Unit Conversions: mph to fps

Date: 03/06/2003 at 19:51:58
From: John
Subject: Converting miles per hour to feet per second

How can I find out how many feet per second I could travel driving 55 
miles per hour?

Date: 03/06/2003 at 23:33:53
From: Doctor Jeremiah
Subject: Re: Converting miles per hour to feet per second

Hi John,

You thought fractions would never be good for anything!

Forget the numbers and concentrate on the units:

  55 miles
  --------
   1 hour

What would you multiply this by to cancel the hours?

  55 miles   hours   miles
  -------- x ----- = -----
   1 hour     sec     sec

What is hours/second? There are 3600 seconds in 1 hour so 
hours/second = 1 hour/3600 seconds which means:

  55 miles   1 hour     55 miles
  -------- x -------- = --------
   1 hour    3600 sec   3600 sec

Now, what would you multiply miles/sec by to get feet/sec?

  55 miles   feet   feet
  -------- x ---- = ----
  3600 sec   mile    sec

What is feet/mile? There are 5280 feet in 1 mile so 
feet/mile = 5280 feet/1 mile which means:

  55 miles   5280 feet   55 x 5280 feet
  -------- x --------- = --------------
  3600 sec      1 mile      3600 sec

This works for any conversion. You just multiply the fractions that 
are appropriate and you end up with the answer.

In this particular instance, as a check, you can use the fact that
60 miles per hour is the same as 88 feet per second. 

- Doctor Jeremiah, The Math Forum
  http://mathforum.org/dr.math/