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!

Vector velocity homework question

  1. May 9, 2005 #1
    On an occasion, John has position vector 40j metres. He wished to skate in a straightline to the point with position vector 30i metres. Given that his speed is constant at 5ms^-1, find his velocity.
    Someone help me on this one please! :confused:
     
  2. jcsd
  3. May 9, 2005 #2

    BobG

    User Avatar
    Science Advisor
    Homework Helper

    You have a magnitude (5 meters per second).

    The starting coordinates (0,40) and the ending coordinates (30,0) supply the direction. Relative to your origin (0,0) your two position vectors are 0i + 40j and 30i + 0j

    Since you're using ijk coordinates, your best bet is to add your two position vectors together. Then you have a scalar problem: x(30i + 40j) = v

    To find x, you have to divide the magnitude of the velocity vector by the magnitude of the resultant position vector. Plug the coordinates of your position vector into the Pythagorean Theorem.

    Once you determine x, do the scalar mutliplication (multiply each of the resultant position vector coordinates by x).
     
  4. May 9, 2005 #3
    Oh!...Thanx
     
  5. May 9, 2005 #4

    OlderDan

    User Avatar
    Science Advisor
    Homework Helper

    You need the difference between the final and initial position vectors, not their sum, to get the direction of your velocity vector

    [tex]
    \overrightarrow {\Delta r} = \overrightarrow{r}_{final} - \overrightarrow {r}_{initial} = 30\widehat{i} - 40\widehat{j}
    [/tex]

    Since you know the magnitude of your velocity vector, you need to find the unit vector in the direction of [tex]\overrightarrow {\Delta r} [/tex]. You can do that by dividing the vector [tex] \overrightarrow {\Delta r} [/tex] by its length. Then multiply that unit vector by the magnitude of the velocity you have.
     
  6. May 9, 2005 #5

    BobG

    User Avatar
    Science Advisor
    Homework Helper

    Oops. OlderDan's right. It's final position minus start position.

    It's -30i +40j , not +30i.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Vector velocity homework question
Loading...