Physics question? Man we're going to start a flame war today.

[Crazy Guggenheim]

This not a trick question.

A person walks at the rate of three miles an hour for one mile. then walks at the rate of four miles an hr. for one mile. What was the average speed?

A lot of people get this wrong. I've have asked Physics PhD.s who have faultered on it.

[RandomKoolzip]

1. Flapjacks.

[Jara sang]

2. Splunge?

[Lars39]

3. 3.5mph

[kick-ass-bob]

8. actually it's less than 3.5 mph.

20 minutes going 3 mph, 15 min going 4.

[Lars39]

15. I just swagged it !

:)

[BigMcLargehuge]

4. 3.5 flapjacks and spunge per hour

[terrya]

5. Wrong answer, but...

1 mile an hour.

Ok, I don't have the slightest fucking idea

[tk2kewl]

6. 3.42 mph

[Balbus]

9. Whipping out my calculator...

I'll have to agree with this person

[tk2kewl]

11. here is how...

1 mi at 3mph took 20 mins

1 mi at 4mph took 15 mins

2 mi in 35 mins = 3.42 mph

[KitchenWitch]

7. 17.5 minutes per mile.

or 3.43 mph (approximately)

[donco6]

10. I'll give it a try.

It depends on what you mean by average speed.

I would say he walked 1 mile in 15 minutes and another mile in 20 minutes. So 2 miles in 35 minutes which reduces to 3.428 miles/hr.

[crispini]

12. Let's see,

He walks three mph for one mile, which would be 20 minutes. Four mph for one mile would be 15 minutes. So he walked 2 miles in 35 minutes for an average speed of (um, gets out calculator) 3.4 mph? Is that right?

[Salviati]

13. Here we go:

when we talk about average speed, we're talking about the average over time, so we need to find out how long they walked at each speed, and then weight each part accordingly. Alternatively, we can find the total distance walked, and divide by the total time walked, which will do the same thing.

3mph for 1 mile = 20 mins
4mph for 1 mile - 15 mins

total time = 35 mins = 7/12 hours

total distance 2 miles

average speed = 2 miles /(7/12 hours) = 24/7 miles/hour = 3 3/7 mph

Which is less than the most likely wrong answer of 3 1/2 mph because the person spends more time walking at 3 mph than at 4 mph.

There was once a question like this on Click and Clack, so you don't need to be a physicist to figure it out, but it does help :)

[mark414]

14. 3.43 mph (after rounding up)

[davidinalameda]

16. better question

why isn't he driving?

[Crazy Guggenheim]

17. Yay! It is 3 3/7 MPH!

Why? Because velocities do not average with respect to distance but time.

Oh shit I didn't realize this. 1/7 = .142857142857~

So does 7 X 142857~ 1? No!

[Balbus]

24. Well...

.142857142857~ * 7 = .999999999~
& 1/7 * 7 = 7/7

therefore .9999999999~ = 7/7
& 7/7 = 1
ergo .999999999999~ = 1

:P (couldn't resist)

[Nicholas D Wolfwood]

18. I could be wrong, however...

don't most physics people get their panties in a bunch over the use of "speed" instead of "velocity"?

[Crazy Guggenheim]

20. I had intentionally used the word speed instead of velocity. Since

I did not want to confuse anyone. Now I could have said "a person walks at a velocity of 3 MPH for one mile and an additional mile in the same direction at 4 MPH. What is the average velocity.

Or a person walks at a velocity of 3 MPH for one mile and an additional mile in the reverse direction at 4 MPH what is the average velocity? That equals zero.

[TXlib]

23. now that second question would trap many people

who don't understand the difference between speed and velocity.

[TXlib]

22. not as long as it doesn't matter

In this problem, the person walks uniformly in one direction, so we only really care about the scalar speed.

Only if directions are changing do we need to worry about the vector velocity.

[TXlib]

19. my solution

t1 = d1 / v1 = 1/3 hr

t2 = d2 / v2 = 1/4 hr

v(ave) = (d1 + d2) / (t1 + t2) = (1 + 1) / (1/3 + 1/4)

v(ave) = 2 / (7/12) = 24/7 = 3 3/7 mi/hr

Barring any stupid arithmetic error, what's so hard about that?

[Crazy Guggenheim]

21. LOL.

[One_Life_To_Give]

25. Approx. 3.42857 n/t

[ProfessorGAC]

28. Approximately? LOL!

Good one!
The Professor

[Ganja Ninja]

26. Here goes

35 minutes = .5834 or 58.34% of one hour
2 miles divided by .5834 = 3.4281796 mph

[mike_c]

27. that would be 2 miles in 35 minutes...

...3.429 miles per hour.

1 mile @ 3 mph = 1 mile in 20 min.

1 mile @ 4 mph = 1 mile in 15 min.

Disclaimer: I'm a biologist..., and before editing this post proved that I cannot ADD!

[C_eh_N_eh_D_eh]

29. 35 minutes, not 25.

A third of an hour, then a quarter of an hour. The rest of your math I can't check at the moment.

[mike_c]

31. yep-- I realized I couldn't add just as I hit "Post..."

Fixed now.

[Rabrrrrrr]

32. I'd say your a biologist

20 + 15 = 35, not 25

:-)

[mike_c]

33. 25, 35-- it's all within the range of expected variation....

LOL, and with a very red face....

[Rabrrrrrr]

38. This engineer would have allowed you a 10% error

but that's a 40% error!

[leftofthedial]

30. 3.448 mph

approximately.

[Rabrrrrrr]

34. Ooh, that was easy - now, do one with velocity, in polar coordinates!

[Crazy Guggenheim]

37. I'll see if I can do that.

[billyskank]

35. I make it 24/7 mph

or 3.429 in decimal to 4 sig figs. Is that right?

[KitchenWitch]

36. How many significant digits do you want?

Gotta love sig figs.

[Coastie for Truth]

39. 3.4483 miles per hour . NT