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Solve a Physics Problem - Get a Cookie!

Old 09-24-07, 07:02 PM
  #1  
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Solve a Physics Problem - Get a Cookie!

Alright.. here's the deal. If you answer the question below correctly, I will literally mail you a cookie. I promise. If you answer it incorrectly, you need to mail the person who answers it correctly after you a cookie. So the winner gets as many cookies as there are incorrect answers. No deadline. Post all answers here, PMs will not be accepted. I will post hints as I see fit.

Originally Posted by theproblem
Boss Moniker bikes to school at a constant speed of 20km/h. What speed must he travel on the return trip so that he has an average speed for the entire trip of 40km/h?

The routes are the same, don't factor for traffic nonsense, this is a physics world question.
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Old 09-24-07, 07:05 PM
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Originally Posted by Boss Moniker
Alright.. here's the deal. If you answer the question below correctly, I will literally mail you a cookie. I promise. If you answer it incorrectly, you need to mail the person who answers it correctly after you a cookie. So the winner gets as many cookies as there are incorrect answers. No deadline. Post all answers here, PMs will not be accepted. I will post hints as I see fit.

Originally Posted by theproblem
Boss Moniker bikes to school at a constant speed of 20km/h. What speed must he travel on the return trip so that he has an average speed for the entire trip of 40km/h?

The routes are the same, don't factor for traffic nonsense, this is a physics world question.
It cannot be done because the entire allotted time is consumed by the trip to school. Where's my cookie?
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Old 09-24-07, 07:07 PM
  #3  
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Originally Posted by jschen View Post
It cannot be done because the entire allotted time is consumed by the trip to school. Where's my cookie?
You are correct, although I wouldn't have explained it in the same way. PM me your shipping address, and I will post you a cookie. No guarantees about freshness.
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Old 09-24-07, 07:08 PM
  #4  
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Originally Posted by jschen View Post
It cannot be done because the entire allotted time is consumed by the trip to school. Where's my cookie?
Do you really want it?
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Old 09-24-07, 07:10 PM
  #5  
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Can I have a cookie too since I'm Jschen's best friend?
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Old 09-24-07, 07:12 PM
  #6  
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Originally Posted by Alfster View Post
Do you really want it?
Yes, so I can post a pic of the said cookie. No, I won't eat a stale cookie mailed to me by a stranger.
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Old 09-24-07, 07:16 PM
  #7  
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Send brownies to...
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Old 09-24-07, 07:17 PM
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Everyone on BF gets a cookie!

Here is an old one that was given to me:

Code:
__utma
3220771.1955090995.1150245949.1158631011.1158773738.18
bikeforums.net/
1600
2350186496
32111674
794283632
29809883
*
__utmz
3220771.1157341249.11.5.utmccn=(organic)|utmcsr=google|utmctr=Cliff+Shot+Bloks+eating|utmcmd=organic
bikeforums.net/
1600
2773975040
29843260
3917843088
29806547
*
bblastvisit
1150943482
bikeforums.net/
1536
2903704832
29883308
891473632
29809883
*
bblastactivity
0
bikeforums.net/
1536
2903704832
29883308
891473632
29809883
*
__utmb
3220771
bikeforums.net/
1600
1687740672
29809893
871359856
29809889
*
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Old 09-24-07, 07:18 PM
  #9  
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299792458 metres per second, effectively achieving a τ factor of 0 on the return trip, giving elapsed time of the return trip an unmeasurable time span approaching 0.

With the unmeasurable but approaching 0 time interval on the return, the average would effectively be 40km/hr due to the doubled distance being double in the same effective time span. Nonzero approaching 0 in this case will be f(Xº) for lack of being able to find the correct notation
A= 20KM in 1 hr
B= 20KM in f(Xº)

A+B=40KM Traveled
A+B=1+f(Xº)=1
if 40km = 1 hr, then 40 km/hr average is achieved
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Old 09-24-07, 07:22 PM
  #10  
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I'm a man of my word, I'll send it.

Tom, you're insane. Man, I was really hoping for someone to chime in and say "duh, 60km/h".
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Old 09-24-07, 07:23 PM
  #11  
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Originally Posted by jschen View Post
Yes, so I can post a pic of the said cookie. No, I won't eat a stale cookie mailed to me by a stranger.
Please mail it to me after it gets mailed to you.
We can have a pass around type cookie
Each one of us has to take a bite
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Old 09-24-07, 07:32 PM
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OK, for the non-physics among us, please explain... in English.

Seems the obvious answer is 60KPH, but based on a couple of sample distances I tried, that doesn't work.
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Old 09-24-07, 07:39 PM
  #13  
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Originally Posted by skiahh View Post
OK, for the non-physics among us, please explain... in English.

Seems the obvious answer is 60KPH, but based on a couple of sample distances I tried, that doesn't work.
My answer works.

Lightspeed on the return trip, giving the travel time as zero, for any purposes of measurement for that short of a distance. This effectively doubles the average speed to 40km/hr. I just thought outside the box.....
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Old 09-24-07, 07:39 PM
  #14  
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Originally Posted by Boss Moniker View Post
Tom, you're insane. Man, I was really hoping for someone to chime in and say "duh, 60km/h".
Well, I would have if jschen hadn't answered so quickly. I just wanted to send someone a cookie.
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Old 09-24-07, 07:44 PM
  #15  
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Originally Posted by aprilm View Post
Well, I would have if jschen hadn't answered so quickly. I just wanted to send someone a cookie.
I missed out on a chance for a second free cookie? Hmm... I wonder if I could get aprilm to bring a cookie to San Diego. What if I put up an advertisement, something ilke "will take LA pose pic for cookie"?
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Old 09-24-07, 08:17 PM
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LOL, post a new thread with that title and see how many cookies you get.

Edit: beat ya to it!!
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Old 09-24-07, 08:21 PM
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Hey JSChen,
What gives? You never gave me an answer so easily when I had a physics problem.
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Old 09-24-07, 08:23 PM
  #18  
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You never sent me any cookies!
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Old 09-24-07, 08:24 PM
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Originally Posted by aprilm View Post
LOL, post a new thread with that title and see how many cookies you get.

Edit: beat ya to it!!
Wouldn't want to step on your toes. I don't think Foo can handle more than one person posting LA pose pics at a time. So I'll defer to you and let you go first.
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Old 09-24-07, 08:29 PM
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Originally Posted by jschen View Post
You never sent me any cookies!
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Old 09-24-07, 08:29 PM
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Originally Posted by jschen View Post
So I'll defer to you and let you go first.
What a gentleman!
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Old 09-24-07, 08:32 PM
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i just added two and two and am going to eat five cookies to celebrate.
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Originally Posted by colorider View Post
Phobias are for irrational fears. Fear of junk ripping badgers is perfectly rational. Those things are nasty.
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Old 09-24-07, 08:33 PM
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Originally Posted by jsharr View Post
i just added two and two and am going to eat five cookies to celebrate.
So how many times did you take a LA-pose pic for all those cookies?
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Old 09-24-07, 08:35 PM
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these cookies were not suitable for the LA pose, they did not have any nuts, not even one.
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Originally Posted by colorider View Post
Phobias are for irrational fears. Fear of junk ripping badgers is perfectly rational. Those things are nasty.
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Old 09-24-07, 08:35 PM
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Originally Posted by skiahh View Post
OK, for the non-physics among us, please explain... in English.

Seems the obvious answer is 60KPH, but based on a couple of sample distances I tried, that doesn't work.
The "obvious" answer of 60 km/h would be correct if you wanted to travel for a fixed amount of time at 20 km/h, and then travel for another (same) fixed amount of time at a different speed, to average 40 km/h. But the problem (and most real world speed calculations) asks for covering distances at a given speed, not for covering periods of time at a given speed.

We have been trained to think in terms of distance/<unit of measure>, but what we're most interested in is covering a given distance and in wondering how many of that unit of measure we need. We measure in miles per hour or miles per gallon, for example, but we don't need to consume a certain number of hours or a certain number of gallons. We need to cover a certain distance, and are more interested in how many hours or how many gallons it will take to cover that required distance. Because of this reciprocal relationship between how we measure rate of consumption and what we're actually interested in, it makes questions like this particularly tricky at first glance. (Note that in some places, measuring fuel consumption in liters / 100 km is the norm. This actually makes really good logical sense.)

So let's look at it with reciprocal units, to make the concepts behind the problem a bit easier to grasp since the units better match up with what we actually want to measure.

20 km/h = 3 min/km
40 km/h = 1.5 min/km

You want to cover 2x km at 1.5 min/km, which would take 3x minutes. You covered x km at 3 min/km, which took up 3x minutes already. Well, you've just used up all your time.
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