# Wrapping my mind around vectors?

1. Jun 19, 2010

### lrl4565

So, I'm assuming that scalars are what I'm used to working with in math. You add, subtract, multiply, etc.; they follow the rules I know. 1 + 1 = 2.

Scalar = Magnitude

Vector = Magnitude and Direction

Now, how do magnitude and direction coexist? Right now I'm just seeing some scalar volume cube being moved in a certain direction. What is the vector DOING? Can you provide me with a solid example of vectors at work?

A vector measures displacement... the distance from start to ending point. So how can they have arrows indicating their direction? They must be finite, but they look like rays.

It doesn't matter where you put a vector on cartesian or polar coordinates... but then we're supposed to calculate the vector using scalar components. If Vector C is 5 long, how can the square root of (really big number A squared) + (really big number B squared) equal really small number 5?

In addition, what ARE vectors? I mean, why are we taking the displacement instead of the distance, and adding some angle?

Last edited: Jun 19, 2010
2. Jun 19, 2010

### lrl4565

Can scalars be negative?

Or are they the absolute value?

Distance (scalar) is always positive, but temperature (scalar) is more confusing... would -30 degrees just be... 30 degrees?

3. Jun 19, 2010

### vanesch

Staff Emeritus
Re: Can scalars be negative?

Yes they can be negative. Think of charge density for instance.

4. Jun 19, 2010

### lrl4565

Re: Can scalars be negative?

Alright... vectors deal with things where location matters.

Can you give me an example of how a vector would be useful? How magnitude and direction can coexist?

5. Jun 19, 2010

### SandeshPhy

Re: Can scalars be negative?

They can be.For Example Work Can be Negative.

6. Jun 19, 2010

### vanesch

Staff Emeritus
Re: Can scalars be negative?

Velocity.

Something that is moving, is moving at a certain speed, in a certain direction.

If an airplane is flying in the air, it has a velocity, say, 600 km/h in a direction north-east.

That's a vector: the magnitude is 600 km/h and the direction is north-east.

7. Jun 19, 2010

### vanesch

Staff Emeritus
BTW, I merged this with your other thread, as I think both questions are related.

8. Jun 19, 2010

### lrl4565

Scalar = magnitude. So can magnitude be negative?

9. Jun 19, 2010

### vanesch

Staff Emeritus
No, scalar is not ALWAYS a magnitude. A magnitude is a scalar, however.

10. Jun 19, 2010

### lrl4565

So, my main goal in asking these questions is to determine WHAT a vector is, why they exist, and how they differ from scalars.

What I have so far:

The purpose of vectors is to make it simpler to deal with "things" that have both a distance and a location... so velocity HAS to have a location? Wow, it really is not at all like speed.

OK, why do I want to know it's location?

Let's see... we put the vectors onto coordinate systems, like the cartesian coordinates. And we use scalars to plot things on the cartesian coordinates, so we break down the vector into components in order to plot it on the cartesian coordinate system.

Does this mean that the cartesian coordinate system is made up of scalars?

11. Jun 19, 2010

### lrl4565

So, magnitude is the absolute value of the displacement?

Err, wait, scalar = distance

12. Jun 19, 2010

### brushman

Last edited by a moderator: Apr 25, 2017
13. Jun 19, 2010

### vandegg

Velocity has a magnitude and direction. This is important because when two vector quantities interact with each other the direction they move is important. It is simpler than you are making it. Lets say that you have a plane flying at x mph and there is a wind directly behind it blowing at y mph. The wind will add to the planes speed. If the wind is blowing directly toward the plane it will subtract from its speed. If the wind is blowing at an angle toward the plane it will push the plane in the direction of the angle. The direction involved in vectors is just an easy way of taking all of this into account when you are solving this problem of how the wind and plane will interact.