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Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: thedoc on 06/01/2015 19:30:01

Title: If I run at 10km per hour inside a plane traveling at 800km an hour what speed am I moving at?
Post by: thedoc on 06/01/2015 19:30:01
Chris Grieshaber asked the Naked Scientists:
   
If I run at 10km per hour inside a plane traveling at 800km an hour what speed am I moving at?


What do you think?
Title: Re: If I run at 10km per hour inside a plane traveling at 800km an hour what speed am I moving at?
Post by: evan_au on 06/01/2015 20:35:47
In Physics, you always need to measure speed relative to some "Frame of Reference (http://en.wikipedia.org/wiki/Frame_of_reference)".

There is no "Perfect" frame of reference, so all of the following answers are equally correct:

The idea that there is no "right" answer was a bit of a shock to some people when Einstein introduced his theory of relativity.
Title: Re: If I run at 10km per hour inside a plane traveling at 800km an hour what speed am I moving at?
Post by: Bill S on 06/01/2015 21:51:29
Of course, one should use the relativistic velocity addition formula to answer your question, but at the low speeds (compared to the speed of light) you mention, the difference would be negligible.
Title: Re: If I run at 10km per hour inside a plane traveling at 800km an hour what speed am I moving at?
Post by: CliffordK on 07/01/2015 05:41:55
Can I just say that 800 kph has one significant figure....  (although it is one of those ambiguous numbers).

So, you're still traveling 800 kph  [:)]
Title: Re: If I run at 10km per hour inside a plane traveling at 800km an hour what speed am I moving at?
Post by: jeffreyH on 07/01/2015 12:37:48
It all depends upon the forces acting on the plane. In a severely time dilated field both the runner and the plane would appear to be traveling faster than would be apparent to a distant observer. Simply because while the object has a specific velocity as viewed remotely time for the passengers has slowed so the relationship between perceived distance traveled over time taken has changed.
Title: Re: If I run at 10km per hour inside a plane traveling at 800km an hour what speed am I moving at?
Post by: syhprum on 19/12/2017 17:25:48
Speed is of course a vector quantity but due to the geometry of the aircraft your speed can only add or subtract from the speed of the aircraft relative to the ground excluding SR.
If you were in a large saucer shaped alien UFO and could run in any direction the angle relative to the craft would have to be taken into account.
Warning do not try this on a Ryanair trip !
Title: Re: If I run at 10km per hour inside a plane traveling at 800km an hour what speed am I moving at?
Post by: alancalverd on 21/12/2017 16:31:31
810 kph if you are running towards the nose, 790 kph if you are running towards the tail.

Now some smartarse is bound to ask "relative to what?" and the answer is "the air around the plane", because that is what planes fly through. Unless the nominal "800 kph" is ground speed measured by a GPS system, of course.
Title: Re: If I run at 10km per hour inside a plane traveling at 800km an hour what speed am I moving at?
Post by: syhprum on 21/12/2017 16:54:07
I don't think many normal commercial flights go as fast as 800 kph I have a theory that pilots go as slow as they dare to gain brownie points by saving fuel
Title: Re: If I run at 10km per hour inside a plane traveling at 800km an hour what speed am I moving at?
Post by: chris on 21/12/2017 19:56:32
In our "Science of Star Wars" programme (https://www.thenakedscientists.com/podcasts/naked-scientists-podcast/star-wars-science-strikes-back) we looked at the issue of relativity and covered this issue - albeit by firing a ball from a moving trolley to prove the point - in this feature about relativity and high-speed travel (https://www.thenakedscientists.com/articles/interviews/getting-grips-relativity-why-luke-and-leia-are-different-ages).
Title: Re: If I run at 10km per hour inside a plane traveling at 800km an hour what speed am I moving at?
Post by: yor_on on 22/12/2017 07:43:45
If you're running back and forth inside that plane I guess you could see it as being still :)
In that case it's 800 km/h
Title: Re: If I run at 10km per hour inside a plane traveling at 800km an hour what speed am I moving at?
Post by: Bored chemist on 22/12/2017 12:23:18
From the point of view of an ant on your head, you are not moving at all.
Title: Re: If I run at 10km per hour inside a plane traveling at 800km an hour what speed am I moving at?
Post by: Bill S on 22/12/2017 17:38:53
Quote from: Alan
Now some smartarse is bound to ask "relative to what?" and the answer is "the air around the plane"

Naturally the smartarse is bound to ask: "What about wind speed/direction?" and we all know where it goes from there!

Did someone say: "to hell in a handcart"?
Title: Re: If I run at 10km per hour inside a plane traveling at 800km an hour what speed am I moving at?
Post by: alancalverd on 22/12/2017 18:50:22
Gotchas!

The rated cruising speed of most jet airliners is 780 - 850 kph depending on type and altitude, but generally in the range Mach  0.75 - 0.9.

No, pilots do not fly slower to save fuel. Jet aircraft are most efficient at their rated cruise Mach number for a given weight and altitude. If you slow down below your filed flight plan speed you may lose your approach slot, with all sorts of expensive and inconvenient consequences, and end up burning more fuel anyway..

Wind speed is irrelevant to airspeed - the plane flies at its TAS through ambient air, regardless of where the ambient air is going, or how fast.
Title: Re: If I run at 10km per hour inside a plane traveling at 800km an hour what speed am I moving at?
Post by: Bill S on 23/12/2017 11:46:35
Quote
Wind speed is irrelevant to airspeed - the plane flies at its TAS through ambient air, regardless of where the ambient air is going, or how fast.

When did mundane facts prevent the crackpots from speculating?