# Can the magnitude of vector be negative?

1. Sep 18, 2009

### Red_CCF

Recently I was told that scalars, although magnitude only, can be negative. Does this mean that the magnitude of a vector can be negative too?

Also, I'm wondering if there's a difference between the absolute value and magnitude of a vector like -3i-4j. Thanks for any help that you can provide

2. Sep 18, 2009

### emyt

no, the magnitude of a vector is computed by sqrt(x1^2 + ...... xn^2)

3. Sep 18, 2009

### Staff: Mentor

I say "absolute value" only when I'm talking about a number, and "magnitude" when I'm talking about a vector. I don't know if it's actually incorrect to say "absolute value of a vector." Nevertheless, I don't think there's any chance you would confuse people by saying "absolute value of a vector," because I can't think of anything else besides the magnitude that it could be interpreted to mean.

4. Sep 18, 2009

### emyt

the absolute value of a vector is the "norm"

5. Sep 19, 2009

### DrGreg

The magnitude (a.k.a. norm or length) ǁaǁ of a vector a is a scalar and is always positive (or zero).

But there are scalars that are not magnitudes of vectors and they can be negative. (For example the scalar product (a.k.a. dot product or inner product) of two vectors a.b).