WEBVTT

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This is Giancoli Answers
with Mr. Dychko.

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There are three torque's shown on
this picture here;

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one due to each of these forces

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and there's also a fourth torque
due to friction

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but we don't know whether the friction is
counter-clockwise or clockwise

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until we have calculated the torque's due to
each of these three

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forces that are shown, these three
applied forces.

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So let's consider the clockwise
direction first:

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there are two forces that are clockwise

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this one and this one are both
making clockwise torques

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and so we have 35 newtons multiplied
by 0.12 meters—

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the perpendicular distance to
the axis of rotation—

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and then add to that 18 newtons times
its perpendicular distance

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to the axis of 0.24 meters

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that gives 8.52 newton meters.

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Counter-clockwise we have 28 newtons applied
at 0.24 meters from the axis

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giving us a counter-clockwise torque of
6.72 newton meters.

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So based on that analysis, we can see that
this wheel is gonna rotate clockwise.

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And so the friction is going to
oppose that motion

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and will be acting counter-clockwise
to oppose that clockwise motion.

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So the friction is gonna get added
to this counter-clockwise torque.

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So we are gonna add to 6.72
this friction of 0.6 we're told

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and that gives a total counter-clockwise
torque of 7.32 newton meters.

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So the total torque is counter-clockwise
minus clockwise because we assume

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clockwise is negative direction just like
for angles measured in

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standard position

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if you have an angle like this starting
from the positive <i>x</i>-axis,

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this angle <i>Θ</i> here is positive when
it's going counter-clockwise

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and the angle is negative if its clockwise

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so this would be like negative 45 degrees

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and this would be positive 45
degrees up here

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so clockwise negative that's why
there's a minus there.

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So we have 7.32 minus 8.52

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that gives a total torque of negative
1.2 newton meters;

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the negative means this net torque is
clockwise, 1.2 newton meters.