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On Feb 8, 2019, at 07:15, John Denker via Phys-l <phys-l@mail.phys-l.org> wrote:
On 2/8/19 2:31 AM, Steve Spicklemire via Phys-l wrote:
This is an interesting conversation!
:-)
Perhaps it’s naive to think it, but I feel that when you can write something like:
dp/dt = Fnet
it means that the *future* p depends on Fnet and p *now*. So:
p_future = p_now + Fnet_now*dt
So I think one could argue that p_now and Fnet_now “cause” p_future, and not the other way around.
no?
What’s the trouble with such a point of view?
The math is not wrong as far as it goes, but it's only half of
the story:
1) Yes, using the differential equation, you can integrate forward in time.
2) You can equally well integrate backward in time! Like this:
p_past = p_now - Fnet_now*dt
Bottom line: Newton's laws tell us just as much about the past
as the future. Since causes must precede effects, Newton's laws
cannot be statements about causation.
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