(BG Spoilers) LTW Conservation of Speed w/ Mass Shift

[Smaug with OCD]

In BG, during the fight with the fire giants, LTW flies at one, changing from a hawk to an elephant mid-flight. According to Dresden, he maintains his flight speed while doing this. I'm not really a math and science guy, but my instinct is to say that something is off about that. The easy answer is "magic." But, I was wondering if I was just wrong, or if shenanigans would indeed need to be happening for that conservation of speed to occur?

A minor detail, I know. But, I'm curious.

[morriswalters]

Your instinct is right.  Take the easy answer.  At a minimum it violates the conservation of momentum in some fashion.

[Mira]

Your instinct is right.  Take the easy answer.  At a minimum it violates the conservation of momentum in some fashion.

Yup, magic is the answer..

[Arjan]

Yup, magic is the answer..

Actually you can get mass from the nevernever as explained in skingame. You can temporarily store it there as well if I remember wok correctly.

[Mira]

Actually you can get mass from the nevernever as explained in skingame. You can temporarily store it there as well if I remember wok correctly.

I think more to the point, mastering the changes in mass and speed are part of the skin changer's art.  It could be that shifting atoms from one dimension to another accounts for it, so flying sparrow LTW can instantly become falling two ton elephant LTW.

[BrainFireBob]

Actually you can get mass from the nevernever as explained in skingame. You can temporarily store it there as well if I remember wok correctly.

Yes, but that mass should be created with potential, not kinetic, energy.

From a physics standpoint, the sparrow would slow down as its momentum and kinetic energy is shared with the new mass, which would then begin falling itself.

Of course, if LTW is smooth, he could time the change so that the elephant is nearly still from the extra mass, but so close that its new acceleration from gravity is still a considerable hit.

Or use extra magic to add matching velocity to the new mass as its added, that would also work.

[Arjan]

Because the new mass comes with acceleration included.

[vincentric]

Adding the mass changes acceleration but velocity is maintained. The only acceleration after the change is provided by gravity.

[BrainFireBob]

Adding the mass changes acceleration but velocity is maintained. The only acceleration after the change is provided by gravity.

Mass doesn't change acceleration at all. Momentum needs to be conserved, though, so unless the new mass is already accelerated to match, the same momentum is distributed over more mass, resulting in a slowdown.

EDIT: If you're interested, the relevant equation is the force of gravitation between two bodies.

F=m1a=G*( m1m2)/d^2

This means the acceleration due to the gravitational force is

a=G*m2/d^2

That's invariant from the frame of reference of m1.

Second edit: It's essentially a 100% nonelastic collision

[vincentric]

I did assume the added mass came with matching velocity. It depends on how magic works

[Avernite]

Clearly the proper frame of reference is the LtW frame of reference, in which the earth is rapidly approaching LtW. in this frame of reference the mass is just added at zero speed.

[morriswalters]

Einstein has officially rolled over in his grave.

[Arjan]

Einstein has officially rolled over in his grave.

Newton. This is all classical mechanics.

[KurtinStGeorge]

Mass doesn't change acceleration at all. Momentum needs to be conserved, though, so unless the new mass is already accelerated to match, the same momentum is distributed over more mass, resulting in a slowdown.

EDIT: If you're interested, the relevant equation is the force of gravitation between two bodies.

F=m1a=G*( m1m2)/d^2

This means the acceleration due to the gravitational force is

a=G*m2/d^2

That's invariant from the frame of reference of m1.

Second edit: It's essentially a 100% nonelastic collision

Correct, but Jim's description bothered me as well, because from what I've seen a hawk that dives on its prey still has its wings spread out a bit; by which I mean it's not in a complete freefall.  I live in a desert area in Southern Utah and often see hawks making lazy circles in the sky when I go on walks in the morning or if I go on a longer hike in the nearby hills.  They drop pretty damn fast when they dive on something, but they don't drop like a rock and I don't believe that they ever reach terminal velocity.  They control their speed and angle of decent, something an elephant could never do.

If the hawk turns into a elephant an instant; let's say two feet, before it hits the target that won't change anything of significance.  If it happens fifty feet away, then maybe it would.  It would take some math skills beyond mine, but I suppose it could be calculated.  I think even an experienced shape shifter would be hard pressed to be able to make such a calculation on the fly, or in this case, during the fall.   

[morriswalters]

Newton. This is all classical mechanics.

No this is magic. Physics need not involve itself. Jim even uses the fact in Cold Days in Harry's fight with Cat Sith on the boat.

Sith bounced off me, which was what I had hoped would happen. In my line of work, I’ve dealt with more than one critter that is faster than fast. When they’ve got their feet underneath them, it’s the next-best thing to impossible to land anything on them—but when they’re in the air, they’re moving at the speed gravity and air resistance dictate, like everybody else. For that one portion of a second, Sith was an object moving through space, not a blindingly fast killing machine. Someone who didn’t know that wouldn’t have known to be ready for it.

Butcher, Jim. Cold Days (The Dresden Files, Book 14) (p. 439). Penguin Publishing Group. Kindle Edition.

It's magic and you can't make it work as anything else.