But I'm sure Speed and velocity are the same thing

[Weissritter]

Yeah, I know the differences between speed and velocity. In common life, however, I may use them as if they were the same. Guess my science-brain isn't that much active sometimes.
While this thread is physicist-targeted, anyone's reaction is welcome.
You know the difference between those two words, right? How do you feel when common people use them as if they meant the same? Have you ever objected at their actions, or you just sigh again, as if no effort could stop the misuse?

 

[jtbell]

Velocity as a vector quantity versus speed as a scalar quantity (magnitude of the velocity) is purely a convention among physicists. I learned just today that it appears to have been started by J. W. Gibbs around 1901. He wrote the first textbook (as opposed to lecture notes) that taught the vector mathematics that we use today.

When I'm dealing with non-physicists in a non-physics context, I don't expect them to know the difference, nor do I make a point of it.

 

[Pythagorean]

I don't expect laymen to recognize the distinction.

 

[TheMadMonk]

The one thing that does get me is when lay persons try and tell me deceleration is not acceleration.

 

[lisab]

When I'm dealing with non-physicists in a non-physics context, I don't expect them to know the difference, nor do I make a point of it.

I don't expect laymen to recognize the distinction.

Same here, and I try not to let them see my eye twitch.

 

[Sdtootle]

I very rarely hear anyone say velocity. I almost always hear then say speed, but I rarely have issue if they misuse it outside of a technical environment.

I get more upset when people mix up your and you're on Facebook.

 

[zoobyshoe]

I get more upset when people mix up your and you're on Facebook.

Some people just don't realize "your" is scalar and "you're" is a vector.

 

[Weissritter]

For now we have no murder desire, so it is kinda good. When it comes to your and you're things do become more annoying.
Do we have more opinions?

 

[Jorriss]

In nonacademic talk, I pretty much use speed exclusively.

 

[jtbell]

Some people just don't realize "your" is scalar and "you're" is a vector.

And that "your'e" is imaginary.

 

[fluidistic]

I don't feel bad when someone misuses velocity for speed, my brain just pops up a "wrong" message. The same happens when I hear/read "weight ... kg".

 

[Pengwuino]

Some people just don't realize "your" is scalar and "you're" is a vector.

Greatest line ever.

 

[Pythagorean]

or "mass.. lbs"

 

[aquitaine]

I don't feel bad when someone misuses velocity for speed, my brain just pops up a "wrong" message. The same happens when I hear/read "weight ... kg".

Or using "lightyear" as a measure of time.

 

[Weissritter]

Or using "lightyear" as a measure of time.

This gives me a controllable murder desire

Also forgot the highest one: saying "degrees kelvin" The worst part is that most people who say this are not common non-physicist. They are usually scientists, who are supposed to know this. It becomes worse when something as big and inspected as "The Avengers" makes that mistake.

 

[berkeman]

So wait. Should we be using the term "escape speed" instead of "escape velocity" now?

Just askin'...

 

[Jimmy Snyder]

So wait. Should we be using the term "escape speed" instead of "escape velocity" now?

Just askin'...

But in this case direction matters.

 

[D H]

The same happens when I hear/read "weight ... kg".

This confusion is our fault rather than a confusion in the lay community. The technical world is the one that had a problem with the term "weight", not the lay community. This technical confusion wasn't resolved until 1901 when, by dint of a bureaucratic decision, weight was deemed to denote "a quantity of the same nature as a 'force'."

The lay community was and is just fine with weight being a non-technical synonym for the technical concept of mass. Legally, the term "weight" is still a synonym for mass in the US. A one pound can of beans weighs one pound at the South Pole, at the tip of Mt. McKinley, and even on the International Space Station.

or "mass.. lbs"

A pound is a unit of mass. Perhaps you are confusing the pound with the pound force.

 

[russ_watters]

But in this case direction matters.

No it doesn't. That was berke making fun of our physicist friends:

The term escape velocity is actually a misnomer, and it is often more accurately referred to as escape speed since the necessary speed is a scalar quantity which is independent of direction (assuming a non-rotating planet and ignoring atmospheric friction).

http://en.wikipedia.org/wiki/Escape_velocity

 

[Jimmy Snyder]

No it doesn't. That was berke making fun of our physicist friends: http://en.wikipedia.org/wiki/Escape_velocity

Remember, escape speed is the speed needed to escape to infinity and as the distance approaches infinity, the angle between your motion and the Earth's center approaches zero, making every path essentially vertical.

Try down. That's a direction.

 

[russ_watters]

Try down. That's a direction.

As long as there's nothing in the way, the trajectory can initially be in any direction, including down. Evacuating a tunnel through the center of the Earth is an engineering problem, not a physics one.

 

[berkeman]

 

[tahayassen]

This confusion is our fault rather than a confusion in the lay community. The technical world is the one that had a problem with the term "weight", not the lay community. This technical confusion wasn't resolved until 1901 when, by dint of a bureaucratic decision, weight was deemed to denote "a quantity of the same nature as a 'force'."

The lay community was and is just fine with weight being a non-technical synonym for the technical concept of mass. Legally, the term "weight" is still a synonym for mass in the US. A one pound can of beans weighs one pound at the South Pole, at the tip of Mt. McKinley, and even on the International Space Station.


A pound is a unit of mass. Perhaps you are confusing the pound with the pound force.

I thought the slug was the unit of mass in FPS?

 

[Pythagorean]

A pound is a unit of mass. Perhaps you are confusing the pound with the pound force.

They're equivalent in US. Slug is the unit of mass (m), lbs is weight (mg), pressure is lbs/in^2 (pounds per square inch)

Nobody ever says pound force.

 

[Pythagorean]

Also, nobody ever says slug because there is no distinction between mass and weight in US laymen.

 

[BobG]

Also, nobody ever says slug because there is no distinction between mass and weight in US laymen.

Also because the slug was added to the units of measurement later than the pound (mass). The term didn't come into common use until the 1920's (common being a very relative term, since pounds (mass) is still more common). Prior to being called the 'slug', the slug was called the 'geepound'. And, prior to the 1890's (approximately) people just used pounds.

I think (but really don't know) that the slug came about to make the US Customary system fit better with standard equations, such as F = ma. With slugs, you can use the actual rate of acceleration in ft/sec^2 instead of a ratio comparing the local gravitational acceleration to the standard gravitational acceleration at the surface of the Earth.

 

[uperkurk]

I am a laymen and may as well take this opportunity for one of you to educate me on the difference.

Mass and weight.
Velocity and speed.

Please give me an example of where each of these should be used.

A man weighs 160lbs.
A car travels at 60mph.

But I don't understand when to use velocity and mass.

 

[Monique]

I don't feel bad when someone misuses velocity for speed, my brain just pops up a "wrong" message. The same happens when I hear/read "weight ... kg".

Why? You expect people to express their weight in Newton or state what their mass is?

 

[Jack21222]

I'm cool with people using speed for velocity in a non-academic setting, but I'm less happy with people using velocity when they mean speed. It's usually a sign that they're trying to sound fancy by (mis)using a less common word.

 

[tahayassen]

I am a laymen and may as well take this opportunity for one of you to educate me on the difference.

Mass and weight.
Velocity and speed.

Please give me an example of where each of these should be used.

A man weighs 160lbs.
A car travels at 60mph.

But I don't understand when to use velocity and mass.

Velocity is just speed with a direction.

A car travels at 60 mph. That's speed.
A car travels at 60 mph north. That's velocity. A car travels at 60 mph to the right. That's also velocity.

Mass is a property of matter. Every single thing has mass. For example: a ball might have a mass of 3 kg (or 0.21 slug if you want to use the FPS system).
A force is a push or pull that causes any object to change its movement or direction. Weight is a force by gravity.

Newton's Second Law:

Force\quad (such\quad as\quad weight)=mass\quad x\quad acceleration\\ Therefore,\quad Weight=mass\quad x\quad g\quad (g\quad represensts\quad acceleration\quad caused\quad by\quad gravity)\\ Weight=(3\quad kg)(9.8\quad m/{ s }^{ 2 })\quad (g\quad is\quad 9.8\quad m/{ s }^{ 2 }\quad at\quad sea\quad level)\\ Weight=29\quad \frac { kg*m }{ { s }^{ 2 } } \\ Weight=29\quad N\quad (N\quad stands\quad for\quad Newtons;\quad 1\quad N\quad =1\frac { kg*m }{ { s }^{ 2 } } )

Mass is constant for an object no matter where the object is in the universe. However, weight decreases as you get further away from Earth. This is because g (which is gravitational acceleration) decreases as your altitude increases above sea level. Astronauts still have the same mass as they do on Earth but their weight substantially decreases.

The FPS system equivalent of a Newton is a pound-force or just a pound. You'll notice that I didn't do the calculations for the FPS system. This is because scientists use the metric system and doing calculations in the FPS system becomes needlessly confusing.

Mod note: following information has been added by request of the poster on 29 december
Edit: I've made a mistake.

While I was correct that g (which is gravitational acceleration) decreases as your altitude increases above sea level, it does not decrease by as much as I made it seem. Astronauts actually still experience 95% of the weight that they would experience at sea level.

Here is why an explanation why astronauts float if you're interested:

 

[uperkurk]

Velocity is just speed with a direction.

A car travels at 60 mph. That's speed.
A car travels at 60 mph north. That's velocity. A car travels at 60 mph to the right. That's also velocity.

Mass is a property of matter. Every single thing has mass. For example: a ball might have a mass of 3 kg (or 0.21 slug if you want to use the FPS system).
A force is a push or pull that causes any object to change its movement or direction. Weight is a force by gravity.

Newton's Second Law:

Force\quad (such\quad as\quad weight)=mass\quad x\quad acceleration\\ Therefore,\quad Weight=mass\quad x\quad g\quad (g\quad represensts\quad acceleration\quad caused\quad by\quad gravity)\\ Weight=(3\quad kg)(9.8\quad m/{ s }^{ 2 })\quad (g\quad is\quad 9.8\quad m/{ s }^{ 2 }\quad at\quad sea\quad level)\\ Weight=29\quad \frac { kg*m }{ { s }^{ 2 } } \\ Weight=29\quad N\quad (N\quad stands\quad for\quad Newtons;\quad 1\quad N\quad =1\frac { kg*m }{ { s }^{ 2 } } )

Mass is constant for an object no matter where the object is in the universe. However, weight decreases as you get further away from Earth. This is because g (which is gravitational acceleration) decreases as your altitude increases above sea level. Astronauts still have the same mass as they do on Earth but their weight substantially decreases.

The FPS system equivalent of a Newton is a pound-force or just a pound. You'll notice that I didn't do the calculations for the FPS system. This is because scientists use the metric system and doing calculations in the FPS system becomes needlessly confusing.

That actually helped a lot thanks :)

 

[surajt88]

As long as there's nothing in the way, the trajectory can initially be in any direction, including down. Evacuating a tunnel through the center of the Earth is an engineering problem, not a physics one.

 

[D H]

I thought the slug was the unit of mass in FPS?

That depends on what engineers you talk to. Some use the slug as a unit of mass and the pound force as a unit of force; they write Newton's second law as F=ma. Others use the pound a unit of mass and the pound force as a unit of force; they write Newton's second law as F=kma, where k has a numeric value of 1/32.1740486.

They're equivalent in US. Slug is the unit of mass (m), lbs is weight (mg), pressure is lbs/in^2 (pounds per square inch)

Not per the US standard bearers, the National Institute of Science and Technology. The word "pound" sans any qualifier designates the avoirdupois pound, a unit of mass. If you mean force it is best to say "pounds force", or lbf for short.

Nobody ever says pound force.

https://www.google.com/search?q="pounds+force"+site:nasa.gov

 

[Pythagorean]

Regardless, lb is used for both mass and force, as is "pound" and is generally assoiated with the word "weight" in laymen context.

Those examples are completely outside the context of this thread... we're talking bout the public here.

 

[MPKU]

In what instance would someone misuse the words? I'm clear in the distinction, but am having trouble thinking of an everyday scenario where someone would misuse either term

 

[Jack21222]

In what instance would someone misuse the words? I'm clear in the distinction, but am having trouble thinking of an everyday scenario where someone would misuse either term

One example: somebody telling me they were driving on a road with curves at a constant velocity. They mean speed, but they're trying to sound pretentious.

 

[russ_watters]

Not per the US standard bearers, the National Institute of Science and Technology. The word "pound" sans any qualifier designates the avoirdupois pound, a unit of mass. If you mean force it is best to say "pounds force", or lbf for short.

I think this is bad advice. We just had a thread where someone asked why pressure is psi and not pounds. Someone else - instead of answering the actual question - gave a criticism on using pounds force instead of pounds, which was very unhelpful.

Most of the time it is better to answer a question according to the meaning that is clearly understood rather than not answer it because a word was very slightly improperly used.

https://www.google.com/search?q="pounds+force"+site:nasa.gov

NASA doesn't always get it right either: https://www.google.com/search?q="po...a0003da4073894&bpcl=40096503&biw=1184&bih=655

NASA sites use both "pounds per square inch" and "pounds force per square inch", as well as "weight" in "pounds". Worse, the second link says "pounds of pressure per square inch"!

 

[russ_watters]

One example: somebody telling me they were driving on a road with curves at a constant velocity. They mean speed, but they're trying to sound pretentious.

Some people do actually use the words interchangeably, so I don't think it is likely the person intended it to be pretentious. Correcting the usage, however, would be.

In the case of "pounds"... I would go so far as to say even in many technical fields it is most often used - unqualified - to indicate force, not mass.

 

[BobG]

I think this is bad advice. We just had a thread where someone asked why pressure is psi and not pounds. Someone else - instead of answering the actual question - gave a criticism on using pounds force instead of pounds, which was very unhelpful.

Most of the time it is better to answer a question according to the meaning that is clearly understood rather than not answer it because a word was very slightly improperly used. NASA doesn't always get it right either:

Good point. It's common (and common sense) to use context as a clue to what's actually meant by a word. When it comes to celestial mechanics/orbital maechanics, etc, how many times do you really want to type out 'specific energy per unit of mass' or 'specific angular momentum per unit of mass'? It doesn't take very long to learn that when a book about orbital mechanics talks about angular momentum, they mean specific angular momentum per unit of mass.

But, it is common to see equations that use mass measured in pounds - uncommon enough that it's kind of annoying when it is expressed in pounds, but common enough that one should know that pounds is a perfectly legitimate unit of measure for mass. (It can also be confusing for students when 'g' pops up in an equation used to calculate the amount of thrust and fuel to escape the Moon, for example - "Why the heck are we using Earth's gravitational acceleration when we're on the Moon?!") In fact, I'd say it's annoying enough to give a person a very, very strong preference for the metric system.

Well, at least until you use the exact same equation in the metric system and divide one term by 'g' to get a special unit (specific impulse) and multiply one term by 'g' to calculate fuel flow rate and start to wonder why 'g' is inserted into the calculation in the first place because all sense of context has been lost.