Momentum

[HeathrowAirport]

If a plane weighs 58Tonnes say A320 with good load and fuel.
58,000kg x 145 knots = 166.86302 mph VREF(74.5944445 m/s) =
58,000 kg x 74.5944445 m/s
Momentum = 4 326 477.78 kg m/s

Does that mean if it hit you, it would hit you with that force? :O

[Agaricus bisporus]

Only if you stopped it. If you didn't stop it then it would retain that figure less what it expended on hitting (and moving) you.

[BOAC]

As AB says, in all likelihood, what is left of you will rapidly acquire a speed of 145kts and the 330 will continue on its merry way with the crew saying "did you hear something...."?:)

[Capt Pit Bull]

"Whoop Whoop" Physics alert....

You're confused between 'momentum' and 'force'.

In this case as your mass is very much smaller than the aircrafts, it really doesn't matter what the aircrafts momentum is.

The only salient fact is that you will go from 0 to 145 knots very rapidly and will therefore undergo a very high acceleration.

Lets do a rough calculation:

Lets imagine you get splatted face on against the radome. Not hugely likely, I’ll admit, but perhaps you’re athletic and manage to leap up in front of it. We could approximate that to hitting a flat plate, rather than the more tricky calculation of being trampled by the undercarriage.

Let’s assume that you go from being 30 cm thick to being 0 cm thick. This would clearly not be survivable, but is the ‘best case’ in that it involves the smallest acceleration.

145 knots is roughly 75 m/s. Therefore, the aircraft would require 0.3/75 seconds to travel the depth of your body, which is about 0.004 seconds.

During this time you would change velocity by 75 m/s, so the acceleration would be 75/0.004 = 18750 m/s^2.

(Incidentally this is about 1900 G, which exceeds the maximum surviveable instantaneous G by a factor of about 50)

Let’s say you mass 70 Kg. Newton 2 tells us the force required to accelerate you that much would be 18750 x 70 = approx 1.4 MegaNewtons.

This is so large that the radome would likely deform as well, actually thereby increasing the time of the collision and decreasing the acceleration required.

So you could rework on the basis that the radome also caves in by 30 cm, leaving a ‘you shaped’ dent.

This would halve the accelerations and forces compared to those originally calculated, but its still clearly a bad day out.

pb

[HeathrowAirport]

I was revising for my Physics exam that I just did it "Quite hard" and I dont know how it came to mind but I was curious as to what would it be like hitting you.

So It wouldnt be 4,326.47778 tones hitting you? So for example a car travelling at the same speed would feel like maybe 100 tonnes due to its weight maybe?

Has there been any cases of a plane hitting someone?

[Capt Pit Bull]

What level physics exam and when are you sitting it?

[HeathrowAirport]

Im doing Additional science its basically more advanced stuff than Core Science, and Ive sat it. Just 2hrs ago. I get two GCSEs for it.

[Lightning Mate]

Good thread.

So what if you stand on a railway line and throw a ball bearing towards the front of an approaching train.

The ball bearing has motion in the opposite direction to that of the train, but when it hits the train it is accelerated in the opposite direction. At some moment it must therefore be stationary. Since this is at the time of impact with the train, then why is the train not also stationary at that moment?

[Capt Pit Bull]

Right. I was just wondering, been a few years since I taught Physics, at that point momentum wasn't in GCSE.

In reference to your original question, the mass of the object hitting you is an issue, but not as much as the man on the street might think.

Of course what you gave was a classic 'collision' problem which is pretty much the bed rock for understanding basic mechanics.

In your example, the question you asked was about the force you would experience.

To calculate that, you need to use Newtons second law (in its F = MA form), so you need to know your Mass (easy enough) and the acceleration you would experience (not so easy).

The method I used to approximate an answer was to say that acceleration = change in velocity / time.

I approximated the time by using time = distance / speed, assuming you were squashed flat.

I approximated the velocity change by saying that you will simply be accelerated to the aircraft's velocity.

Why is this a reasonable thing to do? Well, if a stationary object is hit by a moving object then obviously the velocity of the moving object will be changed. e.g. in you example the aircraft would slow down slightly, so to match its velocity you wouldn't need quite such a large acceleration as I calculated. However the effect is pretty minimal since you only have about 1/1,000 th the mass of the aircraft.

And in fact this is a generally accepted method for solving collisions; if one object is much larger than the other you can usually get away with assuming the larger one doesn't change velocity. In your case 3 orders of magnitude is close enough for ths assumption to be applied

If you're hit by a larger object moving at 145 kts, you'll get accelerated to 145 kts, whether it masses 5 tons, 50 tons or 500 tons. Mutliply the aircraft weight by 10 and you'll only increase the force you experience by a fraction of a percent.

pb

[HeathrowAirport]

Capt Pit Bull (http://www.pprune.org/members/8103-capt-pit-bull),

Thanks very much for the reply, this is indeed a good thread Ive started.

So it does not matter what size the object hitting you, I know its going to hurt either bloody way, but I was wondering what differences there are of the size and weight of the object if it hits you at 166mph, bigger the object the more momentum it has and will hurt.

Lightning Mate, Is that not Resultant force or something like that, I cant remember now :(

[Lightning Mate]

OK then.

Consider a B747 in level flight with autopilot and autothrottle disengaged. It has been manually trimmed in straight and level flight at constant IAS.

This 747 is different. All the seats have been removed and replaced with one thousand bird perches. On each perch sits a parrot.

One of the attendant vets claps his hands loudly and simultaneously all the parrots get airborne.

Question

Does the aeroplane stay level, descend, or climb?

[Heatedfuelreturn]

As the birds exert a force downways equal to their mass, to remain airborne.

next...>

[Heatedfuelreturn]

All the toilet waste is in the aft of the aircraft, so what happens after the rather nice curry in First class, in regard to the C of G , that is....

your move...>

[Lightning Mate]

stays level

Nope.

All the toilet waste is in the aft of the aircraft, so what happens after the rather nice curry in First class, in regard to the C of G , that is....

your move...>

Whose move?

[HeathrowAirport]

Check Mate! Loool! :ok: