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Difference Between Speed and Velocity ?

by Xidike
Tags: difference, speed, velocity
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Xidike
#1
Nov8-12, 11:47 AM
P: 72
I want to as that what is the difference between speed and velocity.. these both seems to be same for me... but i'm in doubt... and similarly what is the difference between distance and displacement ???
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bp_psy
#2
Nov8-12, 12:02 PM
P: 452
Velocity and displacement are vectors meaning that the direction matters. If we face each other and walk in opposite directions with some speed, we will have the same speed but opposite velocities.If you walk in a straight line from your starting point to my starting point and I walk to your starting point we both travel the same distance but we have different displacements.
Nugatory
#3
Nov8-12, 12:03 PM
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Speed is a number: How fast is something moving? Velocity is a vector: How is the position of the object changing over time? Direction matters for velocity, but not speed.

An easy example of the difference:
If you tie a weight to a rope and then swing it in a circle around you.... The object will be travelling at a constant speed in its circular path. However, at one moment it will be moving to the east, and then a moment later to the north, and then a moment after that it will be moving to the west. So its speed is constant, but its velocity is continually changing.

Hetware
#4
Nov8-12, 01:42 PM
P: 125
Difference Between Speed and Velocity ?

Here's something to consider: Einstein's special theory of relativity postulates that the speed of light is the same for all observers.

Two observers moving relatively to one another will both see the same light go at the ~300,000 kilometers per second, relative to themselves. They may, however, see the light traveling in a different direction than what the other observer sees.

So the speed of light is the same for all observers, but not the velocity of light.

Sorry if that's too advanced for you at this point. If it doesn't make sense, but remember the rule so that when you hear a friend say "The velocity of light is the same for all observers". You reply "No, it's the speed of light that is the same for all observers."

Another way of explaining the relationship between velocity and speed is to represent velocity as an arrow pointing in the direction of the motion. The size of the arrow is proportiional to the speed. So speed is called the magnitude of the velocity.
harrylin
#5
Nov8-12, 04:06 PM
P: 3,181
Quote Quote by Xidike View Post
I want to as that what is the difference between speed and velocity.. these both seems to be same for me... but i'm in doubt... and similarly what is the difference between distance and displacement ???
Speed and velocity: that is very subtle. In many languages there is only one word, like there is in English only one word for acceleration. Speed can only mean the magnitude, while velocity can mean, like acceleration, either magnitude or vector (magnitude and direction). It has become customary in physics to use velocity exclusively for the vector quantity but this is not universal and was not common in the past. Moreover, translations from for example German tend to use "velocity" as the German word is non-specific. Thus old translations of some German papers by Einstein have "the constancy of the velocity of light", and the implied meaning (by the context) is in this case speed.

Distance and displacement: that's similar but non-ambiguous: distance is only the magnitude, while displacement has direction.
Xidike
#6
Nov8-12, 10:36 PM
P: 72
why speed can't have direction E.g 44 km/h towards East...??
also why can't distance have direction... ???
K^2
#7
Nov8-12, 10:51 PM
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Quote Quote by Xidike View Post
why speed can't have direction E.g 44 km/h towards East...??
Because speed with direction is velocity. It's just how we name things.
jtbell
#8
Nov8-12, 10:51 PM
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By definition, speed has only magnitude, and velocity has both magnitude (speed) and direction.

By definition, distance has only magnitude, and displacement has both magnitude (distance) and direction.

Sometimes, we need only the magnitude, in which case we use speed and/or distance. Sometimes, we also need to take into account the direction, in which case we use velocity and/or displacement.

[added] This is according to current usage in English-language textbooks. It was also this way forty years ago when I first studied physics. If the original poster is using a textbook that is a hundred years old or in a language other than English, he should clarify this.
Hetware
#9
Nov8-12, 10:59 PM
P: 125
Quote Quote by harrylin View Post
Speed and velocity: that is very subtle. In many languages there is only one word, like there is in English only one word for acceleration. Speed can only mean the magnitude, while velocity can mean, like acceleration, either magnitude or vector (magnitude and direction). It has become customary in physics to use velocity exclusively for the vector quantity but this is not universal and was not common in the past. Moreover, translations from for example German tend to use "velocity" as the German word is non-specific. Thus old translations of some German papers by Einstein have "the constancy of the velocity of light", and the implied meaning (by the context) is in this case speed.

Distance and displacement: that's similar but non-ambiguous: distance is only the magnitude, while displacement has direction.
Latin vs. real English, I guess. When did Gibbs's vector concept really take hold?
Hetware
#10
Nov8-12, 11:02 PM
P: 125
Quote Quote by Hetware View Post
Here's something to consider: Einstein's special theory of relativity postulates that the speed of light is the same for all observers.

Two observers moving relatively to one another will both see the same light go at the ~300,000 kilometers per second, relative to themselves. They may, however, see the light traveling in a different direction than what the other observer sees.

So the speed of light is the same for all observers, but not the velocity of light.

Sorry if that's too advanced for you at this point. If it doesn't make sense, but remember the rule so that when you hear a friend say "The velocity of light is the same for all observers". You reply "No, it's the speed of light that is the same for all observers."

Another way of explaining the relationship between velocity and speed is to represent velocity as an arrow pointing in the direction of the motion. The size of the arrow is proportiional to the speed. So speed is called the magnitude of the velocity.

My previous post appears to have been deleted, silently, and without explanation. Strange that, eh?
hcorbett391
#11
Nov8-12, 11:03 PM
P: 2
Well, speed could have a direction, but then it would be a vector quantity referred to as 'velocity.' The basic distinction I understand between the two is that velocity is a vector - it is a magnitude and a direction. Speed is the magnitude of that vector.

Let's consider a particle that's moving along a plane: It is moving in the [itex]\hat{x}[/itex] direction at some km/h and in the [itex]\hat{y}[/itex] direction at some other km/hr. So, what you've got to do to get to a 'speed' is add these to vectors.

Here's how I think about this, for better or worse: If you draw out the vectors on a sheet a paper, you can see that the velocities in each direction (the vector components) are perpendicular. Looks kinda like a right triangle, where the hypotenuse represents the vector magnitude of the velocity.

So, how do we find the hypotenuse of a right triangle?
jtbell
#12
Nov8-12, 11:07 PM
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Quote Quote by Hetware View Post
My previous post appears to have been deleted
I still see it. No posts have been deleted from this thead as far as I can tell.
Hetware
#13
Nov8-12, 11:12 PM
P: 125
Quote Quote by jtbell View Post
I still see it. No posts have been deleted from this thead as far as I can tell.
I'm talking about the one before that. The nice thing about where I work is that NOTHING gets deleted.
jtbell
#14
Nov8-12, 11:14 PM
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I repeat, no posts have been deleted from this thread. As a mentor (moderator), I can see deleted posts. They are not actually physically deleted, merely "hidden" from ordinary members.
Xidike
#15
Nov9-12, 02:09 AM
P: 72
In the speed-O-meter of Car, why we called it speed, instead of velocity, while car has direction ????
Matterwave
#16
Nov9-12, 02:16 AM
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The speedometer doesn't tell you the direction. Only the speed.
phinds
#17
Nov9-12, 02:18 AM
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Quote Quote by Xidike View Post
In the speed-O-meter of Car, why we called it speed, instead of velocity, while car has direction ????
Because the direction of the car is totally irrelevant to the reading on the speedometer. If you go in a curve, your speed can stay constant while your direction is constantly changing.
Xidike
#18
Nov9-12, 04:54 AM
P: 72
yeh kiya baat hui ?????
but Car runs in direction.. not in arbitrary point.. then why it has speed instead of velocity...
velocity is used where, there is direction.. also car runs in direction.. then why car has speed instead of velocity ???? ///\\\


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