# Will a building smash into me?

Discussion in 'Earth Science' started by Waynewright, Feb 28, 2008.

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8
Brilliant!

3. ### Fraggle RockerStaff Member

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24,690
It isn't because the air is moving. It's because YOU are moving. You are moving at approximately 1000mph (depending on your latitude) in a direction tangential to the earth's surface, because that surface is moving. That's what momentum is and we've all got it.

If you rise from that surface, whether it's by jumping or by the use of a propeller or by shooting yourself out of a cannon, you still have that 1000mph momentum. Newton says you can't scrub it off without applying some force. If you have a rocket engine (perhaps the one you used to lift yourself off of the earth's surface) and aim yourself in the opposite direction of the earth's spin, you can slow yourself down and lose that momentum. If you decelerate at one gravity, which is about all you'd be comfortable with, it will take you about 45 seconds to lose all your momentum. At that point you'll be standing still relative to the local frame of reference, and the earth and everything on it--including the buildings--will be whizzing past you at 1000mph. Surely one will indeed smash you, if not a building then a mountain. Even colliding with a pigeon at that speed will kill you.

But you'll see it coming because you'll be decelerating so gradually. As soon as you see the buildings coming at you too fast to duck, your sense of self-preservation will kick in and you'll use your rocket to lift yourself up to a much higher altitude, where it's much safer to travel at 1000mph relative to the earth's surface.

The point is that you cannot rise above the surface and instantaneously decelerate to a velocity much less than that of the rotating earth. It would violate the conservation of energy.

5. ### NesmRegistered Senior Member

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125
Things smashing into each other...while we're on the topic then:

• A vehicle is traveling at 100 km/h from A to B;
• A self-propelled orb, which will stick / adhere to the vehicle on contact, is traveling at 100km/h from B to A.

When the orb comes in to contact with the vehicle, it:
1. decelerates until reaching 0 km/h;
2. then accelerates, in the direction of A to B, until reaching 100 km/h.

How can the orb (once in contact with & attached to vehicle) have varying speeds (100 -> 0 -> 100) if the vehicle to which it is attached has no speed variation?

Differently: How can the orb be at 0 km/h (traveling in neither direction), if that state is only achieved once it is attached to the vehicle? The vehicle it is a part of never reaches 0 km/h...so how can another part (the orb) of that vehicle, be at a state of 0 km/h?

7. ### EnmosRegistered Senior Member

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43,184
Obviously, that is not what happens. The orb will loose it's energy by deforming on impact. If it doesn't deform it will either shatter or be embedded in the vehicle.

8. ### NesmRegistered Senior Member

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125
(wet blanket)...ok :bugeye: The vehicle cannot be deformed, the orb cannot deform. They're both made in such a way, at a secret factory, for the purpose of such tests.

Last edited: Mar 1, 2008
9. ### decantemixGuest

Snatch no bricks, Ohh, peep of someone's abode.
For the bricks you dwell, my find your home.

You may find yourself, looking through the chair.

Of course it'll be hailing OUT the window, at you...

10. ### Fraggle RockerStaff Member

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24,690
Yes but that's science fiction. It's the old chestnut about the irresistible force meeting the immovable object. No such thing is possible in real life and that's the point! I'm not sure what your purpose is in setting forth this hypothesis since it divorces the example from reality. In reality the vehicle will almost surely deform. If the orb is a rock it might not deform noticeably. The energy of the impact will be dissipated in the deformation of the sheet metal of the vehicle. If it's a large enough orb the energy transfer could result in deformation of the infrastructure of the vehicle and of the occupants.

11. ### AsguardKiss my dark sideValued Senior Member

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23,049
conservation of momentum

god i wish i could think of the forumular off the top of my head

is it

M1+M2=m1m2 ?

that cant be right but i cant think how it goes right now