# The use of the terms speed & velocity

1. Jun 23, 2012

### swampwiz

The use of the terms "speed" & "velocity"

This is a interesting topic concerning when to use these two terms in a discussion. Of course, if it is a vector term, then velocity must be used - however even for a scalar term, velocity could still be used.

It seems to me that aside from the term of the speed of wave propagation (e.g., the speed of light, the speed of sound, etc.), or the measurement of the speed itself (although that measurement would simply give the velocity of that object), then the term velocity should be preferred - even if it is a scalar or applies to only one length dimension, including angular velocity for a planar system. With that said, I could still see speed used instead if talking about the motion of an object subject to set of holonomic constraints - e.g., a car on a roller coaster, or a point on a rotating wheel. etc. I could also see speed used in the term "instantaneous speed", even though it actually represents a vector quantity (in which case the speed would only represent the magnitude.) And of course in any talk about speed when really it means an inverse of the time required (e.g., computer speed, etc.) - rather than the proper parameter of the time derivative of the displacement in space - should only use speed.

2. Jun 23, 2012

### azizlwl

Re: The use of the terms "speed" & "velocity"

Your car speedometer has no negative value but the car can reverse.

3. Jun 23, 2012

### Staff: Mentor

Re: The use of the terms "speed" & "velocity"

Physicists prefer speed for the speed scalar and velocity to mean a velocity vector. Laypeople use them intercchangeably. Decide based on your audience.

4. Jun 24, 2012

### spareine

Re: The use of the terms "speed" & "velocity"

Aside question: Newton did not need the word speed, he used only the word velocity in Principia (translated). I wonder, did the special meaning of the word speed in physics originate in the 18th century (hydrodynamics), in the 19th century (kinetic gas theory), or in the 20th century (when people owned cars with a speedometer)?
Other languages like French and German don't have separate words for speed and velocity.

Last edited: Jun 24, 2012
5. Jun 24, 2012

### Studiot

Re: The use of the terms "speed" & "velocity"

In Newton's day the vector idea was not well developed, and celerity was used for what we now call velocity and some of the modern uses of speed.
The term speed then meant 'fortune' as in God Speed which wished good fortune.

Today the term velocity should be used for a vector, although celerity is still available.

Speed, today, is a much more general term that does not even necessarily involve motion as in

A three speed bicycle
The speed of the bread dough rising
The speed of a camera shutter.
And many more.

It can also be used for the magnitude of the velocity vector. Like any magnitude it has no sign. A separate sign or backwards/forwards indication may be attached however.

6. Jun 24, 2012

### spareine

Re: The use of the terms "speed" & "velocity"

Ok, the special use of the word speed in physics did not originate in Newton's time, the word celerity was used as well. However, the current convention among physicists to prefer speed for the magnitude of velocity presumably originated sometime and somewhere. When and where?

7. Nov 18, 2012

### spareine

Re: The use of the terms "speed" & "velocity"