Speed, Distance, and TimeDate: 19 Jun 1995 12:43:23 -0400 From: carla howard Subject: Speed If your speedometer is incorrect, and you travel one mile in 45 seconds, how fast are you driving? Carla Date: 19 Jun 1995 14:23:29 -0400 From: Dr. Ethan Subject: Speed Hey Carla, If your speedometer is incorrect, and you travel one mile in 45 seconds, how fast are you driving? TOO FAST! No, just teasing. Let's figure it out. When we are done we want miles per hour. Right now we have miles per second. So if we go one mile in 45 seconds, as a fraction that looks like 1 mile ------- 45 seconds Let's change the time (denominator) to hours. First we need to get it into minutes. Well, since there are 60 seconds in a minute, we can just multiply the fraction by 60 seconds per minute. (We haven't changed the value of the fraction because 60 seconds is the same amount of time as one minute.) So now it looks like this... 1 mile 60 seconds --------- * ---------- 45 seconds 1 minute One more step. To go from minutes to hours we do almost the same thing. We multiply by 60 minutes per hour and then the whole thing looks like this. 1 mile 60 seconds 60 minutes ---------- * ----------- * ---------- 45 seconds 1 minute 1 hour Now you multiply all the numbers together and cancel whatever units you can, and whatever doesn't cancel are the units that you keep. In our example the units that do not cancel are mile and hour, so those are the units that we want to keep. So the answer I get when I plug the numbers into a calculator is 80 miles per hour. As I said before, way too fast. Hope this helps. If you do not understand what I did then please write back and I will explain again. Ethan - Doctor On Call |
Search the Dr. Math Library: |
[Privacy Policy] [Terms of Use]
Ask Dr. Math^{TM}
© 1994- The Math Forum at NCTM. All rights reserved.
http://mathforum.org/dr.math/