1. Not finding help here? Sign up for a free 30min 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!

Acceleration vector exercise (coordinate system)

  1. Nov 7, 2012 #1
    1. The problem statement, all variables and given/known data

    Please read the attachment =)

    All the formulas I was given are also on a separate attachment

    I initially thought the answer was the last one because I did rf-ri and subtracted across and got 0 mi + 25mj but im not sure =[
    1. The problem statement, all variables and given/known data
     

    Attached Files:

  2. jcsd
  3. Nov 7, 2012 #2

    PeterO

    User Avatar
    Homework Helper

    rf-ri sounds like a good idea, but 0 mi + 25mj isn't the answer to that.

    ri has a 50mi term
    rf has a -50mi term

    so when you subtract them the answer will not be zero [since they are different]
     
  4. Nov 7, 2012 #3
    Oh thx for the much needed advice. I redid the problem and got the first choice as my answer which was from subtracting -25 from 25 and 125 from 100. Can anyone verify that choice a is correct =[]
     
  5. Nov 7, 2012 #4

    PeterO

    User Avatar
    Homework Helper

    My previous post should have said 25mi and -25mi - you can't look at "pictures" and respond at the same time.

    While the first option is correct - the description of what you did is all backwards.

    You actually subtracted 25 from -25, and subtracted 100 from 125.
     
  6. Nov 7, 2012 #5
    Thanks a ton peter I understand now :)
     
  7. Nov 7, 2012 #6

    PeterO

    User Avatar
    Homework Helper

    By the way: applying the logic of multiple choice questions leads you to this answer.

    If you consider just the i components.

    If you add them [or subtract poorly], you get 0i [only one option has this].

    If you subtract the second from the first you get 50i [only one option has this]

    If you subtract the first from the second you get -50i [two options have this]

    If the answer was not one of the -50i .... examples, you would not even have to consider the j component - possible but unlikely.

    consider the j component. when you subtract here, you either get 25j or -25j

    only one of those possibilities is offered - and there is your answer!!

    Playing the multiple choice game rather than understanding the problem is not a recommended technique - but if all else has failed it is useful.
    Remember: with a normal problem you are trying to find the answer; with a multiple choice question you are only trying to identify which of the options is the answer.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook