# Why does (a.b).c make no sense?

1. Apr 15, 2014

### uzman1243

I was studying the dot product, and it says that (a.b).c makes no sense.

so if you do (a.b) can = to β
and then is it not possible to do β.c?

WHY cant you 'dot' a scalar and a vector? why?

2. Apr 15, 2014

### Staff: Mentor

Because the dot product is defined ONLY for two vectors. You can multiply a vector by a scalar, and this is called scalar multiplication.

3. Apr 15, 2014

### uzman1243

But why? is there any proofs as to why this is defined this way?

4. Apr 15, 2014

### ModusPwnd

Have you tried it? Write down the definition of a dot product. Make up and write down three vectors and perform the calculation.

(Note that definitions are made up, not proved. Can you prove that a cat is not a soda can? No. Its just not defined that way. Theorems and identities are what get proved, under the right definitions.)

5. Apr 16, 2014

### Matterwave

The dot product, in two dimensions (for simplicity) is defined as:

$$\vec{a}\cdot \vec{b}=a_xb_x+a_yb_y$$

Now, this assumes $\vec{a}=(a_x,a_y)$ and $\vec{b}=(b_x,b_y)$ are vectors. What would it mean to turn $a$ into a number? Certainly you can "define" the "dot product" of a scalar and a vector as:

$$a\cdot\vec{b}=a\vec{b}=(ab_x,ab_y)$$

But that's just the same as a scalar product, so it would be supremely confusing to also call it a "dot product". That's why we don't call that the "dot product".

6. Apr 16, 2014

### uzman1243

thank you!

7. Apr 16, 2014

### Matterwave

No problem. =]

8. Apr 16, 2014

### Staff: Mentor

A definition doesn't have to be proved.

9. Apr 16, 2014

### HomogenousCow

What an unexpected place to find such a gem.