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/sci/ - Science & Math

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3139738 No.3139738 [Reply] [Original]

A girl, a boy, and a dog are standing at the same spot on a sidewalk. The girl and boy start walking in the same direction with the girl moving at 3 mph and the boy moving at 4 mph. As they walk the dog runs back and forth between them moving at 10 mph. Assuming that the dog turns around instantaneously each time, which direction is the dog facing after two hours?

>> No.3139748

Yes.

>> No.3139752

Peanut butter.

>> No.3139766

west

>> No.3139774

give me a sec OP

>> No.3139775

a boy, a girl, and a dog cannot stand on the same spot. the question is flawed

>> No.3139793

>>3139738

To be serious:

It depends entirely on which person the dog is facing first.

To be or not to be:

Are you implying that females are inferior to males at walking? You damn sexist pig.

>> No.3139800

MFW in the first second the dog goes back and forth between them over a hundred times

>> No.3139802

At t slightly past 0, the dog would change directions at a near infinite rate so that's a problem.
Also, I'm pretty sure Heisenburg has something to say about.this.

>> No.3139817
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3139817

>mfw I was setting up integrals and actually attempting this problem, in vain, at 1:22 AM.

>> No.3139826

42

>> No.3139875

While walking towards the boy, the dog travels at an effective 6 miles per hour. During this time, the distance increases by the time it takes for the dog to travel multiplied by the speed difference between the boy and the girl. (D/6 * 1).
While walking towards the girl, the dog travels at an effective 13 miles per hour. (D/13 * 1)

Because of this, there has to be some initial separation between them. Otherwise, this is a paradox because both answers are true.

>> No.3139895

>>3139875
wat

>> No.3139902

Forward.

>> No.3139911

Impossible to determine, as the dog constantly fluctuates its orientation at the beginning when the speeds are very close to each other.

>> No.3139918

>>3139817
It's okay, we all know that feeling.

>> No.3139925

>utterly irrelevant bullshit question

Oh god, who cares?

>> No.3139943

OP, where did you find this problem? It's inherently unsolvable.

Working backwards, you know the boy and girl will be 2 miles apart at the end of 2 hours (4-3)*2.

If you attempt to work backwards, then no matter where you assume the dog is at or facing at the end, you can't resolve the beginning The facing of the dog cannot be determined when they are together because the distance is a function of 1/t and the direction reverses every time this distance is crossed. In the limit, it's undefined. There has to be some starting distance or other initial conditions for it to work.

>> No.3139946

>>3139925
It's math, if you don't find it interesting, then don't post in this thread. Math for the sake of math is legitimate. If you want to actually apply something, pick another topic.

>> No.3139958

I'm getting the feeling that the motion of the dog is supposed to be represented by that one trig function that oscillates infinitely as it approaches 0 and then slows down as it gets further away from 0.

>> No.3139972

>>3139958
thats what I'm thinking

sin(20pi/t) or something

Its too late to be breaking my head over this shit.

>> No.3139978

>>3139958
The trig function that oscillates like that is sin(1/x)

thank you google

>> No.3140834

forward

>> No.3140845

What is this "miles per hour" bullshit you decided to pollute this board with?

>> No.3143140

bamp

>> No.3143201

I propose to change the problem a little bit to make it possible

1) The boy goes at 4, the girl at 3 and the dog at 10 m/s. The boy starts at position 2m, girl at 0 and dog at 1 movibg to the right. After 2 hours where's the door going, right or left?

2) Do the same, but with:
Boy (A) goes at Va and starts at Xa
Girl (B) goes at Vb starying at Xb
Dog(C) goes at Vc starting at Xc to the right

After a time of t:
A) Is the dog going to the right or the left?
B) Where's the dog Xc?

>> No.3143211

>>3143201

And there's no "nice" math solution. You have to do a lot of tricks and the second question may be impossible. (For the first you could use a computer...)