## Starting physics

Post any maths and/or physics related questions here.

Moderators: Darobat, RecursiveS, Dante Shamest, Bugdude, Wizard

### Starting physics

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:
Code: Select all
`Speed = total distance / delta time`
Velocity
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`x2 - x1 / t2 - t1`

t i l e x

Posts: 3604
Joined: Wed Dec 03, 2003 3:59 pm

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.
If it wasn't for C, we would be using BASI, PASAL and OBOL.

tomcant

Posts: 3101
Joined: Tue Sep 23, 2003 1:56 am
Location: Colchester, UK

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

t i l e x

Posts: 3604
Joined: Wed Dec 03, 2003 3:59 pm

Yep, but don't forget magnitude.
If it wasn't for C, we would be using BASI, PASAL and OBOL.

tomcant

Posts: 3101
Joined: Tue Sep 23, 2003 1:56 am
Location: Colchester, UK

What is magnitude ?

t i l e x

Posts: 3604
Joined: Wed Dec 03, 2003 3:59 pm

t i l e x wrote: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.
Ryan
Moderator

Posts: 323
Joined: Sat Jun 12, 2004 1:34 pm

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 ?

t i l e x

Posts: 3604
Joined: Wed Dec 03, 2003 3:59 pm

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

Hit the nail on the head!
If it wasn't for C, we would be using BASI, PASAL and OBOL.

tomcant

Posts: 3101
Joined: Tue Sep 23, 2003 1:56 am
Location: Colchester, UK

t i l e x wrote: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?
If it wasn't for C, we would be using BASI, PASAL and OBOL.

tomcant

Posts: 3101
Joined: Tue Sep 23, 2003 1:56 am
Location: Colchester, UK

Laval is in the north of Montreal

t i l e x

Posts: 3604
Joined: Wed Dec 03, 2003 3:59 pm

Then you were correct. Your direction is south.
If it wasn't for C, we would be using BASI, PASAL and OBOL.

tomcant

Posts: 3101
Joined: Tue Sep 23, 2003 1:56 am
Location: Colchester, UK

So let's apply formulas now:

Average velocity:
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`d = x2 - x1 / delta t // where d == displacement100 = 5 / delta t500 = 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 tspeed = 5 / 500speed = 0.01`
Why is there a difference in the two answers ? Where did I go wrong ?

t i l e x

Posts: 3604
Joined: Wed Dec 03, 2003 3:59 pm

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.

GeekDog

Posts: 1160
Joined: Sun Sep 28, 2003 1:42 pm
Location: UK

I know nothing of physics lol
t i l e x wrote: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 ?

t i l e x

Posts: 3604
Joined: Wed Dec 03, 2003 3:59 pm

Code: Select all
`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)

GeekDog

Posts: 1160
Joined: Sun Sep 28, 2003 1:42 pm
Location: UK

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