Transcript Thought Experiments

THOUGHT EXPERIMENTS
Science education
UNESCO PROJECT 2010
THOUGHT EXPERIMENT
The understanding comes through reflection on the
situation.
 Methodology is based on logic rather than
empirical evidence.
 Well-structured hypothetical questions employ
"What if?" reasoning.

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CONSEQUENCES OF
THOUGHT EXPERIMENTATION
 challenge
(or, even,
refute) a prevailing
theory
 confirm a prevailing
theory,
 establish a new
theory,
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NEWTON’S CANNON
In this experiment
Newton visualizes a
cannon on top of a very
high mountain.
 If there was no force of
gravity (as was thought
at the time) the
cannonball should follow
a straight line away from
Earth.

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SO LONG AS THERE IS A GRAVITATIONAL
FORCE ACTING ON THE CANNON BALL, IT WILL
FOLLOW DIFFERENT PATHS DEPENDING ON
ITS INITIAL VELOCITY
 If
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the speed is low, it will simply fall back on
Earth
 If the speed is very high, it will indeed leave
Earth.
 If the speed equals some threshold orbital
velocity it will go on circling around the
Earth along a fixed circular orbit just like the
moon.
 If the speed is higher than the orbital
velocity, but not high enough to leave Earth
altogether it will continue in an elliptical
orbit.
GALILEO'S LEANING TOWER
OF PISA EXPERIMENT
Galileo dropped two
objects of different mass
from the top of the
Leaning Tower of Pisa.
 He did so as an
experiment to disprove
Aristotle's theory of
gravity.
 Objects fall at a speed
relative to their mass.

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GALILEO'S ARGUEMENT
Do heavy objects fall faster
than lighter ones?
 A feather does fall faster than
a marble.
 Will two balls fall at the same
speed?

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NOW IMAGINE THE TWO
OBJECTS JOINED BY A LIGHT
THREAD
The heavier one will start to
pull on the lighter, accelerating
it, and the lighter one will pull
on the heavier, retarding it.
 So you end up with a picture of
the two objects, lightly joined,
falling at a speed in-between
their individual speeds. Is that a
contradiction?

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CONTINUING THE ARGUMENT
Imagine the light thread becoming thicker and
thicker until it becomes a metal rod,
 or the thread becoming shorter until the two objects
are joined
 at what point do you argue that the two objects are
now one?

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JOINED FALLING OBJECTS
You end up with one
object, falling at an
intermediate speed.
 This then becomes a
contradiction, since the
joined single object, with
a mass the sum of the
two initial objects, should
fall, according to
Aristotle, faster than
either of the single
objects.

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20 m
20 m
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20 km/h
Imagine
a pitcher
on a float,
moving at
20 km/h
throwing a
ball to a
catcher on
a float and
on the
ground at
the same
time.
THE EXPERIMENT:
1. Do both balls arrive at the same time?
2. Do both balls travel at the same speed?
3. Setup a series of logical questions that help students find the
answer.
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IF THE TWO BALLS LEAVE THE
PITCHER’S HAND AT THE SAME TIME
AND WITH THE SAME FORCE,
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Questions to assist students
1.Do both balls travel the same distance.
2.What happens to the catcher on the float while the ball is in
motion?
3.If two baseballs are thrown with the same force and one
travels 100m and the other travels 50m. Do both balls have
the same velocity?
4.Which baseball arrives first?
5.Which baseball has the greatest velocity?
6.How would the results differ if the both balls where thrown
with less (but equal) force?
7.How would the results differ is both catchers were behind
the picture?
THE RELATIONSHIP BETWEEN
VELOCITY AND TIME
A person stands directly below
a spaceship with a stopwatch.
 A second person on the
spaceship has a stopwatch.
 As the spaceship flies directly
overhead, both people start
their stopwatches at the same
time.
 After on orbit both watches
are stopped.
 Do both watches record the
same time?

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VAN HELMONT’S EXPERIMENT
He performed an experiment to determine where
plants get their mass.
 He grew a willow tree and meticulously measured
the amount of soil, the weight of the tree and the
water he added.
 After five years the plant had gained about 170
pounds.

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ANALYSIS
 Since
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the amount of soil was basically the
same as it had been when he started his
experiment, he deduced that the tree’s weight
gain had come from water.
 Since it had received nothing but water and
the soil weighed practically the same as at the
beginning.
CONCLUSION
Van Helmont argued that the increased weight of
wood, bark and roots had been formed from water
alone.
 Conduct a thought experiment to challenge his
conclusion.

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VAN HELMONT AND DIGESTION
 Van
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Helmont challenged earlier ideas that
food was digested by the body's internal
heat.
 If such was the case, van Helmont argued,
how could cold-blooded animals live?
 His own opinion was that digestion was
aided by a chemical reagent, within the
body, such as inside the stomach.
 Van Helmont's idea was "very near to our
modern concept of an enzyme.
READINGS
http://edu.technion.ac.il/projects/haptech/publicat
ions/Publications_files/Thought%20experiments.
pdf
 Thought experiments in Einstein’s Work.
http://philsciarchive.pitt.edu/archive/00003190/01/8_norton.pd
f

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