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Author Topic: Could you watch the following and then answer something for me?  (Read 1630 times)

Offline GlentoranMark

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Seen this on the Nasa website, it's the space station boosting its orbit and the astronauts going backwards due to Newton's third law.

http://www.nasa.gov/multimedia/videogallery/index.html?media_id=117824321

Now could someone tell me, when I'm on a plane and I feel the acceleration as it takes off. If I throw something up, why doesn't it shoot to the back of the plane?


 

Offline Geezer

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If you pull a stunt like that these days, you're likely to get kicked off the plane  :D

I believe the answer is that it does, just not enough that you will notice.

An aircraft will require about 1.6 km to achieve a takeoff speed of 250 km/h, or 69 m/s.

v^2 = u^2 + 2as - the initial speed u is zero, so v^2 = 2as.

From that it follows that a (the acceleration) is v^2/2s (v is the final speed and s is the distance travelled)

So, a=69*69/2/1600, which comes to 1.48 m/s/s.

1g is 9.81 m/s/s so the the plane is only accelerating at 0.15g (average).

The object will not come straight down relative to you, but you'll hardly notice the angle because the acceleration of the plane is small relative to the acceleration produced by the Earth's gravitational field.
« Last Edit: 27/10/2011 21:22:28 by Geezer »
 

Offline JP

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Seen this on the Nasa website, it's the space station boosting its orbit and the astronauts going backwards due to Newton's third law.

Geezer's right.  You will notice it, it's just that falling objects well... fall.  And they fall faster than they shoot backwards.  If you put a ball on the floor, you'll see it roll back, though. 

But it's not the third law doing this.  It's the first law.  The astronauts aren't being acted upon by anything while they're floating, so they stay put.  The space station is being accelerated, so it moves.  What you see there is them staying still while the station moves away without them.
« Last Edit: 27/10/2011 23:18:30 by JP »
 

Offline GlentoranMark

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Many thanks, I thought I knew the answer (Newton's 3rd law) before I asked the question but 2 very concise answers put me wrong. While I don't understand the maths I can understand the logic.
 

Offline Geezer

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Mark,

You really don't need to understand the maths, but I'll be happy to explain it by PM if you want to. Trust me, if I can understand it, you will too. I guessed the acceleration was not very great, but I thought I better prove it.

You can also get to the answer by just thinking about the force on your bottom compared to the force on your back while the plane is accelerating. The force on your bottom is much greater. You could actually lean forward during takeoff without much difficulty.
 

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