Starting physics

[t i l e x]

Hi everyone ! I didn't want to go off topic in the "Bouncing ball" topic so here I am with this one, I've found something on gamedev.net and it seems pretty good, it's a tutorial on physics. There's something I don't understand though, what's the difference between speed and velocity ?
Speed:

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Speed = total distance / delta time

Velocity

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x2 - x1 / t2 - t1

[tomcant]

Speed is the rate of change in distance with respect to time. This means it carries no direction information with it.

Velocity is the rate of change in displacement with respect to time. The main thing that separates velocity from speed is that velocity has both magnitude and direction.

[t i l e x]

So... speed is like when driving... 50km/h is a speed indication while 50km/h going south would be velocity, am I close ?

[tomcant]

Yep, but don't forget magnitude.

[t i l e x]

What is magnitude ?

[Ryan]

So... speed is like when driving... 50km/h is a speed indication while 50km/h going south would be velocity, am I close ?

That's right. Magnitude is the "50 km/h" part. Another way to say it is that speed is the magnitude part of velocity.

[t i l e x]

Ok so let's I'm going to Montreal from Laval ( wich is about 5km ) north of it ) and that I take highway 19 and go at 100km/h. My magnitude is 100km/h and my direction south ?

[tomcant]

Another way to say it is that speed is the magnitude part of velocity.

Hit the nail on the head!

[tomcant]

and my direction south ?

Depends what is north of what.... you won't ever get a question that doesn't explain it properly. Is Montreal north of Laval... or is it the other way round?

[t i l e x]

Laval is in the north of Montreal

[tomcant]

Then you were correct. Your direction is south.

[t i l e x]

So let's apply formulas now:

Average velocity:

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d = x2 - x1 / delta t // where d == displacement
100 = 5 / delta t
500 = delta t

Using this I can know that it'd take 500s to go to Montreal from Laval ( if there's no obstacles of course )

Average speed:

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speed = total distance / delta t
speed = 5 / 500
speed = 0.01

Why is there a difference in the two answers ? Where did I go wrong ?

[GeekDog]

You're not comparing the same thing! The first formula is wrong.

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v = x2 - x1 / delta t // v == velocity


So, what do you know to start with, and what do you need to find out?
v == ?
x2 == ?
x1 == ?
delta t == ?

If you're trying to find out delta t, you need to know what v, x1 and x2 are.

[t i l e x]

I know nothing of physics lol

Ok so let's I'm going to Montreal from Laval ( wich is about 5km ) north of it ) and that I take highway 19 and go at 100km/h. My magnitude is 100km/h and my direction south ?

How long will it take to reach Montreal ?

[GeekDog]

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v = delta x / delta t


This rearranges to

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delta t = delta x / v


v == 100km/h
delta x = 5 km
So,

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delta t = 5 / 100 = 0.005 hrs


(If you want that in seconds, multiply by 60*60)