Watch gravity pull two metal balls together 

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. Here's a fun fact: if you scaled down the earth and moon system until the earth was the size of a bowling ball (keeping the density the same), it would still take the moon 27 days to orbit the earth. This is true in general. Like if you scaled down the ISS as well, that would take the same 90 minutes to orbit as it does now. It's true at any scale, not just bowling ball scale! The sponsor is Brilliant: Visit www.brilliant.org/stevemould for 30 days free access. The first 200 people will get 20% off an annual premium subscription.

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

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

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

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.

A spring “oioioing” is the technical term

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.

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...

The experiment brought me here…and the haircut at time stamp 6:15 that happened in under 7 seconds blew me away. Great video and smooth editing for sure!

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.

Mould's Law: a stiffer spring boyoyoings faster

Concerning Mr Lund's experiment : I looked up for common impurities in crude, unrefined lead. I found it typically contains measurable amounts of 6-7 other metals, including up to 1% of nickel. Nickel is ferromagnetic. Could there be some magnetic interaction from that?

"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." So elegant:)

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.)

This is one of my most favorite experiments. As a teenager, I really looked up to Cavendish. The idea that you could "weigh" the earth in a clever setup in a lab was just so mind-blowingly spectacular to me.

I actually did mathematics workbooks as a kid as well, my babysitter would take me to the bookshop and we would pick out newer and harder ones. I loved doing them and I am now one of the top students in my maths and physics classes. It really does make a difference.

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

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

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!

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

“the boi-oi-oing” 😂😂😂 Well said! Efficiently and effectively conveyed what you were talking about. Honestly brilliant. 👏👏👏

Finally, someone on RU-vid 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.

This is such amazing timing. I was just reading about gravitational forces. Seeing such a perfect video released so recently is great

This was one of my lab projects as a physics undergraduate at Imperial College! The apparatus shown here is much nicer than the version I used 20 years ago. Thank you for another interesting video, very nostalgic for me.

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

The surface of that PVC tube separator is very easy to charge and notoriously difficult to discharge -- even friction with dry air or skin can leave a residual charge on it. Since it's highly unlikely that such charge is uniformly distributed over the plastic, then the PVC acts a bit like an electric dipole, so it's an effect you have to try and eliminate. As far as the torsion in the hanging wire goes -- the longer the wire, the better.

6:40 The cause can't be charge. Both objects are metallic, so if it was charge, it would just equallize on impact. But the weights did not rebound.

Great Video indeed. Hope you will bring such interesting videos for the physics enthusiasts. Thank you Steve.

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!

He is so dedicated, his hair cut was oscillating the entire video.

i LOVE this experiment, and have always been fascinated by it. i used to think that gravity could be "overshadowed" by larger masses, and reading about this experiment wrinkled my brain hard. i love it

In Finland, in 80's Tampere University students were defining gravitation constant on a first year laboratory work course just as described here. Nice and easy task to do and result was quite accurate.

We had this experiment as a possible advanced lab during undergraduate. Most people purposefully avoided it because it was notoriously finicky. There, it was pretty big torsional pendulum placed inside a Faraday Cage. Improper grounding will absolutely mess up the experiment. A friend of mine did the experiment and discovered a grounding fault that was the source of all her error; no one knew how long the fault was present. There was also a case when I was taking the class, that one of the members of the group doing the Cavendish experiment came into the common room very animated a cursing. We immediately asked him what was wrong, and apparently a friend in the class though it would be funny to lightly slap the faraday cage. That one impulse set the pendulum oscillating so much it was going to take most of the remaining lab period to settle down (we had three weeks to do each of these experiments); I'm pretty sure the friend got in trouble with our professor, both specifically for doing that to them, and more generally for incredibly inappropriate laboratory behavior and, effectively, data tampering.

2:55 boyoyoing 😂😂😂

When I was in school, I had an intuition that it would be difficult to measure G by ourselves without any precise equipment, but when I saw your setup I thought that it would definitely work because of how heavy the weights were. But as you showed the problems with your setup, one by one, I got a taste of how thorough and precise experiments really have to be for them to be of any credibility, even for such a simple case. Great Video.

I studied physics and there is a lot of stuff you learn. But I think the Cavendish experiment is my favourite "simple" experiment out there.

Getting not just the order of magnitude but also one significant figure in the lab is bloody amazing, top job

Steve, as always, very enjoyable. This one particularly so for me, as I spent 14 years as part of a team working on developing an instrument that was a several-generations-later descendant of Cavendish's torsional pendulum --- a gravity gradiometer, which measures the spatial gradient of the gravitational force field, and is used in geophysical exploration. The inventor of the first gravity gradiometer, the Hungarian physicist Loránd Eötvös, based his design on Cavendish's experiment; with the aid of what is effectively tensor math (although he did his derivation in scalar closed-form equations), we was able to show that by using a modified Cavendish torsional pendulum, making measurements with the base oriented sequentially in several different directions (over the course of several hours, to let the oscillations damp out), he could directly measure several of the components of the gravity gradient tensor, as well as compensating for the instrument's bias term. And with that information, for measurements taken at multiple locations throughout a region, inferences could be made about the subsurface density distribution --- which circa 1900 turned this into a powerful tool for discovering oil & gas deposits. Brilliant work! Our particular instrument (at a company that's now gone, called Gedex) was a variant of his, customized to be able to operate aboard a small aircraft flying low and slow over the ground (!) --- we managed to get it working, and demonstrating far, far greater sensitivity than Eötvös did in his ground-based measurements, despite being aboard an aircraft bouncing around through the sky...and then the money ran out 😞... Anyway, I wonder if there's anything you could do with the concept of a gravity gradiometer, and/or gravity gradients...

This is one of the experiments my uni offers in the lab courses for physics bachelors. It's probably the most hated experiment among students because of the time it takes. Luckily it was taken off the roster when I did my labs so I only heard stories about how tedious this apparently was. I'm glad I got to see the experiment like this, and at a fraction of the time too!

wow this was nice! :D i've been reading up on GR starting from newtonian gravity history, and yours was a help to show what cavendish did was so revolutionary. this was the first experiment that calculated the actual value of G, before which it was just indirectly estimated using the gravitation force F=GM/R^2

If you decrease the mass of the copper balls, the period of oscillation will get shorter; and by the equation, the measured angle will be less. You can counter that using a less stiff wire, but now you'd have a super light system that is more sensitive to things like air currents. The best experiment really is to use heavy masses that are as close together as possible.

2:49 petition to make boi-oie-oing the official scientific name for a spring springing

My A-level physics class had access to a set-up like Cavendish's in the late 1960s. Our measurements were miles out but there that there really was a force was clear.

That's cool, you took me back to an experiment I did during my first year physics university-study. Thanks for re-experiencing me this!

So cool, thanks for the top quality every time!

Did anyone else notice that he got a haircut between shots at 6:25?

Considering you guesstimated both the equilibrium points I’m AMAZED that your number was so close!

After all time and effort these brilliant figures have spent to make this experiment, some people still think gravity doesn't exist

Brings back memories of being a physics student and measuring G at Imperial College. In our lab experiment we had equipment that moved and tracked the laser beam resulting in the positions being recorded, thus making the final analysis easier 😀

I've done this experiment before with something very much like what you used at the end. This experiment is unbelievably sensitive to vibrations. If you plot the position of the laser over time you can see people walking across the room in the plot. That's why you should do it when nobody else is around and have the laser pointing at a surface as far away as you can get it so that the person taking the measurements doesn't disrupt the experiment moving around.

So cool to see practical experiments working how they should!

You're a freaking genius, mate! Absolutely love your videos. Nicely done.

A couple thoughts about what could cause the Lund result, in decreasing order of how likely I think they are: 1. Disrupted the air currents in the room, creating areas of lower pressure between the lead bricks and the hanging blocks, which would compel them to move together. 2. Affected the equilibrium of the hanging blocks by the force of setting down the lead bricks (some motion or vibration in the floor affecting his mounting structure) 3. He says he got them from a cyclotron. Perhaps being blasted with protons had some effect on his lead bricks.

Bro got a haircut mid video 6:12

A very good video, it shows us the difficulties in making accurate measurements and how to prove if experiments are really working.

This takes me back to doing this experiment as an undergrad... they made us derive the equation too, which was a nightmarish construction involving over a dozen variables.

Steve explained gravity so hard, his hair went back inside his head. truly a big brain moment.

In undergrad I had a little project to modify the Cavendish experiment to measure G using driven oscillations. The larger M was oscillated slightly farther from the axis than the smaller m, so that they wouldn't collide; to a first-order approximation the torsion balance would act like a damped driven harmonic oscillator. At the resonance frequency, the amplitude of oscillation would be much larger than a simple stationary attraction. It was a crude setup but I got a decent value for G (with large error bars, lol). Most importantly, it worked as a proof of concept. It's an interesting experimental challenge -- introducing oscillations means the oscillators can sync through all sorts of things other than gravity (the ground, the air, etc.).

The reason why the 2 objects keep moving toward the middle point which is on the extended line from the string or cable used to hang the objects under the bar and above the floor is that each object has a mass, and thus has a weight: W1 = m1*g , and W2 = m2*g Since the string has a tendency force T to balance or neutralise the gravitational force acting on the objects to keep them hanging in the air, the gravitational force is concentrated at the lower end of the string and downward toward the floor, while the string's tension force is pulling upward or opposite to that of gravitational force T = Mg = W1 + W2 = m1g + m2 g Each of cable or string connecting each of the 2 objects to the hanging string also represents a tension force T1 and T2 acting on the 2 objects, pulling them toward the lower end of the hanging string. If you draw a line from the hanging string's lower end downward to the bar which keeps the 2 objects from moving toward each other, you will have 2 equal right triangles, and the drawn segment is the shared height of both of the right triangles. If you remove the bar which keeps the 2 objects apart, they will move toward each other and stay at the drawn segment's position because the shared height represents a vertical force vector pulling upward in the same direction as that of the hanging string's tension force T, and there are 2 horizontal force vectors which points from the 2 objects to the middle point of the bottom sides of the 2 right triangles . Let the 2 horizontal force vectors be F1 & F2, and H be the vertical upward pulling force T1 = √(H^2 + F1^2) T2 = √(H^2 + F2^2) T = T1 + T2 = √(H^2 + F1^2) + √(H^2 + F2^2) = Mg = m1g + m2g Where F1 and F2 are the horizontal component forces of T1 and T2, which pull the 2 objects to the middle point which is on the extended line from the hanging string or H. H is the vertical upward component force which pulls the objects in the direction of the hanging string's tension force T. H = T, and neutralises the gravitational force Mg , which acts on the 2 objects, pulling them downward toward the floor | | ↑ T | | | | * * | * T1 ↗ ↑ H ↖T2 * | * * | * * F1 | F2 * -*---------------------------------->-- |-

We did the experiment with the mirror attached to the thread and the laser (pointer) in our physics 101 university class … in the 1980ies. Still impressive 40 years later. Thanks for this great video showing all the pitfalls of experimental physics.👍😉

The availability of optical fibre makes that a much better material than wire. I did this experiment for the local High School physics class as a guest experiment. The laser pointer was pointed to a ruler taped to the wall. Then with a time-lapse video recording, the students could calculate big G. This was so cool now that everybody has excellent access to time-lapse video.

Thank you, Steve. The Cavendish Experiment from MrLund has always annoyed me because it is very clear that the movement is too big for what one would expect of the actual force of gravity. So many comments on that video think that is real. In fact, even the most subtle air current in your room could make more impact than the force of gravity. I was ecstatic to see how you would deal with the experiment, as it is a very hard to replicate. Watching you use professional equipment in a lab didn't disappoint!

I really enjoyed this - thank you

Your videos are bloody fantastic. Thanks steve. Keep up the awesome educational content. Really bloody hard to make these topics interesting.

steve having a haircut inbetween his two shooting sessions and then editing them together is breaking my object permanence :p

"Just like how a stiffer spring boi-oi-oings more quickly"

I did It in the didactical laboratory when I was a First year undergrad student in physics more than thirty years ago.

Great video! I'd love to see you repeat your home experiment using the laser measurement system to see if you could measure the tiny fraction of change in theta with it.

I looked at your original setup and concluded the value of r for yours was much greater than the other RU-vid one. This may have played a role in not observing rotation. Then, seeing the one with the laser system really cleared everything up. Excellent demonstration and very cool!

This video is special to me. My 8th grade science teacher, who was awesome, made a contraption that had a laser pointer on one end and a copper sphere on the other, with 100ft of "lever" constructed between them in a sort of zigzag fashion. He took another copper sphere and when he slowly brought it really close to the other the laser would move on the wall because of gravity. I don't know how accurate it was with twenty 8th graders around, but that lesson has stuck with me since.

We had that EXACT experiment at my Comp School for 'A' Level Physics in 1974! Unfortunately, the bar & balls were suspended from the ceiling DIRECTLY under the stairs! So, we had to do the experiment on Wednesday afternoon, when the kids were out for Sport. As I remember, we DID get values very close to the known Gravitational Constant. They probably do a "computer simulation" of the Exp, these days. We also had the 'Millikan Oil Drop' equipment for finding the charge on an electron ... ANOTHER result very close to the known charge of the electron!

Very nicely done!

BlueMarbleScience has made a beautiful copy of the Cavendish experiment from scratch (a copy Cavendish's original design) and used it measure G to within a few percent. He has a large number of videos on this in his RU-vid channel. The apparatus is now housed in the physics department of the University of Tennessee.

02:50 I think "boi-oi-oing" should be the technical term for a spring releasing tension

My guess on why Mr. Lund's experiment worked "better" is that he used flat surfaced objects, and you used spherical objects. Therefore, your center of mass was considerably farther away than the physical distance between the objects than Mr. Lund's were (and maybe the wire was just too thick as you stated). As with the lab set-up, it appears more precisely made; more dialed in.

Wow… That’s amazing. Thank you!

To avoid the charge problem, just bind all the conductive balls together through the torsion wire so they're at equipotential. Attach the two balls on the pole electrically to the bottom of the torsion wire. Attach the top of the torsion wire to each of the stationary masses. Now everything is at the same charge.

I worked with an apparatus just like the higher precision one you show in the latter half of the video when I was a grad student and was TAing a 2nd year mechanics course. I'm amazed you were able to get it to work without much more vibration damping. Maybe our building was shaky. We had to have the apparatus sitting in a big tray of sand (to damp out high frequencies) with the tray resting on a layer of tennis balls (to damp out lower frequencies). Without all this damping it just never settled down to an equilibrium. Then a new building started to be built next door. During the construction we just couldn't run the experiment, no matter what we did to try to isolate it from vibrations. The lower precision setups you show in the first half would be less subject to vibrations from the environment because of the large masses and relatively stiff wires. But I'm suspicious that MyLundScience managed to see an effect. I agree that it is unlikely an electric charge effect. Lead is one of the more diamagnetic substances, but that also seems unlikely to be strong enough. So, I'm at a loss to explain how such a large effect was observed. Lucky air currents??

Beautiful! At first i was confusing this with Foucault's pendulum in my mind, but i'm really sure now that these are truly different things. Thanks for challenging us!

i recall when we were doing the streching of brass rod in hot steam we used a similar approach by looking into a telescope, seeing the rod actually stretching by magnitude of 10^-5 degC^-1 blown my mind. Suddenly our TA comes along and sets his elbow on the table the telescope was on.

The Cavendish experiment was my most memorable and delightful moment in undergraduate physics. Truly amazing to witness AND MEASURE the gravity force and constant.

I don't understand why increasing the mass of the hanging objects wouldn't help the experiment. I understand that the additional gravitational "force" would be countered by the additional force required to move the mass, but wouldn't the total force matter when it comes to overcoming the tirsuonal resistance of the wire?

Круто. Спасибо. Не знал что есть такие простые установки для визуализации таких сложных процессов.

I always thought there would be some way to see this, but when I experimented with it 40 years ago as a kid, I had the same results as your project. So, I just figured my thoughts on how it would work were wrong, as it wasn't really taught in any real depth in junior high school at that time, and I was just speculating on how I thought it should work. Glad to know I was right, just lacked a setup sensitive enough to measure it.

You are so good at explaining this stuff. It's really inspiring.

Steve: "I used to be a bad experimentalist." Steve: "I've gotten so much better at experiments."

At this level of such small differences in resulting gravitational attraction forces additional forces caused by light pressure (radiation pressure) can come into effect.

You got big balls doing this experiment.

Love how you have different hairstyles throughout the explanation. You are gifted. Merry Christmas, God bless you.

There is also a problem if only one of the balls is ferromagnetic. Any net magnetization of this ball will lead to forces with most materials even if they are non-ferromagnetic (i.e. paramagnetic/diamagnetic).

Very nice to see an actual demo of this famous experiment. Makes you appreciate how determined and meticulous Cavendish must have been. I guess being one of the richest men in England probably helped a little!

Fascinating- thanks!

That's pretty cool Steve. I'm never disappointed with what you come up with in each video, I always learn something. Thanks

Very cool, this is one of the experiments I missed out on in my undergrad physics labs that I wish I’d done, same for the oil drop experiment. Next up you can take all of Tycho Brahe’s observations and figure out Kelper’s laws from them. Measuring the electromagnetic and electrostatic forces would likely be a lot easier, using them to derive Maxwell’s equations would be pretty neat too.

A nice feature of this equipment is that it self-compensates for changes in the direction towards distant masses, however large, so you don't have to take the tidal force into account.

Great to show the sophisticated modern set-up of Cavendish's balls. Not only we should thank the designer, but also the technician who build it.

What a cool video - wonderful ! I’ve only ever seen a drawing of the apparatus for this classic experiment so to see it for real, and you considering the problems with your “garden shed” version is awesomely illuminating in what’s going on, to be frank more so than if you pulled it off at home- so what might have been frustrating verging on a disaster whilst making the video has made the video a real masterpiece and triumph. Well done!

This is how Henry Cavendish calculated the constant G. Isaac Asimov told this story and at the same time dealt with something that always bugged him about the terminology of what Cavendish did. So he titled his article, "The Man Who Massed The Earth."

An interesting detail about Cavendish's experiment is that you don't necessarily need prior knowledge of the cable's stiffness, as you did qualitatively in the video it is possible to measure the coefficient of torsion of the cable by just analyzing how fast the oscillations are. When I did the experiment last year we weren't told the stiffness of the cable so that was a mandatory step to measure G

Insanely impressive! well done!