Suppose
you
wanted
to
____________________
a
small
rock
,
you
couldn't
do
it
just
by
wishing
rock
with
move
it
,
and
you
couldn't
do
it
by
____________________
,
and
you
couldn't
do
it
by
ordering
rock
to
move
.
There's
only
one
____________________
you
could
make
the
rock
move
and
that's
by
making
a
force
act
on
it
.
What's
a
____________________
?
That's
something
that
pushes
or
pulls
something
else
,
like
the
force
from
a
hand
pushing
the
rock
or
____________________
it
.
It
doesn't
matter
what
does
the
pushing
or
pulling
,
as
long
as
it
applies
a
large
enough
force
it
can
move
the
rock
.
In
fact
,
the
rock
can
even
be
moved
by
a
force
that
you
can't
see
,
if
you
do
this
for
instance
.
When
you
do
that
,
the
force
moving
the
rock
is
called
____________________
.
Gravity
is
a
force
pulling
the
rock
toward
the
center
of
the
____________________
.
Gravity
pulls
everything
else
in
the
world
toward
the
center
of
the
earth
also
,
including
you
.
Gravity
was
pulling
on
the
rock
on
the
land
of
course
but
when
the
____________________
was
there
that
kept
the
rock
for
moving
down
.
If
you
remove
the
table
,
you
allow
the
force
gravity
to
move
the
rock
.
Yeah
!
and
when
you
____________________
the
rock
up
again
,
you
use
another
force
this
time
to
____________________
it
.
So
whenever
you
make
this
rock
move
you
do
it
by
making
force
acted
on
it
.
In
fact
,
anytime
anything
at
all
it's
not
moving
starts
moving
it's
because
some
force
acts
on
it
.
