How do we know how heavy the Earth is? | 3 things we need to know

Want to help me out next time @theslowmoguys 😂 - 08:49

00:00 - Introduction
00:38 - Method 1: Geometry
01:52 - Method 2: Gravity
04:08 - The radius of Earth
07:09 - The acceleration due to Earth's gravity, g
10:39 - Strength of gravity, G
13:55 - Calculating the mass of the Earth

I'm using an iPad mini (with a @Paperlike screen protector) with an Apple Pencil 1st generation to show the maths in this video. The app I'm using is @Goodnotes

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👩🏽‍💻 I'm Dr Becky Smethurst, an astrophysicist at the University of Oxford. I love making videos about science with an unnatural level of enthusiasm. I like to focus on how we know things, not just what we know. And especially, the things we still don't know. If you've ever wondered about something in space and couldn't find an answer online - you can ask me! My day job is to do research into how supermassive black holes can affect the galaxies that they live in. In particular, I look at whether the energy output from the disk of material orbiting around a growing supermassive black hole can stop a galaxy from forming stars.

[Villaboy]

A few years back My little boy transferred school at the start of Year 1, and so had an induction day at the new school

At the end of the day the teacher asked him if he had any questions

“How heavy is the earth?”

Proudest dad moment ever

[dragonfly.effect]

The start of Cavendish's paper, which you flashed briefly around 12:48, gives due credit to John Michell, FRS, for both the concept and the construction of his torsion balance. Michell was also the first to suggest (in 1783) that a sufficiently massive star could be what we now call a black hole. ⚫

[aner_bda]

The fact that you can measure the force of gravity from something simple like Cavendish's experiment absolutely blows my mind. I always think of gravity as such a small force that you wouldn't really be able to notice it with such relatively small masses compared to the Earth. That's crazy!

[davidtripp]

I retired a little of a year ago from 28 years of teaching high school physics, and watching this video was a lot of fun and kind of nostalgic. Thanks.

[MaryAnnNytowl]

"Standing on the shoulders of giants." How all progress has ever been made, in any field. We should always remember to be humble, because without them, we would not be where we are today.

[cyris]

You can also calculate g using a pendulum, but the slo-mo camera method is pretty sweet.

[davidsingleton]

42 km wider at the equator? 42! Everything else suddenly make sense!

[KonekoEalain]

Truly excellent video, a big science question answered by breaking down the math and tying in history, I plan on showing this to my kids when they are older.

[futureboy]

Cavendish is probably my favourite historical scientist. From Bill Bryson's (excellent) 'A Short History of Nearly Everything':
"He suffered, in the words of one of his biographers, from shyness to a degree "bordering on disease." Any human contact was for him a source of the deepest discomfort.
Once he opened his door to find an Austrian admirer, freshly arrived from Vienna, on the front step. Excitedly, the Austrian began to babble out praise. For a few moments Cavendish received the compliments as if they were blows from a blunt object and then, unable to take anymore, fled down the path and out the gate, leaving the front door wide open. It was some hours before he could be coaxed back to the property. Even his housekeeper communicated with him by letter."

[cytonicstarspren]

Really love these type of videos! They don’t state the facts but tell us how we know the facts. A great representation of the scientific method and really interesting to watch!!

[denisdaly]

I'm like the earth. I bulge at the equator slightly.

[t.c.bramblett]

Dr. Becky you are awesome. I'm so glad you share your enthusiasm and not only that, but help to recognize the astronomers who were lost to history

[joshyaks]

1:10 - "Now if we remember back to high school geometry..." I've just encountered our first problem.

[jah_hern]

Becky, just came across your channel and I immediately became a huge fan. Thank you for doing what you do.

[croco]

I love the use of the iPad and Pencil to draw your notes and formulae 👍
makes it both clear and look a bit more interactive (as opposed to post-prod animations) as we see you draw and reason about it live

[oscresson]

You are so much fun! These simple ways of experimenting and explaining are what drew me to science 70 years ago, and has held me ever since. Here's a story I heard as a child about the earth's shape. Around 1900 a ship was loaded with gold in Alaska, but when it was weighed in San Francisco they thought some had been stolen. It took a while before someone figured out the problem was the curvature of the earth!

[brunolima]

Hey becky by 13:08 you mean more* dense than water! It is written more but you said less.

[folkmaster]

Dr. Becky, I enjoy all your videos, thanks for your hard work in producing them.

[sacredkinetics.lns.]

I like the deep philosophical context in the statement about the last 2 thousand years... And on the shoulders of the Giants.
💫👽💫

[FamCh]

Fascinating !!! straight to my top 10 science Videos !!! Love it !!! Thanks Dr B. !!!