# Math Help - Projection Vectors Problem

1. ## Projection Vectors Problem

The angle between two vectors a and b is ∅, where ∅≠90°. Under what conditions will |Projab|2 + |Projba|2 = 1?

Don't really know where to start on this one.

2. ## Re: Projection Vectors Problem

Originally Posted by anonymoususer20
The angle between two vectors a and b is ∅, where ∅≠90°. Under what conditions will |Projab|2 + |Projba|2 = 1?

Don't really know where to start on this one.
Hi anonymoususer20!

I'm not entirely sure what you mean with Projab.
Presumably it is:

$\text{Proj}_{ab} = \mathbf a \cdot \hat{\mathbf b}$

where $\hat{\mathbf b}$ is the unit vector in the direction of $\mathbf b$.

If that is the case you can substitute $\mathbf a \cdot \hat{\mathbf b} = a \cos \phi$.
And you can substitute something similar for the other projection.

Have you tried that?
If so, what did you get?

3. ## Re: Projection Vectors Problem

Projab is a projection vector.

|Pab|= |a.b|/|b|.

4. ## Re: Projection Vectors Problem

Originally Posted by anonymoususer20
Projab is a projection vector.

|Pab|= |a.b|/|b|.
Aha! Then it is exactly what I suspected, since $\mathbf{\hat b} = \frac{\mathbf b}{|\mathbf b|} = \frac{\mathbf b}{b}$.

5. ## Re: Projection Vectors Problem

Originally Posted by anonymoususer20
Projab is a projection vector.

|Pab|= |a.b|/|b|.

Originally Posted by ILikeSerena
Aha! Then it is exactly what I suspected, since $\mathbf{\hat b} = \frac{\mathbf b}{|\mathbf b|} = \frac{\mathbf b}{b}$.

There seems to be somewhat of confusion on notation here.
I have never seen the notation $\text{Proj}_{ab}$.
In North American text books $\text{Proj}_{\vec{b}}\;\vec{a}=\frac{\vec{a}\cdot \vec{b}}{\vec{b}\cdot\vec{b}}\;\vec{b}{$

If we use that definition then $\left\|\frac{\vec{a}\cdot \vec{b}}{\vec{b}\cdot\vec{b}}\;\vec{b}\right\|^2=| \vec{a}\cdot\vec{b}|^2$

Now that makes a really interesting question.

6. ## Re: Projection Vectors Problem

Originally Posted by Plato
There seems to be somewhat of confusion on notation here.
I have never seen the notation $\text{Proj}_{ab}$.
In North American text books $\text{Proj}_{\vec{b}}\;\vec{a}=\frac{\vec{a}\cdot \vec{b}}{\vec{b}\cdot\vec{b}}\;\vec{b}{$

If we use that definition then $\left\|\frac{\vec{a}\cdot \vec{b}}{\vec{b}\cdot\vec{b}}\;\vec{b}\right\|^2=| \vec{a}\cdot\vec{b}|^2$

Now that makes a really interesting question.
That does make more sense and is probably what it means.
I didn't think to include the direction.

However, $\left\|\frac {\vec{a}\cdot \vec{b}}{\vec{b}\cdot\vec{b}}\;\vec{b} \right\|^2 = \frac{|\vec{a}\cdot\vec{b}|^2}{|\vec{b}|^2} = (a \cos \phi)^2$

7. ## Re: Projection Vectors Problem

Originally Posted by ILikeSerena
That does make more sense and is probably what it means.
I didn't think to include the direction.
However, $\left\|\frac {\vec{a}\cdot \vec{b}}{\vec{b}\cdot\vec{b}}\;\vec{b} \right\|^2 = \frac{|\vec{a}\cdot\vec{b}|^2}{|\vec{b}|^2} = (a \cos \phi)^2$
Actually no, it equals $\left\|\frac {\vec{a}\cdot \vec{b}}{\vec{b}\cdot\vec{b}}\;\vec{b} \right\|^2 =|{\vec{a}\cdot \vec{b}|^2$.

So we get $|{\vec{a}\cdot \vec{b}|^2=\frac{1}{2}$

8. ## Re: Projection Vectors Problem

$\left\|\frac {\vec{a}\cdot \vec{b}}{\vec{b}\cdot\vec{b}}\;\vec{b} \right\|^2 = \frac{|\vec a \cdot \vec b|^2} {|\vec{b}\cdot\vec{b}|^2} ||\vec b||^2 = \frac{|\vec a \cdot \vec b|^2} {||\vec{b}||^4} ||\vec b||^2 = \frac{|\vec a \cdot \vec b|^2} {||\vec{b}||^2}$