With a little help from a free program, we're going to demonstrate the principle of Conservation of Momentum. The program simulates a 5-ball pendulum, that looks like this:

You can download this free program and play with it yourself. But more on that later; for now, let's look at what you can do with this pendulum. Start by raising one of the balls, and then let go:

The single ball has mass  m. When you let it go, it will hit the other balls with velocity  v.

What will happen when it hits?
Amazingly, exactly one ball will fly off the other end. This ball also has mass  m, and velocity  v.

The total momentum before the collision was  m · v
The total momentum after the collision is also  m · v

Momentum was conserved.

Try the same experiment with two balls. You will find that two balls fly off from the other side:

And again with three balls, momentum is still conserved. Whatever momentum existed before the collision must also exist after the collision. If three balls were moving before, three balls with the same total mass, and velocity, will be moving afterward.

We'll leave it to you to try this with four balls. The program simulates a real device available in your Physics classroom, and works exactly the same way. You can get this little pendulum program right here.

The program does NOT take friction into account. So unlike
a real pendulum, it will oscillate forever until you change something.

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