A physics question about JFK's reaction

[Guest Mark Valenti]

Thanks to Robin Unger for posting the clear close-up of the shooting sequence in the Z film in another thread.

For the first time I really paid attention to JFK's right arm flailing in the air. My question: if he was shot from the front, wouldn't his arm have involuntarily straightened in front of him as his body fell, rather than bending toward himself as it does? Admittedly, I have no skills in this area - but getting hit from behind, it feels more natural that his arm would react as it did. Any thoughts from smart people out there?

[Jon G. Tidd]

In order to analyze the arm movement in terms of physics, one needs a good physical model for the arm (including its connection to the shoulder) and a complete description of the forces acting on the arm. I bet a bioengineering student could tackle this. I sure can't.

[Guest Mark Valenti]

Here's the sequence. Jackie's holding JFK's left arm, so it doesn't show any reaction.

But at 313, JFK's right arm flails TOWARDS his body, indicating a shot from behind.

[Ron Ecker]

It looks to me like his arm flails upward as if shot from the front. Why would it flail upward if shot from behind? Seems to me it flails toward his body, as you say, only because his whole body is going backward.

[Guest Mark Valenti]

Ron, that's what I was wondering about. Say you're pushed from behind. Your arm, which was not the object that was pushed (your back was) will appear to move toward your body as your body is pushed forward. If you had been pushed backward, wouldn't your arm straighten out in front of you? Also, it's possible this is a silly group of questions, I have no background in this area but just had a thought.

[Jon G. Tidd]

Mark,

I get your argument at post #4. It's based on Newton's First Law, which says a body remains either at rest or in constant motion unless and until acted upon by a force. That is, if JFK's body was flung forward by a bullet, the arm (the "body" in Newton's First Law) would tend to stay in place, ignoring the fact the arm is connected to the rest of JFK's body at the shoulder. So JFK's body would slam into the arm. This is loosey goosey, but it's an argument based on a bedrock law of physics.

FWIW, I don't believe any sound conclusions can be based on the extant Z-film, because I think it's a fake.

Edited May 24, 2015 by Jon G. Tidd