On 22/02/2015 21:09, Doctor John wrote:
> On 22/02/15 19:28, Alain wrote:
>>
>> The 10^-9 range is for our spacial neigourhood, or relatively close to a
>> star. As you travel farther, it drops conciderably. Between the Sun and
>> Proxima Centaury, the vacuum around the moon looks like high pressure.
>>
>> In space, to have a drag equivalent to 100 Km/h on a road, using the
>> 10^14 factor for Earth region, you would need to travel at over 100 000
>> 000 000 000 Km/h (10^16 Km/h). That's  /way/ faster than the speed of
>> light ! (about 10^9 Km/h)
>>
>>
>>
>> Alain
>
> How the hell did you calculate that? I'm not saying you're wrong -
> you're probably right - but can I see the equations?

Drag force is directly proportional to the fluid density, and velocity 
*squared*. So if the density is 10^14 times lower, the velocity "only" 
needs to be 10^7 times higher to get the same force. I don't know where 
Alain got the 10^16 km/hr from.

But it gets worse, the power (rate of doing work) required to maintain 
the velocity against drag equals the drag force times the velocity. So 
if you want the same *power* requirement as on Earth, the velocity only 
needs to be about 50000 times higher.

Post a reply to this message