/sci/ - Science & Math

[Anonymous No.8329224]

/g/ can't solve this. Can /sci/?

An old car has to travel a 2-mile route, uphill and down. Because it is so old, the car can climb the first mile - the ascent - no faster than an average speed of 15mi/h. How fast does the car have to travel the second mile - on the descent it can go faster, of course - in order to achieve an average speed of 30 mi/h for the trip.

[Anonymous No.8329227]

it has to teleport back down the hill because in travelling up the hill at 15 miles an hour it has already used up all the time it has available to make the total journey up and down the hill at an average speed of 30 miles an hour.

a 2 d journey done with an average speed of 30 miles per hour takes 2 d / 30 hours which is equal to d / 15 hours, which is how long it takes to go up the hill of length D travelling on average at 15 miles per hour

try v next time, brainlet

[Anonymous No.8329230]

>>8329227
Average speed = (ascent speed + descent speed) / 2
30 = (15 + X) / 2
X = 45

Sit down kid

[Anonymous No.8329232]

>>8329224
-1/12 mph

[Anonymous No.8329639]

>>8329230
>>8329227
I'm confused

[Anonymous No.8329695]

>>8329230
brainlet detected

15mi/h for 1 mile is 4 minutes
45mi/h for 1 mile is 1,33... minutes

total
45mi/h for 25% of the time
15mi/h for 75% of the time

0.75*15+0.25*45=22.5mi/h avg is what your answer gets

[Anonymous No.8329700]

>>8329224
how much does the car weight?

[Anonymous No.8329706]

the answer is infinity miles per hour

[Anonymous No.8329925]

885 mph average downhill leads to an average of 29.5 mph

Since speedometers don't show decimal numbers this will be rounded up to 30 mph

[Anonymous No.8329988]

>>8329232
Lol!

[Anonymous No.8330246]

60mph easy

[Anonymous No.8330325]

>>8329224
what is the speed limit

[Anonymous No.8330388]

>>8330325
Oh look, it's the guy that would get hired at Google

[Anonymous No.8330601]

[Anonymous No.8330609]

>>8329224
(15mph*1)/2
Then you just plug in 9.8m/s^2 to account for gravity and multiply by pi
The answer is 78mph
Take that /g/

[Anonymous No.8330746]

>>8329230
Speed is a weighted average, dumbass, if you spend 90% of your time going 2 mph, and then 10% of your time going 50 mph, then your average speed isn't (50 + 2)/2 = 26 mph. Your average is going to be 0.9*2 + 0.1*50 = 2.3 mph. In terms of the problem, because the car can go no faster than 15 mph on the 1 mile ascent, we know that the car will take 0.0666... hours or more to reach the turn point. After the turn point, the car drives down the 1 mile stretch at some speed x mph such that the average speed of the trip is 30 mph. To compute this, we calculate the time associated with the descent, which is 1/x hours. Now, we need to convert this into a fraction of the total time, where the other stretch of time is 1/y (where y≤15 mph), which gives us (1/x)/((1/x)+(1/y)) for 1/x and (1/y)/((1/x)+(1/y)) for 1/y So the equation we have to solve is [(1/x)/((1/x)+(1/y))]*x mph + [(1/y)/((1/x)+(1/y))]*y mph = 30 mph, y≤15 mph. Simplifying, we get 2/((1/x)+(1/y)) = 30 mph, 1/15 = (1/x)+(1/y), but y≤15, so (1/y)≥1/15, and (1/y)+(1/x)≥1/15 + (1/x)>1/15, hence (1/y)+(1/x)>1/15, a contradiction. Therefore, there is no value x that satisfies this.

[Anonymous No.8331603]

>>8330746
but you dint take the curvature of the earth into account?? or if the driver really needed a poo???

[Anonymous No.8331628]

>>8329224
this is easy as fug

[Anonymous No.8331682]

If the car went 15 miles in one hour it already used up all its time. It would have to instantaneously be at 30 miles to have gone 30 miles in one hour. Parameterizing this with distance doesn't change the fact that you are already out of time.
This is just like saying If John runs twice as fast as Jane, after one lap of a two lap race, how much faster does Jane have to run than John to win? John has already won.

[Anonymous No.8332100]

>>8331682
What makes you think going one mile at 15mph takes an hour?

>>>/out/

[Anonymous No.8332111]

Didn't account for coriolis effect or the force exerted on the car by the skyscraper sized electromagnet at the top of the hill.

[Anonymous No.8332119]

1 mile at 15mph uses up 4 minutes, meaning the car would have to clear the next mile at a speed equivalent to 29 miles in 28 minutes, ergo, just over 60mph.

[Anonymous No.8333069]

explain this faggots.

[Anonymous No.8333084]

negro please i am failing physics

[Anonymous No.8333097]

>>8329224
45 mph, I'm sure /g/ had no problem answering this.

[Anonymous No.8333116]

>>8329224
(15 + x) / 2 = 30
x = 45 mph

[Anonymous No.8333131]

>>8329639
We are looking find the an average speed of thirty miles per hour. Average is found by dividing the sum of all component speeds traveled by the number of various speeds traveled. So 30= (15 + x ) / 2, because the desired average speed of 30mph is a result of the ascent average speed of 15 plus and unknown average that sum divided by two (the number of component average speeds).

30 = (15 + X) / 2
30 = 15/2 + X/2
45/2=X/2
X= 45/2 * 2/1
X= 90/2
X=45

[Anonymous No.8333139]

>>8329925
Where did you get that?

[Anonymous No.8333142]

>>8333131
>>8333116
>>8333097

brainlets detected.

See >>8329695

/sci/ is dumber than I thought.

[Anonymous No.8333146]

>>8333142
How is the correct answer found?
If the first half is traveled at 15 mph then wouldn't the total journeys average speed be determined by the first half plus the second half divided by two?

[Anonymous No.8333182]

>>8333142
fuck off, the problem is bait and doesn't have an answer, go back to >>>/x/

[Anonymous No.8333193]

>>8329224
kek 9/10 b8

To travel a distance of 2 miles at an average speed of 30 mph would take (2/30)*60= 4 minutes

If you travel for 1 mile at 15 mph you spend (1/15)*60 = 4 minutes

So, it is pretty obvious my kohai, the speed of light itself.