Kind: captions
Language: en
[Applause]
You have made your prediction and now
it's time to see what happens when I
release the balance. Ready? In three,
two,
one. The balance tips towards the right
towards the hanging heavier ball. But
why does this
happen? Well, the best way I can think
of to explain this is that both balls
displace the same amount of water. So
they both experience the same upward
buoyant force which is equal to the
weight of the water they displace.
That's just Archimedes principle. But by
Newton's third law that means there must
be equal and opposite forces down on the
water in both beers. So you would think
that both be heavier by this same
amount. Now for the hanging ball, the
beaker does get heavier by this amount
because the buoyant force is now
supporting some of the weight that used
to be supported by this tension in the
string. but it's now reduced and so the
beaker actually has more weight. But for
the ping-pong ball, the downward force
on the water is almost entirely
counteracted by the upward force of the
tension in that string on the bottom of
the beaker. And therefore, the weight of
this beaker only increases by the weight
of the ping-pong ball itself. Whereas
for the hanging ball, the weight
increases by the weight of water it
displaces. So obviously, this beaker is
going to end up being heavier. Now, I
want to propose an additional
experiment. What if instead of tethering
the ping-pong ball to the base of this
beaker, I just got a free ping-pong ball
and submerged it with my finger just
barely under the surface of the water?
In that case, what do you think would
happen when the scale was allowed to
rotate? Would it tilt down A towards the
hanging acrylic ball or B down towards
the ping pong ball, which is now just
barely submerged under the water? or C,
would the balance remain perfectly
balanced? So, I want you to make your
selection, make your prediction by
leaving a comment starting with either
A, B, or C, and then giving me your
explanation. and I'll tally up the votes
and let you know the answer next time.