7

to emphasize physical in tuition a nd to make o ur treat-

men t as simple as it could b e made (but not sim-

pler). Surely a question ab out what L. A. Delsasso

called, according to F eynman’s recollection, “a freshman

exp e riment ”

4

deserves a n answer presented in a langua g e

at the cor resp onding level of complica tion. More imp or-

tant is the principle, famously put forward by F eynman

himself when discussing the spin statistics theorem, that

if we can’t “reduce it to the freshman le vel,” w e don’t

really unders tand it.

39

W e also hav e commented on the p erplexing history o f

the reverse sprinkler problem, a history which is interest-

ing not only beca use ph ysicists o f the stature of Mac h,

Wheeler, and F eynman enter into it, but als o b ecause

it o ﬀers a startling illustra tio n of the fallibility of great

scientist s faced with a question ab out “a freshman ex-

per iment .”

Surely , as the Duc hess said to Alice during one of her

adven tures in W onderla nd, “everything’s g o t a mor a l, if

you ca n only ﬁnd it.”

40

Ackno wledgments

The historical section of this pap er ow es a great deal to

the list of references on the r everse sprinkler that is given

at the W eb site for the Universit y of Maryla nd’s Physics

Lecture Demonstration F acility .

26

Thanks are due to sev-

eral rea ders who commented o n this pap er a fter it ﬁrst

app eared in manuscript form, particular ly to J. M. Dlu-

gosz, who too k it up on himself to clarify the rela tionship

betw een this discussion a nd the account of the exp eri-

men tal res ults at the Univ e rsity of Maryland. The result

of his inquiries was a useful exchange with R. E. Berg.

∗

Electronic add ress: jenkins@theory .caltech.edu

1

R. P . F eynman, Sur ely Y ou’r e Joking, Mr. F eynman , (Nor-

ton, N ew Y ork, NY , 1985), pp. 63–65.

2

R. P . F eynman, Ibid. , p . 63.

3

It has not b een p ossible to identify the b o ok to which

F eynman was referring. As we shall discuss, the matter

is treated in Ernst Mac h ’s Me chanik , ﬁrst published in

1883.

10

Y et this b o ok is not a “hydro dynamics b o ok” and

the reverse sprinkler is presented as an example, n ot a

problem. In Ref. 21, J. A. Wheeler suggests that the prob-

lem o ccurred to them while discussing a diﬀeren t question

in the undergraduate mec hanics course t h at Wheeler was

teac h ing and for which F eyn man was the grader.

4

R. P . F eynman, Ibid. , p . 65.

5

In the literature it is more u sual to see this problem iden ti-

ﬁed as the “F eynman inv erse sprinkler.” Because the prob-

lem did not originate with F eynman and F eynman never

published an answer to the problem, w e hav e p referred

not to attac h his name to the sprinkler. F urthermore, even

though it is a pedantic p oint, a query of the Oxfor d English

Dictionary sugges ts that “re verse” (opp osite or co ntrary in

c haracter, order, or succession) is a more appropriate de-

scription than “inv erse” (turn ed up-side dow n) for a sprin-

kler that sucks water.

6

This observ ation might seem trivial, but its consequences

can b e counterin tu itiv e. The Zapruder ﬁlm of the 1963

assassi nation of U.S. president J. F. Kenn edy , shows

Kennedy’s head snapp ing bac kwa rd after the fatal shot,

even though th e oﬃcial theory of the assassinatio n asserts

that the shot was ﬁred from behind Kenned y b y g unman L.

H. Oswal d . F or several decades, conspiracy theorists hav e

seized on this elemen t of the Zapruder ﬁ lm as evidence that

the fatal shot could not hav e been ﬁred by Oswa ld and

must hav e come instead from in front of the president’s

motorcade. In 1976, L. W. Alv arez published an analysis

of the Zapruder ﬁlm in whic h he explained that th e jet

of brain tissue that emerged from president’s exit wound

migh t easi ly ha ve t hro wn his head in the direction opposite

to that of the incoming b ullet. Alv arez d emonstrated this

to h is satisf action b oth theoreticall y and ex p erimen tally ,

the latter by ﬁring at a melon and photographing it as it

mo ved in the direction opp osite to what one would naive ly

hav e ex p ected.

7

7

L. W. Alv arez, “A physicist examines the Kennedy assas-

sination ﬁlm,” Am. J. Phys. 44 , 813–827 (1976).

8

Tw o intere sting p roblems for an introductory un iversit y -

leve l physics course suggest themselves. O n e is to show

that the center of mass of the bullets-and-ship system will

not mov e in the horizontal d irection regardless of th e ﬁr-

ing ra te, as one exp ects from momentum co nserv ation. An-

other would b e to analyze this problem in the light of Ein-

stein’s relativit y of sim ultaneit y .

9

A. K. Sch ultz, “Commen t on the in verse sprink ler p rob-

lem,” A m. J. Phys. 55 , 488 (1987).

10

E. Mac h, Die Me chanik in I hr er Entwicklung Historisch-

Kritisch Dar gerstel lt (1883). First published in English in

1893 as The Sci enc e of Me chanics: A Critic al and Histori-

c al A c c ount of i ts Development (Op en Court, La Salle, IL ,

1960), 6th ed., pp. 388–390.

11

E. Mach, I bid. , p. 390.

12

In Ref. 23, P . Hewitt prop oses a physical setup identical

to the one shown in Fig. 8(b), and observes that the de-

vice turns in opposite directions dep end ing on whether the

ﬂuid p ours out of or into it. Hewitt’s discussion seems to

ignore the imp ortant diﬀerence b etw een such a setup and

the reverse sprinkler.

13

E. Mach, O p. cit. , p . v .

14

P . Kirkpatrick, “A neglected lesson from the Cartesian

diver,” Am. J. Phys. 10 , 160 (1942).

15

H. S. Belson, “‘Empt y’ hero’s en gine,” Am. J. Phys. 24 ,

413–41 4 (1956).

16

Proceedings of the National Science F oundation Confer-

ence on Instruction in Fluid Mec h anics, 5–9 September

1960, Exp. 2.2, p. I I–20.

17

A. T. F orrester, “ Inv erse sprink lers: A le sson in the u se of a

conserv ation p rinciple,” Am. J. Phys. 54 , 798–799 (1986).

18

A. T. F orrester, “Comments on a letter by A. K. Sc hultz,”

Am. J. Phys. 55 , 488–489 ( 1987).

19

L. Hsu, “Tw o simple exp erimen ts and the resolution of