Hi. It’s Mr. Andersen and this is AP Physics
essentials video 75. It is on systems. And
this is the first of 25 videos that are going
to deal with the conservation laws which are
best described by Richard Feynman as things
in science that stay the same. So his analogy
is imagine you have a kid whose playing with
blocks. The number of blocks, we will say
is equal to n. So let’s say it is 20 blocks.
And so this kid, no matter how they put those
blocks together the number of blocks, 20,
is going to be conserved. It is going to stay
the same. And if we were to go in there one
day and find 19 blocks that would mean that
one of them has been lost. And if we find
30 blocks they must have been brought in from
somewhere else. And so figuring out in physics
what are the blocks and then more importantly
what is the system, what is the room where
these blocks are contained is important if
we ever hope to solve the problems. And so
a system is going to be two or more objects
that are separated from their environment.
But if that whole system has no internal structure
then we just count that as an object. And
so if we have a system, a series of two or
more objects separated from their environment,
whatever is inside that system is going to
be conserved. It is going to behave according
to these conservation laws. Now those blocks
or those objects or that energy could be energy
that is conserved. It could be charge. It
could be momentum. It could be linear. It
could angular momentum. They are going to
stay the same within that system. And if it
is not, then we have ourselves an open system.
And so a system is a collection of objects.
And so in this case we have a pulley system.
So we have the pulley, we have the rope, we
have the weights on either side of it. So
we count that as a system made up of objects.
But if we were to take one of those objects
and look inside it, we know that it is made
up of iron which is atoms. And those atoms
have subatomic particles. And then we have
fundamental particles inside of that. And
so why do we treat this as an object? It is
because it has no internal structure, at least
no internal structure that is important in
solving this problem. So we simply treat it
as an object within this greater pulley system.
And so did you learn that a system is a collection
of objects that have not internal structure?
I hope so. And I hope that was helpful.
