Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: labview1958 on 03/09/2008 15:31:28
-
i have done an experiment to prove hookes law with a spring and some weights. However when I plot extension vs. force the graph does not pass through the origin. Is hookes law WRONG?
-
Hooke's law only applies if you do not exceed the elastic limit, (i.e. do not overstretch & permanently deform the spring).
http://home.clara.net/darvill/enforcemot/springs.htm
-
For what you have said to be true the spring's extension must have been not equal to zero with no weight on it. Did you measure the change in length, or just the length?
-
This is common with the springs used in School labs. Remember what the spring looked like when it was given out you. It was probably tightly coiled up. It would have needed a fraction of a Newton to get the coils to move apart for a start - hence the kink in the curve (straight line) at the origin.
-
i have looked up in the internet and it appears that hookes law does not apply for forces of less than 1
-
Hookes law does not apply for forces of less than 1 Newton?
-
.
[ Invalid Attachment ]
http://home.clara.net/darvill/enforcemot/springs.htm
-
Yes, RD, that is the standard graph you get when you permanently stretch a spring. But when it comes out of the box with the coils are squashed together you get a (force / extension) graph which doesn't go through the origin however little you stress it. This is because a fraction of a Newton is needed before the spring starts to stretch at all.
[diagram=364_0]
i have looked up in the internet and it appears that hookes law does not apply for forces of less than 1
That is a naive way of stating what I said and giving the wrong explanation!
If you permanently stretched the spring a bit - so the coils start off 'open' - you would get a very good straight line. With care, the 1N steps will sit within a pencil's width of a straight line. You'd see Hooke's Law all the way.
-
My springs were used by the students for the last two years. They are not out of the box. I believe that it is in the nature of springs.
-
Spring fever, you mean?
From your remark, I take it you teach Science. The phrase "in the nature of springs" , reads like some medieval statement - like "nature abhors a vacuum". Should you be promoting this sort of view to kids you want to grow up knowing about Science?
There will be a very simple reason why your particular springs appear to behave the way they do.
1. they could be as I suggested (have you looked at one recently and is the graph like my example?).
2. they may not be uniform. Hooke's law doesn't in fact refer to springs - it refers to the material. It sometimes requires some effort to identify, accurately, the actual Stress and Strain when deforming a material. If the shape is not uniform then you should not expect a straight line graph. In fact, even stretching a wire will reduce its cross sectional area. This changes the actual Stress for each length - you don't get a perfectly straight line for even a simple wire, without some compensation in your results.
3. Your measurement of length (a ruler / traveling microscope / laser interferometer/eyeball) may be introducing error.
4. Are you calculating your extension by subtracting the first value from subsequent measurements (sorry about that question - it is probably a sucking raw eggs question but has to be asked).
-
If the coils of the spring were actually pressing on each other you would need some initial force before they seperated which would cause the effect you observe this probably happens because the springs are made by winding them round a former that is significantly smaller than the final size of the spring this would probably just involve running them tightly round a pulley with some gripping rollers keeping it in place while the spring curls off behind. This probably makes a "straight" spring not a spiral one so the coils would like to lie on top of each other so here's a little prediction to add to the experiment.
If this was true the extra force required to start stretching the spring would be approximately that needed to stretch each coil of the spring by an amount equal to the thickness of the wire in the spring. put that ito the equation for stretching and see how it works out. Another way of doing this would be to say that the spring is actually zero length unloaded. Remember too that yo need to factor in the weight of the spring itself because that will do some stretcing particularly to the coils at the top if the spring.
-
Lets say I put a weight of 0.1N on the weight holder. The spring does not stretch. So is it with 0.2N ......0.5N. Then it will stretch nicely according to Hooke's Law. The point is that the graph will NEVER go through the origin howsever carefully the experimewnt is done.
-
http://it.stlawu.edu/~koon/classes/221.222/221L/SampleFormalLab.pdf
The above site explains that hookes law should be extension is prportional to force and NOT extension is directly proprtional to force.
-
Hooke's law states that stress/strain is a constant (The Young Modulus). That implies direct proportionality - because you start with your sample at a given shape / size and then measure the (extra) forces needed to change it.
The relevant thing is the straight line; No physical object will be of zero dimension with no external applied force - obviously - so expecting 'direct proportionality' is a bit of a nonsense.
-
If you look at the site given in my message, it is obvious that experimentally Hookes law should be that extension is linearly proportional to force and NOT extension is directly proportional to force.
-
The link just describes a standard experiment in which the spring has (obviously - from the data) its coils pressed together when under no load. Under these condiditions, if you start from the unloaded condition, the spring is Non-Linear because of its geometry.
If you started your run with a load which already stretched the spring (or if you gave it a good hard hank and opened up the coils) and you measure extension from that first length then you get direct proportionality, which shows that Hooke's law applies.
However, if you really insist on starting with the spring fully compressed then you cannot just take the 'length' of the spring as representing the strain and the load you hang on it as the stress. You need to calculate the stress involved with the detailed shape of the coils as they press together (effectively an extremely stiff spring near its unstretched length).
As I said - your simple spring extension experiment is not a true measurement of Young Modulus and, if you read precisely what Hook says, it is the modulus of the material which counts. If you look at the 'small print' you will see that he refers to a UNIFORM spring, which your spring (and the one in the link) initially is not.
I'm afraid that you haven't found a flaw in an established bit of Science - so we can all relax. You should always approach this sort of thing assuming 'they' are right and you may have got it wrong - it's just a matter of seeing where.
-
Even if you start with a spring after it has been given a good hank, and put a load of 1 gram, the extension is still 0. Thus there is no direct proptionality as the graph will NEVER pass throuh (0,0).
-
Do you mean that, whatever the size of the spring, there is an initial 0.01N offset in its characteristic? That would be very strange.
When an open spring is hanging down, it is already supporting its own weight and that may account for a small error in your very simple Hooke's law experiment. BUT your method is not really a proper test, is it? You are adding a fixed mass to a distributed mass (the spring itself). Everyone does it - it proves a point within a certain accuracy.
If you do the experiment properly and measure the ACTUAL stress and strain, you will get a very good direct proportionality. I have made this point before but you seem to ignore it.
You still seem to think you have found a hole in the fabric of Science. You have not. All that has happened is that the practicalities of measurements need to be considered.
-
Let's forget the spring. Take a piece of wire. Add 1 gram load to it. It definitely does not extend. Thus the graph will never pass through (0,0).
-
Let's forget the spring. Take a piece of wire. Add 1 gram load to it. It definitely does not extend. Thus the graph will never pass through (0,0).
The extension of the wire, ΔL, is given by ΔL=FL0/(A0E), where L0 is the original length, A0 is the cross-sectional area of the wire, and E is Young's modulus (which is a constant for a given material). For copper, E≈100x109, which is a huge number. For a typical wire of 1 meter length, and 1 mm^2 cross-sectional area, with your applied mass of 1 g, I get a stretch of 10^-4 mm. Have you done this experiment with a measuring device sensitive enough to pick that up?
-
Thank you jp.
-
http://it.stlawu.edu/~koon/classes/221.222/221L/SampleFormalLab.pdf
The above site is right. My experiments prove it. Sir Hooke was wrong.
-
You have not answered the primary objection that Hooke, when being precise, actually discusses stress and strain (do you understand what they are?).
How can you think that two sets of data, obtained by students with poor quality equipment, can disprove one of the most basic and well proven bits of Physical Science?
Rather than looking at those graphs, you should look at some evidence which has been gathered in properly conducted experiments and look at the actual theory. You are not encountering 'typical reaction against new ideas' here; you are coming face to face with some real, correct, Science.
-
www.covenantchristian.org/bird/Smart/Physics1/Hooke's%20Law%20Lab.doc -
More experimental proof at the above website.
-
www.covenantchristian.org/bird/Smart/Physics1/Hooke's%20Law%20Lab.doc -
More experimental proof at the above website.
Did you actually read the paper or did you just look at the graph?
You should read the part in the conclusion: "If your line of best fit has a y-intercept of something other than zero, that demonstrates error. Explain a solution to this discrepancy. "????
It seems to me that the document implies that you are wrong and not right.
-
It shows that the experimental results NEVER confirm with the theoretical results. It is assumed that there are errors when there are NONE.
-
OK
So, if he did get it wrong, perhaps you could say for what actual amount of stress and what actual strain would the law stop working?
If I were to use a watch hairspring, what load would you need to get it to extend? Then, for a car suspension spring, what load would be needed?
And, of course, the laws describing how materials distort must work for compression as well as stretching. And then there's the question of what is your actual definition of the 'no load' condition. Does it include the effect of the weight of the spring itself?
Rather than keeping on about Hooke getting it wrong, I suggest that you either repeat some measurements with a view to serious accuracy - taking all the forces into account -and also that you read how the interaction at molecular level relates to the measurements on a macroscopic scale.
Don't just mindlessly bat on about how conventional Science seems to be at fault. Do you really think it is another huge conspiracy? If you really do, then you owe it to yourself to get a lot better informed. (About Physics and about the basics of experimental measurement technique.)