The answer is that the speedometer will read a lower speed than the true linear speed.

How is this so?

Also for my own sanity, what is the difference between speed and velocity? I always thought they were the same, but apparently not.

EDIT: I'm going to take a nap (final tomorrow) - be back in a few hours.

Last edited by MasamiAkane at 1:23 am, Dec 16 2009

well first of all speed is a scalar quantity which refers to "how fast an object is moving. While, velocity is a vector quantity which refers to the rate at which an object changes its position.

or in another words:
While speed is the rate of change of distance with time, velocity is the rate of change of displacement with time. Or while speed is the first derivative of distance with respect to time, velocity is the first derivative of displacement with respect to time.


as for your question:
since it measures the angular speed, when i made the tired bigger. same distance will be taken with larger tires meaning less rotation. and since my speedometer measures according to rotation rate, when i decrease the rotation rate, speedometer will read a lower value.

well i don't think i explained well but O_O

Hi, the first thing you should have known is the relation between angular speed and linear speed in pure rolling motion (of the wheels).

How speedometer works: (i totally made this up )

The speedometer measures the linear speed through the angular speed of the tires, : v = r w In production, each speedometer in a car has a designated value of r in its "memory" O_O. So, it measures v through w by a factor r (which is kept constant unless u reprogram it)

Now we use larger tires r' > r : v' = r' w is the true linear speed but the speedometer is not synced with the new tires , it still computes the linear speed through the default formula, so it reads the value r w instead of r' w and thus the value is lower than the real linear speed.

Speed and velocity are two veerryy different quantity.

1/ Speed is a scalar quantity, refers to the rate of the change in the distance traveled (path taken). v = delta s / delta t
Velocity is a vector quantity, refers to the rate of the change in the position (aka the displacement). v = delta r / delta t

2/Speed and velocity are different not only because they are scalar vector, but also because the are measured differently. this is what you should know before going any further in physics
Edit : Yeah, kind of agree with the above, except for the later part though, speedometer reads linear speed through angular speed, but not the angular speed explicitly.

Last edited by silencer at 2:16 am, Dec 16 2009

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To answer the difference between speed and velocity. People like to use big words to discern these two things.
SO Let me make it absolutely simple and clear: Velocity is speed with direction. Or Speed is the magnitude of velocity.

In 1 dimentional space, if velocity is -50m/s then speed is just 50m/s

In 2-d, if velocity is -50m/s in the x direction and -50m/s in the y direction, then the speed is the magnitude of this vector arrow.
speed in this case is square root[50^2+50^2]

Yep, it's that simple.


remember, that the more you learn, the more you learn. New stuff make everyone uncomfortable, but only those with conviction can learn! So push forward in the rain of arrows and learn!!


Last edited by BoxBox at 12:46 pm, Dec 16 2009

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Gay book discussion thread

Erm, actually it's not that simple >,> if we think the term "velocity" and "speed" only involves the "instantaneous velocity" and "instantaneous speed" then you are right.
If we think they also refer to "average velocity" and "average speed" on a path than you are half correct.
An object move in a zigzag trajectory from A to B with constant speed u (to make it simple) The magnitude of its (average) velocity on this path is not u (the speed)
To sum it up, the difference between "speed" and "velocity" is like the difference between "distance traveled" and "displacement"- they are no big words =_=



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If an object moves in a zig zag form of path, you then divide the path into vectors. Each vectors have a magnitude, That is to say if a vector does not have a same magnitude all around the path, it does not have constant speed. I'm not sure what you are trying to get there....perhaps more descriptions.

Besides....when objects zig zags as you said, there's no way it can have constant speed. The thing is accelerating and decelerating as it turns/

There are always big words spoken by small people.

Last edited by BoxBox at 10:01 pm, Dec 16 2009

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And that 's not average velocity on the path AB ( i know that you always tend to think of instantaneous velocity )

Okay, to make it clearer, a car moves in a circle from A and back to A with constant speed u, its average velocity in this orbit has magnitude of 0. As i said before, distance traveled is not the change in position (the displacement)

Are you sure ? ( that when acceleration exists the speed can not be constant ? O_O im kinda surprised)


If the terms "velocity" and "speed" only refer to the instantaneous velocity and instantaneous speed then their relation is like a vector and its magnitude. This doesn't hold if we use a broader concept as (average) velocity and (average) speed on a non-linear path, Hmm, however, people like to separate those things, so if you say average velocity/speed of a object on its path is not its velocity/speed, Then the other things you said are correct, too.

Yeah, there are always big words =D we just have to use them appropriately. Though that is also a relative concept O_O it depends on people...how about we accept them as something new to learn ?

Edit : since you edited yours, too

Last edited by silencer at 10:30 pm, Dec 16 2009

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Speed is just how fast an object is moving.

Velocity measures rate change in position.

The magnitude of velocity is just the speed at which it's position has changed from point A to point B.

Speed with direction just makes it a vector. You'd need to get all of the vectors that an object that has moved to calculate it's final position. From there you can find it's velocity.

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Sometimes I forget the ability of the collective to know FREAKING everything! Makes me proud!

@ silencer

When objects move in zig zag motion, it accelerates and decelerates. The speed will not remain constant. In physics, not one cares about the average velocity, it's got nothing. No information at all. People always use the speed and velocity of the path parts.

By the way, a car moving in a circle is not a zig zag motion as you previously mentioned. A car moving in circle now have circular velocity, momentums and etc etc. It's an entirely different problem.....

I graduated middle school, I think I know what I'm talking about




Thank you..

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Gay book discussion thread

Ok guys...

Thanks to everyone, nonetheless.

Disappointed.

Disappointed again.
It looks like you can't distinguish between instantaneous velocity and "velocity of the path parts" o_o
Do some research on this and we'll discuss again =D (if you ever want to)


Even in middle school i wouldn't make this mistake i thought you'd realize after i mentioned it. The concepts are acceleration, velocity and speed, so you should have learned all in Middle school, actually i'm not sure how the system is in US.

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Dude, let's try to be civilized around here. It's not 4chan where you rant and flame people. Besides, a particle travelling in a zig zag motion will definitely change in acceleration. There needs a change in it's X and Y component to change direction. I'm sure YOU of all people should agree. And the average speed of such a particle cannot be found just by looking at the start and end point of the path. It's a bit more complicated. Why don't you read up on circular motions. There are linear velocity and there are angular velocity. When you talk about a particle travelling in a circle, you are talking about angular velocity

p.s. please do not say that your country's education is superior to the U.S's education system....it's racist and mean....I hate it when people bad mouth the U.S. when they haven't even been to the U.S. before

To mod: I'm sorry to rant, but it's just when I see people giving other people false information, intentionally or not, I like to correct them. I mean this is a place where people get their homework helped....and I try to maintain the quality of the answer....


Last edited by BoxBox at 5:26 pm, Dec 17 2009

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Gay book discussion thread

In the US, the student has the option of taking physics in High School in 11th and/or 12th, but unfortunately most decide not to take this third year of science... or even any regular science classes but rather something we called "general science"... which allowed kids to fulfill their science requirement w/o really needing to take a science class.
Of course, this is just for public education in Southern California... I don't know how it is for the private sector (mostly religiously based) or other states.

So, if you were able to take physics in middle school... you're lucky in my opinion.

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"Officially, this machine doesn't exist, you didn't get it from me,
and I don't know you. Make sure it doesn't leave the building."

Will anyone here be willing to help me with Calculus? It's a few topics because I have a chapter quiz tomorrow.

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