Newton’s laws are a ubiquitous topic in introductory physics instruction. One common problem involves asking what will happen if you stick your finger into a cup of water sitting on a scale (Fig. 1). A way to solve the problem would be to first recognize that the water exerts a buoyant force upward on the finger, which students can recognize as being the reason why they feel lighter when they are in a swimming pool. By Newton’s third law, the finger must hence exert a downward-pointing force on the water (labeled FB in Fig. 2). Since the water is not accelerating, the normal force from the beaker acting upward on the water (FN2) must increase to equal the sum of the weight of the water (FW) and FB (Newton’s second law). By Newton’s third law, the water must exert a force equal to FN2 down on the beaker. However, since the beaker is not accelerating, Newton’s second law tells us that the normal force from the scale is equal to this increased FN2. Via these chains of Newton’s second and third laws, we can see that the reading on the scale will increase.

1.
e.g.,
B. W.
Dreyfus
 et al, “
Chemical energy in an introductory physics course for the life sciences
,”
Am. J. Phys.
82
,
403
411
(
2014
).
2.
e.g.,
E. F.
Redish
 et al, “
NEXUS/Physics: An interdisciplinary repurposing of physics for biologists
,”
Am. J. Phys.
82
,
368
377
(
2014
).
3.
H. C.
Hamaker
, “
The London—van der Waals attraction between spherical particles
,”
Physica
4
,
1058
1072
(
1937
).
4.
Forces in this article are recorded in grams, as this is the unit used by the scale. It should be made clear to students early on that scales measure force and not mass, regardless of the unit reported. If available, the scale should be set to measure in millinewtons instead of grams, to reinforce to students that force, and not mass, is being measured.
5.
P. L.
Du Noüy
, “
A new apparatus for measuring surface tension
,”
J. Gen. Physiol.
1
,
521
524
(
1919
).
6.
Readers can access the Appendix at TPT Online at https://doi.org/10.1119/5.0064927, under the Supplemental tab.
7.
e.g.,
D.
Hammer
, “
Two approaches to learning physics
,”
Phys. Teach.
27
,
664
670
(
1989
).
8.
e.g.,
K. A.
Strike
and
G. J.
Posner
, “
Conceptual change and science teaching
,”
Eur. J. Sci. Educ.
4
,
231
240
(
1982
).
9.
First of all, polar water molecules can induce polarity in otherwise nonpolar molecules (the Debye interaction). The hydrogen atoms of the water molecules make the ensuing “hydrogen bonds” particularly strong. In fact, even in Experiment 3, which involves nonpolar fluids, London dispersion forces exist between instantaneously induced dipoles. Furthermore, not just energy but also entropy should be considered. When water adheres to the flask, it spreads out in volume, similar to how a gas will expand to fill its container, increasing in entropy.
10.
e.g.,
A.
Elby
, “
Helping physics students learn how to learn
,”
Am. J. Phys.
69
,
S54
S64
(
2001
).
11.
e.g.,
L. R.
Snyder
, “
Classification of the solvent properties of common liquids
,”
J. Chromatogr.
92
,
223
230
(
1974
).
12.
D.
Sokoloff
and
R.
Thornton
, “
Using interactive lecture demonstrations to create an active learning environment
,”
Res. Sci. Technol. Educ.
35
,
340
347
(
1997
).
13.
e.g.,
D.
Kuhn
,
K.
Iordanou
,
M.
Pease
, and
C.
Wirkala
, “
Beyond control of variables: What needs to develop to achieve skilled scientific thinking?
Cognit. Dev.
23
,
435
451
(
2008
).
14.
e.g.,
H.
Hammer
, “
Misconceptions or p-prims: How may alternative perspectives of cognitive structure influence instructional perceptions and intentions?
J. Learn. Sci.
5
,
97
127
(
1996
).

Supplementary Material

AAPT members receive access to The Physics Teacher and the American Journal of Physics as a member benefit. To learn more about this member benefit and becoming an AAPT member, visit the Joining AAPT page.