• Support PF! Buy your school textbooks, materials and every day products Here!

Help with physics demonstration

  • Thread starter brad sue
  • Start date
  • #1
281
0
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
 

Answers and Replies

  • #2
EnumaElish
Science Advisor
Homework Helper
2,304
124
What is "a x b" vs. "a.b"?
 
  • #3
Integral
Staff Emeritus
Science Advisor
Gold Member
7,198
55
What are your definitions of The dot and cross product?
 
  • #4
281
0
Integral said:
What are your definitions of The dot and cross product?
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
and
"a.b" mean a dot b
Thank you
B
 
  • #5
EnumaElish
Science Advisor
Homework Helper
2,304
124
brad sue said:
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
and
"a.b" mean a dot b
Thank you
B
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:

Related Threads for: Help with physics demonstration

  • Last Post
Replies
6
Views
2K
  • Last Post
Replies
0
Views
762
  • Last Post
Replies
10
Views
2K
  • Last Post
Replies
2
Views
2K
  • Last Post
Replies
2
Views
901
  • Last Post
Replies
2
Views
929
  • Last Post
Replies
9
Views
686
Replies
10
Views
1K
Top