1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook