What is the difference between velocity and speed

[mreq]

I don't understand the difference between them !
Any help ?

 

[marcusl]

Velocity has a magnitude and direction. Speed is magnitude only with no indication of direction.

 

[lostinaworld]

Velocity is speed and direction. :)

 

[Danger]

There's no such thing as a "negative direction". An increase in speed is a positive acceleration and a decrease is a negative one.
Bear in mind as well that the speed can remain constant and there will still be an acceleration if the direction changes. For instance, a satellite undergoes perpetual acceleration because it never stops changing direction as it falls around the planet.

edit: Actually, I'm not entirely sure about that last statement. By Einsteinian physics, a curved path through a gravitational field is actually a straight line in curved space, but I think that Newtonian terms apply to this subject.

 

[Doc Al]

There's no such thing as a "negative direction". An increase in speed is a positive acceleration and a decrease is a negative one.

For straight line motion, positive and negative can define directions with respect to some axis. An acceleration is a vector. If it points in the positive direction, it is positive.

 

[Danger]

I don't quite follow that, Al. Do you mean that someone can just arbitrarily deem "north" to be positive and "south" to be negative? If so, how do multiple observers decide whose opinion is correct?
(Not trying to be a smart-***; I honestly don't get it.)
No rush for an answer; it's 5:30 am and I've been up for 20 hours. I'm going to sleep now.

 

[mikeph]

Both are correct. Say you define "positive" wherever you happen to be facing. Two people won't disagree on the velocity of a car just because they happen to be facing different directions.

 

[A.T.]

Do you mean that someone can just arbitrarily deem "north" to be positive and "south" to be negative? If so, how do multiple observers decide whose opinion is correct?

It's just a convention. It's neither correct not wrong.

An increase in speed is a positive acceleration and a decrease is a negative one.

This is the layman definition of "acceleration". In physics "acceleration" is the time derivative of velocity, not of speed.

 

[Doc Al]

I don't quite follow that, Al. Do you mean that someone can just arbitrarily deem "north" to be positive and "south" to be negative? If so, how do multiple observers decide whose opinion is correct?

They must all use the same convention, of course. You can also just use "north" and "south" themselves to specify the direction of a vector.

But it's quite useful to use positive and negative for one-dimensional problems. For example, for a ball tossed straight up in the air, often "up" is taken as positive and "down" as negative. Makes the algebra easy to do. (But you are free to choose your description of the direction as you please.)

 

[Danger]

In physics "acceleration" is the time derivative of velocity, not of speed.

My phrasing might have been ill-advised. I deliberately separated velocity into its two components of speed and direction, but perhaps didn't stress enough that they can't be physically separate from one another. (Nuts! I'm still not expressing myself properly. I think that I'll just go home now. Thanks for the responses, guys.)