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Here we have a little block
that sits on that surface.

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And well, what can
one do with a block?

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You can push it,
or you can pull it.

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And that's exactly what
we're going to look at now.

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So I can exert a pushing force
onto this block here, F push.

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But I could also pull
it like this, F pull.

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And the question is, how can
we formalize this a little bit

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more?

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We can now also look at a small
piece of rope or a string.

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And I could, in the tug of
war, I'm going to pull here.

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And I'm also going to pull here.

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And we'll see who
is going to win.

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So we have two opposing
forces here on either side.

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In a slightly
different scenario,

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where we're going to put both
of these things together,

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we have a block here
sitting on a surface.

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And we have a little
string attached to it.

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And let's say we
have a pulley here,

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and the string goes
around there and has

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a little mass hanging here.

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We want to now describe
what this force is here

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that's pulling things.

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And for that, we have
to look at what's

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going on in that little string.

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So let's draw another string.

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And this is our string.

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And let's take an imaginary cut
right through the middle here.

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And I'm going to draw
both pieces here.

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This is the left part, and
here is the right part.

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And what's happening
in this rope here now?

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Well, there is a force
acting on the left object

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due to the interaction
with the right one.

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And here we have a
force on the right one,

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due to the interaction
of the left piece.

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And that, of course,
happens anywhere.

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I take a cut here
along the line.

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And we can even formalize
that a little bit more

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by just placing our
coordinate system here.

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And let's say x equals 0 here.

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And so for all x
along this line,

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we always have these
pairs of forces.

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So they are an interaction pair.

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And if I look at the rope from
afar, they will cancel out.

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But if I look at what's
going on inside the rope,

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then this is what they are.

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And we know from
Newton's third law

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that F RL equals minus F LR.

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So they're forces of
the same magnitude,

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but the opposite direction.

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If they weren't the same, then
my rope would get in trouble.

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But what we want to
define now actually

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is tension, the
tension force, that

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is along, that's
happening along,

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this rope here in our
tug of war if someone

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pulls from the outside.

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And for that, we
first got to look

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at the magnitude of our
interaction pair here, F RL.

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And that, of course, equals
the magnitude of F LR.

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And we're actually going to
define now this magnitude here

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as the tension force.

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And that is true for
all x along this line

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here, that we have
this, the magnitude,

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that this is the magnitude
of this force here.

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And from now on,
we're going to call--

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when we talk about
tension in the rope,

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then we talk about the magnitude
of one of these internal forces

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here.