Question

If `bar"a" = hat"i" + hat"j" + hat"k"  "and"  bar"c" = hat"j" - hat"k"`, find `bar"a"` vector `bar"b"` satisfying `bar"a" xx bar"b" = bar"c"  "and"  bar"a".bar"b" = 3`

Answer 1

Given: `bar"a" = hat"i" + hat"j" + hat"k" ,  bar"c" = hat"j" - hat"k"`

Let `bar"b" = "x"hat"i" + "y"hat"j" + "z"hat"k"`

Then `bar"a".bar"b" = 3` gives

`(hat"i" + hat"j" + hat"k").("x"hat"i" + "y"hat"j" + "z"hat"k") = 3`

∴ (1)(x) + (1)(y) + (1)(z) = 3

Also, x + y + z = 3     ...(1)

Also, `bar"c" = bar"a" xx bar"b"`

∴ `hat"j" - hat"k" = |(hat"i", hat"j" , hat"k"),(1,1,1),("x","y","z")|`

`= ("z - y")hat"i" - ("z - x")hat"j" + ("y - x")hat"k"`

`= ("z - y")hat"i" + ("x - z")hat"j" + ("y - x")hat"k"`

By equality of vectors,

z - y = 0     ...(2)

x - z = 1     .....(3)

y - x = - 1     ...(4)

From (2), y = z.

From (3), x = 1 + z

Substituting these values of x and y in (1), we get

1 + z + z + z = 3

∴ z = `2/3`

∴ y = z = `2/3`

∴ x = 1 + z = `1 + 2/3 = 5/3`

∴ `bar"b" = 5/3hat"i" + 2/3hat"j" + 2/3hat"k"`

i.e. `bar"b" = 1/3(5hat"i" + 2hat"j" + 2hat"k")`