Watch gravity pull two metal balls together

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The cavendish experiment shows that even the very week force of gravity can be seen between two room scale objects. Even with the naked eye.

Chapters:

00:00 the beginning
00:44 The Cavendish experiment
07:30 I get it working!

Corrections:

4:53 This isn't a fair comparison. Actually, if you changed the mass of the hanging masses in the experiment, it WOULD change the deflection angle. That's because the value of T in the equation would change. The torsion pendulum would oscillate more rapidly with lighter masses. Thanks James Gilbert.

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[SteveMould]

EDIT: I want to address the comments that say gravity isn't a force, it's the curvature of space time. There's an interesting philosophical point here. The way I think of it is this (I'm not the first to say this but I can't remember who was): physics just gives us models for how the univers works. None of them are "true" but some of them are useful. Newton's model of gravity that describes it as a force is really useful. It doesn't work in certain circumstances. Einstein's model, that describes gravity not as a force, works in more circumstances but is more cumbersome. You pick the mode that best suits what you're doing. In this vide Newton's model is the most appropriate in my opinion. So talking about gravity as a force is perfectly reasonable. Like, imagine being in a physics lab with some springs and pulley or whatever, and you're trying to balance the forces, and every time you mention the force of gravity, someone pipes us and says "I think you'll find gravity isn't a force". That person is unhelpful. Other commenters are saying gravity isn't a force for another reason, which I believe is related to a non spherical model of the earth that they believe in. We can safely ignore those comments.

[Rubrickety]

I’m glad you showed your homemade experiment even though it didn’t work. That took balls.

[Impatient_Ape]

Great job, Steve! 10% error is typical for what physics majors get when they do this lab experiment using the 2nd apparatus you used.

[thegamesforreal]

In my first year at university physics, we did the exact same Cavendish experiment you did to measure G, laser pointer and all. By sheer statistical wonder, despite the extreme finickyness of the experiment, me and my lab partner somehow got the value nigh-bang-on at 6.68*10^-11. The professor simply didn't believe we were that close until he looked at our measurements directly. He said it was the first time he'd seen that anyone measured it to within 0.02*10^-11 accuracy. But then when we calculated the error margin on our measurements, it turned out we had a margin of error of nearly 10x that...

[jasonpatterson]

The Cavendish experiment and Millikan's oil drop experiment were the two historical experiments that really struck me when I was studying physics. Being able to see gravity directly, or being able to see the influence of a single electron's charge - it's just mind blowing. I love Von Jolly's version of the thing as well.

[FelipeKana]

Remember folks, an experiment with a failed result is still an successful experiment! It's very important in science to not hide the mistakes, but to document them throughly and try to understand the failure. Great video

[reinerwilhelms-tricarico]

We did this experiment in a physics lab many years ago using a small mirror attached to the center of the horizontal bar so we could use a light beam to more accurately observe the deflection, noting the angle every minute or so to make a graph. It took several hours. When it was finished we found it was a damped oscillation as expected, but there was a moment when the amplitude increased instead of continually decreasing. We later found out that a small earthquake had occurred during the experiment.

[ssskillzzz]

2:51 "...Boi-oi-oings more quickly." 🤣🤣

[KnowArt]

I vote for a sister series to Matt's calculating pi by hand series in which you calculate g in more and more elaborate ways

[robadkerson]

Mould's Law: a stiffer spring boyoyoings faster

[anmolagrawal]

2:51 That boi-oi-oing was so well delivered, I felt the springiness within

[DrSimonFoster]

I helped set this experiment up and I can tell you that Steve has the patience of a saint! He very much downplays just how tricky and finickity this experiment was to set up! I had to leave as I thought I was losing my mind and it reminded me why I am not an experimentalist! Frankly, I am blown away with how well this came out!

[smitajky]

I did this same experiment in a classroom with my year 12 class. I used piano wire and 50 kg of suspended masses. It had an oscillation frequency of over an hour from which we could know the stiffness of the spring. By introducing the stationary masses we found the shifting of the centre of the oscillation. That gave us G to one significant figure. It was the only time that I was actually able to demonstrate Cavendish experiment. Taking many hours to achieve a result.

[kriscollinstunes]

“the boi-oi-oing” 😂😂😂

Well said! Efficiently and effectively conveyed what you were talking about. Honestly brilliant. 👏👏👏

[roy]

I did this experiment during my undergrad at IUP. We found that we could get extremely accurate results if we set the device (which took results electronically instead of via laser) to work overnight. Even in the basement of a concrete and brick building, the footsteps of people inside threw off the results. We took results overnight on two nights. About 140k datapoints if I remember correctly. We ended up off by 1.4%. The next group used the procedures we had come up with and were off by about 1% as well. Basically, it was a good lesson in looking into sources of error.

(In that course, we had to design our own labs with given equipment to reproduce some of the most famous discoveries in classical physics.)

[IngmarSweep]

Thank you for not disturbing your nice video's with background music, like so many others do.

[carykh]

That idea of using laser reflections to get a finer measurement of rotation is so clever! It reminds of, when I was in a car on a sunny day playing with a reflective Rubik’s Cube, even the tiniest turn of a layer of the cube (like under 1 degree) would send the reflections of the 9 squares of a side way out of alignment!

[exel]

Finally, someone on YouTube has calculated the proper deflection angle! Props to the author. :)
Before that, I watched a bunch of videos about someone quickly cobbling together a setup to observe gravity in their room without even realizing how tiny the deviation should be. That includes the video the author showed as example.

[MusicBent]

I did this experiment for my 8th grade science fair and had the same issue with sensitivity. I replaced the wire with fishing string, used 2 pound lead weighs on my bar, and 15 pound lead weights on the floor. I used a small paddle hanging from the bar into a dish of water to dampen the noise from air currents.

My setup meant I couldn’t use the torsional rigidity of the string, which was now almost zero, but using a time lapse I could measure the position of the bar as the bar swung from ~80 degrees off, to the lead weights touching. Position -> velocity -> acceleration. I think I was quite off, but within 2 orders of magnitude. Basically just confirming gravity’s pull was measurable, but very weak.

That was pushing my limit of understanding of physics at the age. I remember being really awed at being able to see gravity behave in a way I’d never seen before

[Afdch]

"Watch gravy pull two meatballs together". I'm being totally honest, that was what I read and now I am a bit dissappointed it wasn't that. Dissappointed and hungry, apparently.