Kind: captions
Language: en
But this is going to be really tough to
see. So, how are we going to actually
determine what the right answer is?
Uh,
if I were to drop it now, it would
happen so fast you wouldn't really see
clearly what's happening. So, I've
brought along my slow motion camera and
you'll see it at 300 frames pers and
it's quite spectacular.
Well, that's ultra slow-mo. So, that's
exactly what we need to sort out this
problem.
We'll give a a countdown cuz it happens
really fast.
All right. Three, two, one, drop.
Wow. Did you see that?
I I I I didn't really see which happened
first. No, I needed need to slow that
down.
Well, let's go to a slow motion replay
and see what actually happened.
[Music]
So from the slow motion camera, we can
clearly see that the bottom end stayed
completely stationary even after you you
let go of the top. It waited until the
whole slinky had collapsed down to the
bottom before it itself moved downwards.
How do we explain that? Well, there are
a number of explanations, but the
simplest explanation is that the bottom
end is sitting there minding its own
business with gravity pulling it down
and tension pulling it up. Equal and
opposite forces, no motion at the bottom
end until
the bottom end gets the information that
the tension has changed. And it takes
time for that information to propagate
down through the slinky to reach the
bottom end. So, it's propagating down as
a compressional wave. And we saw that
compression wave travel down and it has
to reach the bottom before the bottom
even knows that you've let go at the
top.
Correct.
And that's when it knows to start
falling.
Correct.
That's a really remarkable finding. I
mean, does this apply to any other
objects or is it just slinkies?
Uh, no. It applies uh to the real world
uh particularly in sports, which is the
field I'm interested in. For example,
when a player hits a ball, there's a
huge force at the business end, but that
force is not felt at the handle end
until the the ball is well on its way.
So that a wave has to propagate from the
business end down to the handle end and
then it propagates back again. And what
you actually feel down this end is
considerably less than what the ball
felt.
So wow. If you're playing tennis or
something, you only feel that you've hit
the ball after you've actually hit the
ball.
Yeah, the ball's nearly to the net by
the time you actually feel what's
happened.
That goes against, you know, all your
intuitions that you really can feel it
as soon as the ball's on your on your
racket.
Uh it's the same in golf. If you whack a
golf ball, you often find golfers will
finish with a a nice flourish thinking
that it has some effect on the ball, but
of course the ball was halfway to the
hole by the time that happens.
Of course. Now, what if we wanted to do
a little extension activity? I want you
to make a prediction. If we attach this
tennis ball onto the base of the slinky
and we uh drop it again, what will
happen to the tennis ball? Will it do
the same as the base of the slinky and
just stay there? Or will it fall with
the acceleration due to gravity uh g or
will it go upwards?
Well, I have to try it and find out.
All right, I'd like you to make your
prediction now. Quick.