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Homework Help: I dont understand how to calculate the speed in this problem ?

  1. Sep 15, 2008 #1
    Driving along a crowded freeway, you notice that it takes a time t to go from one mile marker to the next. When you increase your speed by 5.0 mi/h, the time to go one mile decreases by 13 s. What was your original speed?
    (in mph)

    I did R= 1/T
    and R= 1/T-13 -5/3600

    However, my answer didnt make any sense at all when i combined the 2 equations. please help
  2. jcsd
  3. Sep 15, 2008 #2
    Make sure it's 1/(T-13). Also, from the way you wrote the equations, your speed R is in miles/second. Maybe that's why the result seems strange to you.
  4. Sep 15, 2008 #3


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    Those equations look right to me. What answer did you get for R and T?

    Did you check if your numbers were consistent with the two equations?

    p.s. Welcome to Physics Forums!

    p.p.s. Smallphi is giving good advice here.
  5. Sep 15, 2008 #4
    I still dnt get it. Im really bad at physics :(
    SO my answer was in miles/ sec opposed to miles/ hr?
    i got -103.4651999 miles/ sec then
    but how could it be neg, and if it is right, how do i convert it?
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