# The Naked Scientists Forum

### Author Topic: If I run at 10km per hour inside a plane traveling at 800km an hour what speed am I moving at?  (Read 2070 times)

#### thedoc

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##### If I run at 10km per hour inside a plane traveling at 800km an hour what speed am I moving at?
« 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?
« Last Edit: 06/01/2015 19:30:01 by _system »

#### evan_au

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##### Re: If I run at 10km per hour inside a plane traveling at 800km an hour what speed am I moving at?
« Reply #1 on: 06/01/2015 20:35:47 »
In Physics, you always need to measure speed relative to some "Frame of Reference".

There is no "Perfect" frame of reference, so all of the following answers are equally correct:
• as seen by someone standing on the ground, you are traveling at 810 kph across the sky.
• as seen by to someone sitting on the plane, you are traveling at 10 kph down the aisle.
• as seen by the mobile phone in your pocket, you are effectively stationary.
• as seen by someone standing on Pluto, you are traveling at 100000 kph around the Sun.

The idea that there is no "right" answer was a bit of a shock to some people when Einstein introduced his theory of relativity.

#### Bill S

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##### Re: If I run at 10km per hour inside a plane traveling at 800km an hour what speed am I moving at?
« Reply #2 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.

#### CliffordK

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##### Re: If I run at 10km per hour inside a plane traveling at 800km an hour what speed am I moving at?
« Reply #3 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

#### jeffreyH

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##### Re: If I run at 10km per hour inside a plane traveling at 800km an hour what speed am I moving at?
« Reply #4 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.

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##### Re: If I run at 10km per hour inside a plane traveling at 800km an hour what speed am I moving at?
« Reply #4 on: 07/01/2015 12:37:48 »