Provided you know the weight of the phone, how would you weight any object using the phone?
Edit: Ok what if you had a device that would let you compare the weights?
A:
How can you when the phone is not equipped with some sort of scale?? You need the necessary hardware for that. It's not possible with just the phone imo.
nkr1pt
2010-09-03 06:38:51
@nkr1pt: Yup, it's not possible with just the phone, you need at least one other object, such as a string, a pencil, a book, rocket motor, or a human assistant, etc.
hotpaw2
2010-09-03 06:47:31
+1
A:
The only thing you can do is compare the weight of the object to the phone's. This way, you would have a vague idea of whether the object is lighter or heavier than XXX grams for instance.
However, I don't think you could do anything programmatically, provided that the phone has no scale.
Wookai
2010-09-03 06:44:45
+1 for being the only answer mentioning programming therefore the only valid answer on a SO point of view :p
DrDro
2010-09-03 08:32:29
+2
A:
How about using the accelerometer to measure the time taken to drop the iPhone from a known height. Then tape the object you need to weigh to the iPhone and drop from the same height. Using some pretty simple equations you can work out the difference in end velocity using the known gravitational acceleration, and hence the difference in weight. Subtract the weight of the iPhone and hey presto, you have probably the worlds least accurate weighing device (but without any additional hardware).
Edit
As pointed out by Seth The above will clearly not work as the objects will accelerate at the same speed and therefore have the same end velocity (other than any minimal impact of air resistance). In order to measure the weight the force that the object hits the ground needs to be calculated which can then be used to calculate the weight (using F = ma). One way of acheiving this (although not necessarly a realistic way) would be to have a uniformly compressible landing material where the distance it is compressed can be measured. The force that the iPhone alone will exert on the material is known as both m and a are known, and as such can be used to work out the force exerted by the combined iPhone and object.
As for the material that could be used to drop the iphone onto, any suggestions?
Macros
2010-09-03 07:01:20
I imagine that the acceleration will be about 9.8 m/s^2, assuming you are performing this test near the surface of the earth.
Seth
2010-09-03 07:02:59
The wording is indeed better, but the acceleration of your two objects will be the same (presuming that the second object is also significantly less massive than the earth).
Seth
2010-09-03 07:12:27
Seth, that is an excellent observation! +1 for taking into account that the second object needs to be sufficiently small :)
mnemosyn
2010-09-03 07:18:08
Doh - I've just got off a plane and lack of sleep doesn't seem to bode well for my simple physics abilities. It's taken me that long to realise that both objects will have the same end velocity! Thanks Seth... Have updated with another possibility
Macros
2010-09-03 07:33:35
If you place both the iPhone and other object into identical lightweight spheres of known aerodynamic drag, and de-orbit both from the ISS, then time how long it takes the heat of reentry to melt each, it might be possible to calculate the relative masses from how long it took to reach terminal velocities.
hotpaw2
2010-09-03 07:55:30
@hotpaw2 - You may also need to run the 'accelerate-iPhone-to escape-velocity-and-point-at-ISS' app prior to de-orbit. Pretty sure there are a couple of those in the app store
Macros
2010-09-03 08:00:58
+4
A:
Write a program that measures the phones acceleration, using the accelerometer sensor.
Throw the phone at the stationary object with the unknown weight. Using the change in acceleration recorded on the phone and the distance that the unknown object moved, you should be able to compute the appropriate vectors to determine the object's weight.
More accurate results will be achieved by minimizing friction - I suggest sliding the phone across a smooth surface (say, a solid block of carbon dioxide ice).
Additionally, tests like these are best done with your friend's phone.
Seth
2010-09-03 07:01:56
+3
A:
If you can compare weights, then accelerate the lighter object to a fraction of the speed of light until the weights compare equal. Since e = m * c^2, convert the energy of acceleration into mass, then add or subtract that mass to calculate your unknown weight.
hotpaw2
2010-09-03 07:08:55
If you can find an anti-matter iPhone and an anti-matter object, crash each into its anti-object, measure the energy produced by each total annihilation, and compare the ratio of energies. (This one would also work best with a friend's iPhone.)
hotpaw2
2010-09-03 07:33:23
+1
A:
Take a solid spring bar mounted to the ground. Attach the iPhone and the object to be measured to the bar. Measure the acceleration created by the spring flexing back and forth. Take the given flexion and acceleration and <insert math that I don't know here> in order to obtain the mass. Conversion from mass to weight is left as an exercise for the reader.
Ignacio Vazquez-Abrams
2010-09-03 07:10:53
Nifty idea! That should work indeed, but it's not exactly easy: The equation is basically `m*a=k*x` which is a 2nd order diff. eq. The issue is that it involves x, the position, which can't be measured by the phone so you'd have to integrate discrete timesteps of acceleration... Also you need to make sure the object moves only on one axis.
mnemosyn
2010-09-03 07:26:30
+9
A:
Place both objects motionless in outer space a known distance apart over a massless ruler. By mutual gravitational attraction, they will attract to the common center of gravity. Use the iPhone camera to record the starting point and meeting point over the ruler. From the c.g. point and the starting points calculate the relative masses. Bonus if you can also do this using the iPhone's clock instead of camera.
hotpaw2
2010-09-03 07:27:07
+1
A:
The spring idea (By Ignacio V-A) is nice and doesn't need any more PDE's other than knowing that the frequency will be ((k/m)^(1/2))/(2 pi).
(is there no LaTeX markup here?)
So if you know the weight of the phone, and you can measure the frequency using the internal accelerometer, measure the K of the spring once using the phone on its own and then put both phone and object on spring and measure the frequency. Solve for the mass of the two and subtract iphone mass to get mass of object!
Note that the nice thing about this method is that you do not need to know the spring constant in advance...you can measure it!
Yossi Farjoun
2010-09-03 08:12:46
Seems very reasonable as long as the spring's extension stays near enough the linear region for Hooke's law to apply. The unknown object has to be heavy enough to be above the spring's thermal noise level, and light enough not to distend or melt the it.
hotpaw2
2010-09-03 08:26:30