# General question about directions of v.

1. Feb 14, 2010

### theel

I chose not to use the template because this is not a specific homework question. However, it is a problem I face often in my homework problems:

How do I determine the direction of velocity? For example, if I have an object fall from rest at a height, is its final velocity a positive or negative value? Likewise, if I throw an object in the air, is its final velocity when it hits the ground positive or negative?

Basically, I want to know what is the general rule of thumb for determining the direction of v?

Thanks.

2. Feb 14, 2010

### pgardn

The general rule of thumb is down is negative (usually y-axis). And up is positive.
So velocity as a rock hits the ground is usually negative.

But you can set up your own cooridinate system as long as you hold to it. Later on, you might get some problems where it is easier to do so. Get used to seeing situations where you make up the most reasonable frames of reference to solve certain problems if you continue in Physics.

3. Feb 14, 2010

### ideasrule

Positives and negatives are a human invention. If you want a falling rock's velocity to be negative, then it's negative. If you want it to be positive, then it's positive. It's the same way with charges: why should a proton be positive? No reason; somebody arbitrarily chose it to be positive.