# The Naked Scientists Forum

### Author Topic: Does a boat weigh less when the tide is out?  (Read 5070 times)

#### na na

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• Posts: 8
##### Does a boat weigh less when the tide is out?
« on: 08/05/2008 11:34:18 »
Background:-
Gravitational or tidal drag is caused by the mutual gravitational attraction between the Earth and the moon. It is slowing the Earths rotation and driving the tides.

Tides are caused by a physical "bulge" or high-point following the Moon. When this "bulge" is located over the ocean the tides are out on land due to the sea's surface being "pulled" upward. When this gravitational "bulge" is over land the tide is in, i.e. the sea's surface is not being "pulled" upward.

Questions:-
Is the gravitational attraction sufficient to flex the surface of the Earths crust when the gravitational "bulge" is over land, or is the viscosity of the Earth too stodgy to be affected by the moons gravity? (Does the Earth get any "tidal heating" from internal friction caused by gravitational stretching/squeezing?)

If a boat is out at sea when the tide is out, does the boat and occupant(s) weigh less than when on land or when the tide is in?

Thanks

#### lyner

• Guest
##### Does a boat weigh less when the tide is out?
« Reply #1 on: 08/05/2008 12:20:04 »
There is a lag (varies from place to place) between the position of the Moon and the phase of the tide so you can't say your weight would vary in an obvious way. But, aside from the state of the tide, with the Moon overhead or 180degrees away, you would weigh a tiny bit less than with the Moon at +/-90degreees.
The other factors, like the shape of the Earth and the distribution of rocks beneath you will be large.
As for a tidal movement of the crust - the approx 12hr period of the tidal force is to short to cause any significant flow - I don't think it's been detected. The constant force difference due to the Earth's spin has made a slight difference between the NS diameter and the diameter across the equator, but that has been constant for millions and millions of years; plenty of time to creep.
The tidal heating (causing melting) effect is thought to account for the lack of impact craters on some of the satellites orbiting close to the large planets. I don't remember which ones but NASA will have pictures. I think that, in the Earth's case, the available energy is not enough.

#### na na

• First timers
• Posts: 8
##### Does a boat weigh less when the tide is out?
« Reply #2 on: 08/05/2008 12:35:08 »

So basically the answer is yes! - Weight gain and loss does occur due the moons gravitational influence, depending on location on Earth and location of the moon.

Doesn't sound like it will save me from going on a diet, but very interesting.

#### lyner

• Guest
##### Does a boat weigh less when the tide is out?
« Reply #3 on: 08/05/2008 15:14:33 »
Yes- but ignore the bit about the tides- even over the UK, they can be hours different. Even in mid ocean, there is a phase difference.

#### LeeE

• Neilep Level Member
• Posts: 3382
##### Does a boat weigh less when the tide is out?
« Reply #4 on: 08/05/2008 16:13:39 »
I believe that most of the heating of Io, one of the moons of Jupiter and the most geologically active body in the solar system, is caused by gravitational tidal heating.

#### lyner

• Guest
##### Does a boat weigh less when the tide is out?
« Reply #5 on: 08/05/2008 16:21:19 »
Yes - its' too small to have its own internal heat source.

#### daveshorts

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• Physics, Experiments
##### Does a boat weigh less when the tide is out?
« Reply #6 on: 09/05/2008 09:08:00 »
I think in the UK the tides are about 90 degrees out of phase - on a full moon - spring tide - high tide is at about 6pm and 6am but the moon is right above you at midnight. This makes sense if the position of the water is limited by inertia, as in this case the moon will be pulling water from the sides, but by the time the earth has turned around 90 degrees the bulge is now where it wasn't but still at 6pm.

This means that at low tide there is less water below you and more moon pulling you upwards so you will be lighter, and at high tide there is less moon and more water, so you are heavier.

#### lyner

• Guest
##### Does a boat weigh less when the tide is out?
« Reply #7 on: 10/05/2008 00:49:38 »
High tide varies by about ten hours between Penzance and North Scotland so there is no particular phase relationship and you can't generalise. If the tides were all the same they wouldn't have to calculate and publish different tables for every port in the country.
All you can say is that, in any given place, high spring tides will always happen around about the same time of day (about noon in Chichester Harbour) and high neaps about six hours different.
It's a wave / resonance phenomenon, the excitation comes from Sun and Moon.
The situation might be more like the above post if the Earth was one big ocean, all of the same depth.
« Last Edit: 10/05/2008 21:16:10 by sophiecentaur »

#### The Naked Scientists Forum

##### Does a boat weigh less when the tide is out?
« Reply #7 on: 10/05/2008 00:49:38 »