever you can get them!

billmoakhl · Feb 1, 2011, 10:30 PM

I went back over my numbers and found I had used 4.28 sec instead of 4.428. Thanks for putting up with me.

jcaron2 · Feb 2, 2011, 07:40 AM

I guess that would explain it. Great job!