Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: chris on 06/09/2019 23:10:51
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I need a way to get across the concept of tidal locking to a group of school-age children.
Can anyone advise?
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Take a bicycle into class. Hold up the front and spin the wheel. Tell them to pretend it's the (a) Moon spinning, close to a planet.
You'd need to explain that due to friction in the bearings and with the air, the wheel will slow down; but in space the Moon would just keep spinning.
Now stick a big blob of plasticine or blu-tack on a spoke, near the rim.
Explain that this represents that the density of the moon is not perfectly even (see mascon), and that the moon isn't perfectly smooth (see mountains). So the mass is not perfectly distributed.
Spin the wheel, it'll (if the bearings are good!) always stop with the extra weight down.
... there's lots wrong with this, but in a "lies to children" way I think it's a reasonable graphic demonstration.
(e.g. The worst thing here is it implies a stop to rotation, but a tidally locked Moon is of course still spinning, just the rate has matched its orbit. You'd need to explain that the bicycle is stationary on the floor of the classroom and you can't demonstrate that part, but if they can imagine the bike orbiting the Earth they might be able to see what would occur.)
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Thanks; that's a nice idea / demo, but, like you, I foresee a few problems. There are several analogies in that demo that could confuse. The Earth / moon system is in orbit around each other, and there are two elements - the moon's orbit around the Earth and the moon's own rotation. The wheel kind of conflates these two.
The other wrinkle is that we need to do this on the radio, so a visual aid isn't going to work, unless it's something we can easily describe and get the audience to imagine clearly.
Not sure how to get around this...
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Maybe describe it in terms of friction?
- If you have a spinning wheel (like the front wheel on a bike), it gradually slows down.
- The wheel rubs on the axle, and loses energy. This is called friction.
- If you have a good bike, this slowing down happens very gradually.
- But eventually, the energy of the spinning wheel gets turned into heat, and the wheel stops spinning.
You are probably familiar with the way the gravity of the Sun and Moon make the oceans on Earth move up and down, to create ocean tides(?)
- The seawater scrapes on the bottom of the ocean, and that loses energy due to friction
- It's less obvious, but the rocks of Earth also move up and down in response to these gravitational tugs. The rocks of Earth rub together, and this creates friction, too.
At present, the Moon always presents the same face to the Earth.
- But in the past, scientists think the Moon was spinning on its axis as it orbited the Earth.
- That meant that the Earth produced tides in the Moon
- There were no oceans on the Moon, but the rocks still rubbed together, turning the spinning energy into heat
- So the Moon slowed down
- Until eventually it stopped, with the same face to the Earth all the time.
- If we waited long enough, tidal friction will stop the Earth spinning too, and the Earth will be tidally locked, with the same face to the Moon. Half of the Earth won't be able to see the Moon!
This didn't just affect the Earth's Moon - many planets in our Solar System also have moons which are tidally locked to their planet.
- In fact, the dwarf planet Pluto and it's large moon Charon are are already tidally locked to each other.
What this leaves out (for other discussions):
- Conservation of angular momentum: As oceanic friction slows down the Earth, the Moon's orbit moves farther out.
- The Sun will turn into a red giant before the Earth gets fully tidally locked to the Moon
As always, see: https://en.wikipedia.org/wiki/Tidal_locking
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If you stretch a rubber band and release it several times, it gets hot. Some of the energy you put in to do the stretching is turned to heat by friction inside the band.
The effect of the moon's gravitation on the sea is obvious - quite simply the water bulges towards the moon, and as the earth rotates, so the bulge appears to sweep round the earth as a tide.
The earth has a similar effect on the moon - it's slightly stretched towards the earth, but being solid and not liquid, the tidal bulge is tiny. If the moon were to spin relative to the earth, there would be a moving solid bulge. This movement would generate heat, which is gradually lost by radiation into space, so the moon loses energy if it spins in the earth's gravitational field, and over a very long period of time the spin slows down.
There are other moons in the solar system that are still visibly spinning and flexing. Io, a moon of Jupiter, flexes so much that huge volcanoes are blown off its surface by the heat.
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Imagine two spinning tops next to each other. They are obviously not tidally locked. Now imagine one of the tops moving around the stationary one just fast enough so that the same part of it is always towards it. Tidal locking.
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Problem is, Jeffrey, as with many school textbooks, you have described the effect without ascribing a cause. Why does the spin of our moon so precisely match the rotation of the earth? If it was accreted from space dust, or blasted off the earth by an asteroid impact, it would have an entirely random spin vector compared with ours. If it was simply blown outwards from our protoplanet by a single huge volcanic event it would look different on the other side.....which it does, come to think of it…. The clue is in the word "tidal".
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Well this is for 10 year olds and not undergraduates. You could state the following from wikipedia.
https://en.m.wikipedia.org/wiki/Tidal_locking (https://en.m.wikipedia.org/wiki/Tidal_locking)
"The effect arises between two bodies when their gravitational interaction slows a body's rotation until it becomes tidally locked. Over many millions of years, the interaction forces changes to their orbits and rotation rates as a result of energy exchange and heat dissipation. When one of the bodies reaches a state where there is no longer any net change in its rotation rate over the course of a complete orbit, it is said to be tidally locked.[2] The object tends to stay in this state when leaving it would require adding energy back into the system. The object's orbit may migrate over time so as to undo the tidal lock, for example, if a giant planet perturbs the object."
I think the spinning top idea is much simpler to take in.
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It's a simple image, but where does tide feature in the behaviour of a spinning top? And if I spin two tops (because 10-year-olds are familiar with the equipment) why don't they synchronise before they fall over? Why would one of them want to orbit around the other? Why doesn't it?
On the other hand, you can do the experiment with a rubber band and observe the real tide and the fact that the moon is somehow locked to the earth, so why not explain what really happens?
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I've always struggled to grasp the mechanism too before now, but I got an idea from the first reply. How about making a really extreme moon to amplify the effect and make the mechanism easier to grasp. Let's introduce a long pole sticking out from the wheel/moon and have a mass on the far end of that which we can put near the Earth. The Earth will pull more strongly on this mass at the end of the pole, and it will swing about like a pendulum over the Earth, being pulled back in every time it tries to move away. That's tidal locking. Even as the moon moves further away due to other tidal effects, the corrections are applied automatically to keep the same side of the moon facing towards the Earth, slowing the rotation of the moon. You can imagine shortening the pole and the effect still applying, so just keep on shortening it until the pole has no length and you just have a greater mass concentration at the near side of the moon. The same mechanism will act as you shorten the pole, but the moon has to be closer and closer to rotating at the right speed to start with if it is to become tidally locked.
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How did the explanation go, Chris?
Do you have a link, or was it "off the record"?
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An explanation for 12-14 year old kids:
First explain how the tides between the Earth and the Moon work (explain the resulting friction). Then explain how things follow the zero path with action equals reaction (the sum of the forces equal zero gives the trajectory of an object). Then explain that the rotation of the Moon on its axis relative to the Earth produces a rotation of the tidal wave. So now you have friction across an axis which rotates around the Moon (there are a radial and a transverse component of the tidal wave). Any friction dissipates the energy of the wave and in the case of gravity, this gives the shortest path between two masses because it is an attractive force. As the Moon's mass is not distributed uniformly, the rotational energy will dissipate and result in a tidal lock with the Moon's center of mass having the shortest distance to the Earth. The initial quantity of the Moon's rotational energy compared to the tidal friction due to its rotation is small enough to have dissipated completely and stopped it relative to the Earth. The tidal force on the Earth due to the Moon has the same effect but the energy of Earth's rotation is so high relative to the tidal friction that it takes much more time to lock with the Moon.
A minimal concept of energy as motion is also necessary.
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Simple, it's the water wanting to kiss the moon.
And that's that..