Does distance divided by time give you a constant speed?

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glassynails
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Does distance divided by time give you a constant speed?

Post by glassynails » Mon Nov 28, 2016 2:39 pm

As I was laying in bed last night trying to fall asleep I got to thinking about the old distance, time and speed equation and pictured myself having to drive 100 miles in a car (not that I do, but just for example).

So I thought of something like this: If I have to drive 100 miles in 1 hour and the first 1/2 hour I end up only going 20 miles I must've "averaged" 20mph. So in order to get to my 100 mile mark in 1/2 hour remaining I have to drive 160 mph.

So I had always taken it that distance / time = constant speed (mostly because I never think about these things much anymore), but I believe it actually means "average speed". Am I right?

For example, If someone drove 100 miles in 1 hour we wouldn't have any idea what speed he was always traveling for that hour, just that he got there in 1 hour and drove 100 miles. So I'm now assuming that - distance / time = average speed.
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Re: Does distance divided by time give you a constant speed?

Post by FJ25 » Mon Nov 28, 2016 4:54 pm

glassynails wrote:So I had always taken it that distance / time = constant speed (mostly because I never think about these things much anymore), but I believe it actually means "average speed". Am I right?
Well, the average speed is the speed you would need to go at if you were going to travel at a constant speed, I think that's the link. Otherwise what would it mean to say it equates to a constant speed... that every time you cover a certain distance in a certain time, you must have been travelling at a constant speed?

As you know, if someone set out just after you going twice as fast, they could never catch up with you, because they would first have to get to the point you were at when they started, and by the time they'd got there, you'd have moved on a bit, so they would have to get to that point before they could catch you, but by the time they did, you'd have moved on a bit, so they'd have to get to *that* point before...

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Re: Does distance divided by time give you a constant speed?

Post by pogmoor » Mon Nov 28, 2016 6:19 pm

FJ25 wrote:As you know, if someone set out just after you going twice as fast, they could never catch up with you, because they would first have to get to the point you were at when they started, and by the time they'd got there, you'd have moved on a bit, so they would have to get to that point before they could catch you, but by the time they did, you'd have moved on a bit, so they'd have to get to *that* point before...
You're trying to fool glassynails with one of Zeno's paradoxes :shock:
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Re: Does distance divided by time give you a constant speed?

Post by Andrew Pohlman » Mon Nov 28, 2016 6:34 pm

glassynails wrote:As I was laying in bed last night trying to fall asleep I got to thinking about the old distance, time and speed equation and pictured myself having to drive 100 miles in a car (not that I do, but just for example). ...
Distance divided by time is average speed. But it is a fake out, because to measure "instantaneous speed" you still need a point of reference, i.e., passing some physical location at a certain time. Or using radar you would measure length of time for the radar waves to travel from source to object and back again.

So to travel 100 miles in one hour, your average speed needs to be 100 mph. Your instantaneous speed could be anything.
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Steve O
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Re: Does distance divided by time give you a constant speed?

Post by Steve O » Mon Nov 28, 2016 7:49 pm

In China some of the freeways we travel on photograph your licence plate at the on-ramp, and again at the off-ramp. They calculate your average speed over that distance, and if you've been speeding you get a ticket.

The penalty must be severe because our drivers rarely speed on those roads, but they continue to drive like homicidal/suicidal maniacs the rest of the time.

There's one particular road that has photo radar on every second overpass. We drive 160 km/h for 2 km, slam on the brakes to slow down to 80 km/h to pass the photo camera, then back up to 160 km/h for another 2 km, and so on until I want to slap the driver so hard that my head explodes.

stevel
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Re: Does distance divided by time give you a constant speed?

Post by stevel » Tue Nov 29, 2016 1:54 am

I always wondered about this one:

At 60 MPH it will take me an hour to drive 60 miles.

If I drive 65 MPH, how much time will it shave off?

Or more importantly, if I've only got 45 minutes to drive that 60 miles, how fast do I need to drive to get there in time?

These are all those math "word problems" that I struggled with in high school.

All I know is 2 8th notes equal a 1/4 note.

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Re: Does distance divided by time give you a constant speed?

Post by chiral3 » Tue Nov 29, 2016 2:02 am

>Or more importantly, if I've only got 45 minutes to drive that 60 miles, how fast do I need to drive to get there in time?

I'd wait around for 35 minutes daydreaming and then try and go 600mph for 10 minutes. That's kinda how I practice guitar some days.
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astro64
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Re: Does distance divided by time give you a constant speed?

Post by astro64 » Tue Nov 29, 2016 4:05 am

stevel wrote:I always wondered about this one:

At 60 MPH it will take me an hour to drive 60 miles.

If I drive 65 MPH, how much time will it shave off?

Or more importantly, if I've only got 45 minutes to drive that 60 miles, how fast do I need to drive to get there in time?

These are all those math "word problems" that I struggled with in high school.

All I know is 2 8th notes equal a 1/4 note.
- first question: about 5 min
- second question: 45 min is 0.75 hr, so you need to go 60/0.75=80 miles per hour.

Brian M
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Re: Does distance divided by time give you a constant speed?

Post by Brian M » Wed Nov 30, 2016 3:29 am

Slightly tangential: there's a book called "A Tour of the Calculus". It's sort of "Math Appreciation". I loved it.

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Re: Does distance divided by time give you a constant speed?

Post by Brian M » Wed Nov 30, 2016 3:38 am

glassynails wrote:As I was laying in bed last night trying to fall asleep I got to thinking...
Lay off the caffeine after 5 PM. If I have caffeine after 5 PM, I get to thinking... deep thoughts. Like, "Whoa, in The Music Man, Goodnight My Someone and Seventy Six Trombones are the same tune, which means that she's already..."

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Re: Does distance divided by time give you a constant speed?

Post by chuckinphoenix » Wed Nov 30, 2016 3:30 pm

I wonder if Glassy spends as much time on guitar practice as he does conjuring up these life's imponderable questions.
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Andrew Fryer
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Re: Does distance divided by time give you a constant speed?

Post by Andrew Fryer » Wed Nov 30, 2016 4:21 pm

No offence, but there's nothing imponderable about it - it's just basic applied maths.
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chiral3
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Re: Does distance divided by time give you a constant speed?

Post by chiral3 » Wed Nov 30, 2016 4:28 pm

Andrew Fryer wrote:No offence, but there's nothing imponderable about it - it's just basic applied maths.
I just learned a new word - imponderable. Reminds me of that line from that older movie 3 Amigos:

Dusty: “What does that mean? Infamous?”
Ned: “Ah, Dusty! Infamous is when you're more than famous! This guy El Guapo is not just famous, he's IN-famous!”
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robinfw
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Re: Does distance divided by time give you a constant speed?

Post by robinfw » Wed Mar 22, 2017 3:51 am

All questions are answered.

Terry Pratchett Author of Thief of time.

And for those that want the perfect guitar.

Soul Music.

By the same author.



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Re: Does distance divided by time give you a constant speed?

Post by PeteJ » Wed Mar 22, 2017 11:34 am

chiral3 wrote:>Or more importantly, if I've only got 45 minutes to drive that 60 miles, how fast do I need to drive to get there in time?

I'd wait around for 35 minutes daydreaming and then try and go 600mph for 10 minutes. That's kinda how I practice guitar some days.
Lol. Post of the day.

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