t hought e xperiments science education unesco project 2010

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THOUGHT EXPERIMENTS

Science educationUNESCO PROJECT 2010

ThoughtExperiments @

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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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STE.caSO LONG AS THERE IS A GRAVITATIONAL FORCE ACTING ON THE CANNON BALL, IT WILL FOLLOW DIFFERENT PATHS DEPENDING ON ITS INITIAL VELOCITY

If 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.

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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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STE.ca 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.

20 km/h

20 m

20 m

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THE EXPERIMENT: IF THE TWO BALLS LEAVE THE PITCHER’S HAND AT THE SAME TIME AND WITH THE SAME FORCE,

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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Questions to assist students1.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?

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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 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.

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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 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.

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READINGS http://edu.technion.ac.il/projects/haptech/

publications/Publications_files/Thought%20experiments.pdf

Thought experiments in Einstein’s Work. http://philsci-archive.pitt.edu/archive/00003190/01/8_norton.pdf