# Re: starship-design: Re: Tidal forces

```Due to mail-deamon error, this message might have arrived before.

To Kelly,

>>>Tidal forces varry acording to size.  Given a structure a dozen times the
>>>size of earth (and extrenmly unlikelt situation) it would have larger tidal
>>>force.  Certainly enough to keep it edge on to the sun.
>>
>>Why? Can you use a description without using "tidal forces"? That term seems
>>
>>I understand that part that are at different distances from the Sun will
>>have a different gravitational pull, but believe me these differences are
>>REALLY small. You cannot compare those forces to those which a meteor
>>experiences when flying close by a planet.
>
>But the forces are acting on a lever hundreds of thousands of miles long.
>Thats a lot of torque!

Distance Mercury-Sun: 5.79E10 m
Mass of the Sun     : 1.989E30 kg
G=6.67E-11 N m^/kg^2

The gravitational acceleration at distance r is:
a=G M/r^2=0.0395734 m/s^2

Now go 100 thousant km further, (1E8 meters)

so r=5.79E10+1E8=5.8E10 m

which gives a=0.0394371 m/s^2

The difference is da=0.0001363 m/s^2

If my guesses are right, even over a 1E8 meter long lever the force is well
below the breakingpoint of steel.

The problem is that I don't know how to do an acurate structural analysis,
so I can't give you a more clear answer at the moment.

Tim

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