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!

Help with physics demonstration

  1. Oct 16, 2005 #1
    Hi ,
    Please can I have some help with this demonstration:
    Let a#0 ( #= different)
    Show that if a x b = a x c and a.b=a.c, then b=c ( Hint: Cross both sides of the first equation with a)
    When I cross by a ( as the hint suggests) , I found 0=0 ??
    thank you
  2. jcsd
  3. Oct 16, 2005 #2


    User Avatar
    Science Advisor
    Homework Helper

    What is "a x b" vs. "a.b"?
  4. Oct 16, 2005 #3


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    What are your definitions of The dot and cross product?
  5. Oct 17, 2005 #4
    Hi , I don't know what you mean but I guess you want to know that
    "a x b" means a cross product with b
    "a.b" mean a dot b
    Thank you
  6. Oct 17, 2005 #5


    User Avatar
    Science Advisor
    Homework Helper

    Right. So, is "dot" vector multiplication? How is "cross product" different than vector multiplication? Some examples would be useful. What is (1,2) x (3,4)? What is (1,2).(3,4)?
    Let's say a, b, and c are vectors. Let a=(1,2), b=(1,2) and c=(c1,c2).
    ab = ac ---> a1b1 + a2b2 = a1c1 + a2c2 ---> 5 = c1 + 2 c2 ---> c1 = 5 - 2 c2, which means that although c = (1,2) would satisfy the equation, so would c = (5,0). Therefore what you want to demonstrate is not demonstrable on the basis of vector multiplication alone. That is why more info is needed on the exact definitions of the cross and the dot.

    My guess is they are inner and outer products: a.b = a1b1 + a2b2, axb = (a1b1, a1b2, a2b1, a2b2). But can you verify this so we won't be making a mistake?
    Last edited: Oct 17, 2005
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Help with physics demonstration
  1. Irrational demonstration (Replies: 10)