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: Diffrential calculus; distance problem

  1. Dec 8, 2007 #1
    1. The problem statement, all variables and given/known data
    Two cars approach an intersection at the same time. The red car is 300m east of the intersection and traveling at a speed of 60km/h. The blue car is 450m north of the intersection and traveling at a speed of 75km/h. When are the two cars the closest?

    2. Relevant equations
    c^2 = a^2 + b^2

    3. The attempt at a solution

    I converted the speeds into m/s first then used it in a pythagorean theorem equation and found the derivative.
    75km/h = 20.8m/s 60km/h = 16.7m/s
    c^2 = (20.8t - 450)^2 + (16.7t - 300)^2
    and when i do the derivative i get 20.2. But I was just wondering, does it make a difference whether i do 20.8t - 450 or 450 - 20.8t??
  2. jcsd
  3. Dec 8, 2007 #2


    User Avatar
    Science Advisor

    Here it doesn't matter because of the squares. 20.8t- 450 is the negative of 450- 20.8t but squaring removes the difference.

    But you get "20.2" for what? This problem asks when the two cars will be closest. That requires a time answer. If you mean "The two cars will be closest 20.2 seconds after the starting time (when they are at the given positions)", you must include the "seconds".
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook