WEBVTT

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So now that we've combined
pulley A, string 2, platform,

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and washer as our system, we
can now address our question.

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If we measure the
acceleration of the person,

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what is the force that the
person pulls the rope down

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with?

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Well, of course, that will just
be the tension in the string.

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And with this simple
system, we can now

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apply Newton's second
law, F equals ma.

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But recall, we need
some directions.

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So suppose we expect that the
acceleration of the platform

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and person is up.

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So we'll choose a unit vector j
hat in the positive direction.

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And now the problem
becomes tension one,

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three different tensions,
3T1, and gravitational force

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minus mp plus mw times g.

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Now, what is the mass
that we have to consider?

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Again, what is the
mass of our system?

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Well, the platform
and the person,

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and we assume the pulley and
the string, too, were massless.

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So we have simply mp plus mw a.

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And so we can now solve for the
tension in the string, which

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is equal to mp plus mw
times g plus a divided by 3.

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And recall that this tension,
that the string is pulling,

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this is what we called
the force that the person

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[? of ?] the string
on the person.

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And by Newton's
third law, that's

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also, on the washer, that's
also the force that the washer

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applies to string 1.

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So this was our goal.

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It's the force that
the washer applies

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to string 1 by the third law.

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And so by thinking about
how to choose a system, what

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could be a very
complicated problem,

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with lots of equations,
is simply one equation.