(bouncing) - Okay, let's dive into
the use of particles for simulating water. (water splashing) Before talking about how
lots of particles move, let's start by looking at
how one particle moves. If I drop a particle, or a ping pong ball, it accelerates towards the ground. We also saw this kind of
motion in the animation topic. Notice that it bounces back up, but not quite as high as it started. It loses some energy during the bounce, and eventually when it loses
all of its energy, it stops. Now let's look at our
ping pong ball simuator where we model each ping pong
ball using a virtual particle. The virtual particle accelerates downward until it hits an obstacle, and then it bounces back up,
only with a little less energy. With this simulator,
we can control how big the particle is using this slider. We can also control how strong the gravity is using this slider. A small gravitational value
similar to the gravity found on the moon causes the particle to fall more gradually. A larger gravitational value, like the gravity here on Earth, causes the particle to fall
at a more familiar rate. And sending it to a very
large gravitational value, like on Jupiter, makes the
particle fall really quickly. We can also control how much
energy the particle loses during a bounce, by
controlling its elasticity. It describes a fraction of energy that is retained when
the particle bounces. Elasticity is a number
between zero and one. A value of one means the
particle loses no energy. It'll bounce forever. This is called perfect elastic collision, and it's something that
doesn't happen in real life. A value of zero means the
particle loses all of its energy. In this case, sticking to the floor. Intermediate values give the
particle more realistic motion. Elasticity is also used
to control the energy loss when particles hit each other. If we set elasticity to zero,
then the particles will lose all energy when they collide. Like this, similar to a pile of marbles. And as we increase the elasticity, we get something that looks
more like rubber balls. And of course, you can also change the size of the particles. Let's pause here so you
can use the next exercise to get some experience with
the range of particle behavior you can achieve by
changing the particle size, gravity, and elasticity. (playful music)