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100

Having my own 6-year-old and having successfully explained this, here's my advice from experience: Don't try to explain gravity as a mysterious force. It doesn't make sense to most adults (sad, but true! talk to non-physicists about it and you'll see), it won't make sense to a 6yo. The reason this won't work is that it requires inference from general ...

75

The misconception likely comes from a misunderstanding of "down". Making a 2-D drawing of the earth with buildings, people, and trees might help. For example,

68

The Foucault pendulum is a great experiment which does demonstrate that the Earth is rotating, but it was only introduced in 1851. The Earth had been known to rotate for several centuries before that, probably stimulated by Copernicus and Galileo pushing the heliocentric model of the solar system during the 16th century. A couple of decades before Faucalt's ...

56

The Earth's climate isn't quite as stable as you think. The Earth's climate has toggled back and forth between a greenhouse Earth and an icehouse Earth for the last 600 million years or so. During the icehouse Earth phases, the climate can enter an ice age, an extended period of time during which the climate in oscillates between glaciations and ...

54

Hiroshima exploded 67 terajoules of energy i.e. $6.7\times 10^{13}$ joules. We may calculate the mass of the Russian meteor assuming that the speed is $v=20,000$ m/s: $$\frac{1}{2} mv^2 = 6.7 \times 10^{13}$$ We obtain 335 tons. The numbers aren't precise but they're in the ballpark and reasonable. The Russian academy of sciences actually estimates 10,000 ...

53

Foucault pendulum. I don't know how the ancients did it, but it is surely pure classical mechanics. The animation describes the motion of a Foucault Pendulum at a latitude of 30°N.

52

Your intuition about spinning fluids is wrong for a couple reasons. Angular momentum is conserved so an isolated system of any shape will keep on spinning unless it has a way to transfer that momentum elsewhere. If you spun in egg levitating in a vacuum it would spin forever. The more bumps, flaws, or non-spherical features your container has the faster ...

51

The moon does not fall to Earth because it is in an orbit. One of the most difficult things to learn about physics is the concept of force. Just because there is a force on something does not mean it will be moving in the direction of the force. Instead, the force influences the motion to be a bit more in the direction of the force than it was before. ...

40

Wrap a ball (like a tennis ball) with a rubber band. Tell her to put her finger between the ball and the rubber band and try to move her finger away from the ball. Have her do this on all sides of the ball. Now explain to her how the rubber band is like gravity.

39

I generally take "foreseeable future" to mean "in my lifetime" in which case the answer is No. However, if you are really asking, "What does science say about the solidification of earth's core?" then we can answer this. In the core of the planet, we actually have this pretty picture: The core is heated by radioactive decays of Uranium-238, Uranium-235, ...

38

Barring whatever fantastic energies would be required to stop the mass of the Earth from rotating and then changing the direction of the rotation, one of the major things I can see changing would be the expectations of weather patterns. Part of what affects our weather is known as the Coriolis Effect. While there would certainly be effects from the ...

37

This is a fun what-if. I really wish I was xkcd right now so that I could include pictures and humour, but I'm not. If we built a giant ring around Earth at just the right height everywhere, then in theory, the ring would just float there once the supports were removed. However, (and this is the reason not to try it) this is an unstable state. If you so ...

32

Briefly: Because the moon's orbit "wobbles" up and down, so it isn't always in the plane of the earth's orbit around the sun. There's a 2D plane you can form from the ellipse of the earth's orbit and the sun. This plane is known as the ecliptic. The moon's orbit is not exactly in the ecliptic at all times; see this (slightly overcomplicated) picture from ...

31

Why not try this at home by using the Coulomb force or magnetism instead of gravity? Although all those forces are different in nature, mathematically they are the same: $F(r) = -\frac{const.}{r^2}$ Edit: Magnetism is different as pointed out in the comments, but it still works for illustrating the problem. In fact, the result is the same for any ...

29

Simplest, you say? There are two that strike me as being simple to demonstrate. Luckily someone on the internet has already spent some time to help us here to make these easy to illustrate: Shadows differ from place to place: Eratosthenes carried out this experiment to determine the circumference of the Earth, already assuming its spherical shape; ...

29

The atmosphere of the Earth is mainly composed of nitrogen (N2, 78%) and oxygen (O2, 21%) molecules, which together make up about 99% of its total volume. The remaining 1% contains all sorts of other stuff like argon, water and carbon dioxide, but let's ignore those for now. As you probably know, the oxygen we breathe is produced by plants from water and ...

26

Whether the sun "rises" in the east depends on your position on earth, and the time of the year. In northern latitudes, during the summer, the sun rises significantly North of East, and in the winter it rises in the South. For example, today's sunrise/sunset directions in Umeå Sweden, look like this (source: www.suncalc.net) The yellow line shows the ...

25

The other answers provide a first-order approximation, assuming uniform density (though Adam Zalcman's does allude to deviations from linearity). (Summary: All the mass farther away from the center cancels out, and gravity decreases linearly with depth from 1 g at the surface to zero at the center.) But in fact, the Earth's core is substantially more dense ...

23

Correct. If you split the earth up into spherical shells, then the gravity from the shells "above" you cancels out, and you only feel the shells "below" you. When you are in the middle there is nothing "below" you. Refrence from Wikipedia Gauss & Shell Theorem. {I am using some simplistic terms, but I don't want to break out surface integrals and ...

23

A slightly simpler version of David Hammen's (as usual excellent) answer: Earth is "big enough" to have sufficient pull on the atmosphere: gravity stops it from escaping Earth is "close enough" to the sun to keep liquid water (and liquid core) Core is sufficiently magnetic that it acts to protect against solar wind (which would otherwise strip the ...

20

Those code names all come from certain catalog. For example, NGC means 'New General Catalogue'. There are various catalogs aiming at different objects, like stars, nebulae, galaxies, etc, but not for the Earth, at least not yet. You can find almost all known astronomical catalogs and tables at CDS

19

I suspect the question may be unanswerable, and possibly even meaningless. As I understand it, the giant impact that resulted in the formation of the Moon would almost certainly have also completely liquefied whatever crust the Earth had at the time, producing a global "magma ocean". Thus, there would've been no traces of the impact left — or rather, ...

19

The day/night cycle is the first obvious effect: Days and nights would both be longer (EDIT Or shorter, apparently I got it backwards) if the speed of rotation is the same. All life on the surface of the planet has evolved for day/night cycles of roughly the length we experience now, with leeway for the difference between summer and winter. A sudden ...

19

This is a really rough calculation that doesn't take into account the realistic direction of the bow shock, or calculation of the drag force. I just take the net momentum flow in the solar wind and direct it so as to produce the maximum decceleration and see what happens. Apparently the solar wind pressure is of the order of a nanoPascal. As I write this ...

19

Rub a balloon with a cloth to induce a charge which will demonstrate "static cling", then attract small pieces of paper with the balloon. Once they see that static electricity attracts the small pieces of paper to the bottom of the balloon, then you can start explaining to them about 'the force'.

19

Ask the child what "down" would mean to the people on the "other side" of the Earth.

18

When you're trying to understand the mechanics of a system it's usually convenient to choose coordinates that reflect the symmetry of the system. The solar system is roughly centrally symmetric because the Sun is by far the largest mass in it, and the coordinates that reflect this symmetry are polar coordinates with the Sun at the centre. For example in ...

17

Yes, helium can leave the Earth, and yes, we will run out of helium, but because of different reasons. When you buy a helium balloon and its contents get released, this helium goes into the atmosphere. It isn't gone, and it could in principle be purified out of normal air. However, the total amount of helium in the atmosphere is so small it is ...

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