# Experiments at home

1. Jun 29, 2015

### micromass

Staff Emeritus
The idea of this thread is to make a kind of collection of experiments of important scientific facts that you can do at home.

Many students in high school are quite frustrated with science, because they see it as a collection of facts to memorize. The purpose of this thread is to show how many of those facts can be derived or experimentally tested at home. For example, the earth is round is a scientific fact. While showing pictures of astronauts indeed shows it, it seems more satisfactory to have an easy experiment that you can try yourself. These experiments should have the following characteristics:

1) You must in principle be able to verify it yourself, or with a limited amount of help from others.
2) You should not spend much money for it (for example, building a particle accelerator)
3) You should not spend too much time for it (for example, testing something every day for 50 years).
4) Appeal to authority is not allowed in this thread
5) Underlying scientific knowledge should be kept to the minimum possible.

Here is a list of possible scientific facts. Some of this will be very easy to show using the criteria. Some will not be possible (this is interesting information too!). Please feel free to add some facts, because that is what this thread is all about!

1) The earth is round
2) The earth orbits around the sun
2.5) The earth rotates
3) The moon orbits around the earth
4) Finding the radius of the earth
5) Finding the mass of the earth
6) Finding the radius of the moon
7) Finding the mass of the moon
8) Finding the radius of the sun
9) Finding the mass of the sun
10) Finding the distance between earth and sun
11) Finding the distance between earth and moon
12) Finding the speed of sound
13) Finding the speed of light
14) Proving the existence of atoms and molecules

Last edited: Jun 29, 2015
2. Jun 29, 2015

### Staff: Mentor

15) Measuring the speed of a passing car using only a cell phone with a sound spectrum analyzer app (using the Doppler shift)...

3. Jun 29, 2015

### Orodruin

Staff Emeritus
This is probably the easiest one. You need:
• A ca 1 m transparent plastic or glass pipe with a diameter of ca 5-10 cm.
• Sand or other material which is easily moved.
• Loudspeaker system connected to a frequency generator (there are good free frequency generators online).
• Ruler.
Do the following:
1. Set up the pipe horizontally with the sand inside (just on the bottom, enough to be visible as it moves).
2. Better plug one of the ends of the pipe not to spill too much sand.
3. Put the speaker on the other side.
4. Tune the frequency of the tone generator so that a standing wave is obtained in the pipe.
5. Measure the wavelength of the standing wave.
6. Relate to the speed of sound by $c = \lambda\nu$.
Positives: You also get a demonstration of the relation between different eigenfrequencies by changing the frequency of the generator to generate different standing waves.
Negatives: It is not a direct measurement of the speed in terms of travel time, but relates the speed to the frequency and wavelength.

Only allowed after you did 12

4. Jun 29, 2015

### Staff: Mentor

Good point. Otherwise they would be relying on authority. Okay, they should download a sound recorder app and find an empty gymnasium for that measurement...

5. Jun 29, 2015

### Staff: Mentor

How basic do you want? I have a few elementary and junior-high level examples in the list below
- Combustion consumes something in the air (easy to show with a burning candle standing in a bowl of water when you put a glass over it. I'm still bitter that my second-grade teacher accused me of parroting the encyclopedia when I tried explaining this).
- Atmospheric pressure (fun to do with water because you need a fairly respectable height).
- Heat of fusion and vaporization of water (a good high school exercise, especially when you consider how to add a known amount of heat to a container of water when your only tools are a thermometer, weight and volume measuring tools, and a heat source).
- Behavior of falling objects when air resistance is involved (helps to have access to multi-story stairwell).
- Verify $E=(mv^2)/2$ by measuring automobile stopping distances from various speeds (need access to a suitable venue, but a teacher who can pull that off will be worshipped forever).

6. Jun 29, 2015

### micromass

Staff Emeritus
But now you would have to show somehow that the spectrum analyzer app and the frequency generator work as advertized. You would know it by building one yourself, but that seems too intensive.

7. Jun 29, 2015

### Orodruin

Staff Emeritus
You would need a height difference of ca 10 m. Sounds like something you could do with a garden hose and a fairly high balcony.

8. Jun 29, 2015

### Staff: Mentor

yep - and that's enough to make it fun if you watch a bunch of kids working out the logistics :-)

9. Jun 29, 2015

### Orodruin

Staff Emeritus
You would also need to trust the ruler manufacturer ... I think you have to accept things like this to some degree ... And the clock maker for experiments measuring time as a part, you can hardly expect someone to build a clock based on the definition of the time units.

I found it: c = 1. Also known as the "check the calibration of this ruler"-experiment. Coincidentally, my answer to my professor after measuring the additional time delay between photons from electron positron annihilation after moving one of the detectors a certain distance was "we have now calibrated your ruler, it seems correct to $\pm 3$ mm.

10. Jun 29, 2015

### micromass

Staff Emeritus
I don't have to trust the ruler or clock manufacturer. I can just build my own ruler, or I can easily check that the provided ruler is good enough (up to a certain degree). For example, I can build my own ruler using a compass and geometry. I can build my own clock using candles and a sun dial, or I can at least check the provided clock using this. Yes, there are conceivable ways where those things might be in error (such as the candle not burning uniformly over time). But if I check the clock in many different ways, then there is only one possible conclusion.

Ah yes, it is true that we have predefined units for time and distance. But I am not saying that we should use those, since that might indeed involve an authority. Instead, we will be free to define our own time units and distance units (but not in a way that it makes the problem trivial, i.e. we should still be able to measure the length of a table easily using our distance units). This then provides us with the task of showing that our units are (approximately) consistent.

I guess the spirit of this thread is by assuming you're back in 100 BC and want to derive all these things yourself without using (much) outside help. But don't take this too strictly, we won't forbid the use of cars or cell phones.

11. Jun 29, 2015

### Orodruin

Staff Emeritus
So how do you propose to calibrate the ruler without relying on authority? It seems to me that using the older meter definition in terms of the distance from equator to pole would be relying on authority. As would the assumption that the compass is pointing north due to a geomagnetic field. And perhaps more important, this definition assumes the Earth is round. In addition we would have to take on authority that this meter definition is compatible with the current definition in terms of how far light travels in a second.

The alternative would, as you say, be to define an "alternative" system of units based on the length of a "meter" stick, call it m', and refer to lenghs using that definition.

This is a tough one. You would essentially have to repeat the Cavendish experiment, which is not exactly a tabletop experiment and involves very large balls of lead. The alternative would be to take the gravitational constant on authority.

12. Jun 29, 2015

### micromass

Staff Emeritus
Right, so we ditch the entire meter concept and just invent our own.

13. Jun 29, 2015

### anorlunda

To prove the Earth is round (1), simply do it the way the ancients did. Observe a sailboat approaching from seaward. First the top of the mast becomes visible, then lower and lower parts become visible as it gets closer.

Or you could do it a modern way using phones. Call friends all over the world and ask them where the sun is in the sky right now. Friends can send cell phone pictures of the sky right now. Discuss the answers with the students. What would the answers be if the Earth was flat? What shape Earth would produce the answers?

Good luck with the others. Sounds like fun. Learning might even be enhanced if you ask the students to think up the experiments themselves, then discuss why the proposals will or will not work.

14. Jun 30, 2015

### eifphysics

"125 Physics Projects For the Evil Genius" is a great book for this purpose. It has different levels of experiments, but very suitable for high school students.

15. Jun 30, 2015

### nasu

@micromass

Do you really have in mind experiments that can be done at home and proving that the Earth is round and it orbits around the sun?
And this without any prior knowledge (recourse to "authority") and in a relatively short time.

I think the thread is a good idea but the conditions are too drastic, without a real benefit.
Asking the students to learn and do everything from scratch is not realistic.
This is how knowledge advances, using the previous generations knowledge.

16. Jun 30, 2015

### anorlunda

I disagree. The goal could be to teach students the scientific method, and how to gather evidence from observations of the world around them, and attempting to make conclusions based on that evidence.

Looking up at the sky, or looking at pond water in a microscope, are equivalent for those purposes.

17. Jun 30, 2015

### nasu

Yes, but defining even your units of measure seems a little too drastic.

Do you mean them to build a microscope first and of course grinding their own lenses for that purpose, after making their own glass in an oven? (an oven is by itself something to be "discovered")
And of course, this only if they are lucky enough to observe at a bonfire that sand mixed with some other minerals may melt and produce a weird, glassy mass and so "discover" glass. And then the principles of the optics. And this in just a generation?

I am not against discovering yourself, I just feel from the way the discussion went that it goes too far with "start from scratch", so far that it looses its purpose .
It's OK to "rediscover" for yourself but you have to start on a realistic base, one that gives you as much insight as possible in a life time.

Besides, for some "discoveries" experiment by itself is not enough. You need the models and theories developed sometimes by several generations of trial and errors.
Classifying the use of all previous knowledge as "recourse to authority" seems to me going too far.
Even in 200 BC there was a base of knowledge on which people based their further investigation of nature.

18. Jun 30, 2015

### anorlunda

19. Jun 30, 2015

### micromass

Staff Emeritus
Don't fix yourself at the "at home" part. Going to the sea and watching a ship move below the horizon gradually would show the curvedness of earth. That is already good enough. It just shouldn't take too much knowledge, money or time.

I agree, you cannot do science by doing everything from scratch. You need to rely on authority now and then. I have never claimed that science teaching should be done in the way outlined in my OP. But you can take aspects from it. You can show the students that it is possible to do it this way without actually doing it completely. This would teach students to think critically, to follow the scientific method, to come up with experiments and to experiment themselves. Besides, it is also fun. So I might integrate parts of it in a curriculum for high school students, but of course not completely.

Besides, outside of teaching, I find the questions asked in my OP to be philosophically pleasing. To see where you can get without accepting any authority. And to see where you cannot get without accepting it. I have nothing against authority. I have done scientific research myself and I have accepted enough results on authority. But this thread takes the philosophy to not accept authority and see where you get. I find it instructive and philosphically pleasing. If you disagree with this, you have the full right to do so and I would certainly understand where you're coming from.

20. Jun 30, 2015

### micromass

Staff Emeritus
A unit of measure is arbitrary. It is good to point that out to students by telling them that it is possible to define your own. You can then let them come up with proposals and discuss problems in their proposals. And then you can go on to say how the standard units now are defined and how it deals with the problems. I am not saying the lecture should be like this, but it is a possibility to do it like this and I think it would be great to have discussions such as this occasionally (but not always!).

No, that was not my point at all. I am happy to accept the existence of a microscope since it is just a mechanical instrument and it is clear how it works. I am less happy with accepting the existence of a spectrum analyzer. I can't really explain the difference.

But using previous knowledge is recourse to authority. If you learn GR, then you accept on authority that experiments have been done that show that it works. That said, that doesn't mean that recourse to authority is a bad thing. It is a necessary thing to do science. A science student should learn when to accept authority and when not to, this is a valuable skill. But the spirit of this thread is not to accept authority and see how far you get.

Likewise, I don't mean to say that everybody has to go on and discover everything themselves. This would be a very inefficient way of learning. But doing it occasionally is very helpful.

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