hey guys this video is the second or the
third installment of the rub Goldberg
video series that's going to help you
prepare for simple machines in your root
Goldberg project and this video is going
to be on the pulley it should be pretty
short and sweet uh just to remind you of
a few quick things we were looking at a
few different concepts where remember
that work is constant as long as there's
no friction so the input work is equal
to the output work that you do right and
so if we if we write out the equation
for work input force time input distance
equals output force time output distance
we can see that uh if you kind of
remember the different variables and
what they mean the box that you're
moving is the output force that's known
as the load right so it's like the
weight of the box that you're moving the
distance output is how high you end up
raising that box off the ground so it
could be say this little distance over
here um the input for is of course how
hard you're having to pull or push and
the input distance is over which
distance you pull or push so we're going
to look at some pulley here and let's
flip to the next slide and I'll add a
little picture so we can just start
talking about why pulley useful and why
are they considered simple
machines okay so let's get a picture of
a pulley
up so you can see here this guy is
pulling he's lifting a barrel right and
so right now let's label before we know
anything about pulley let's label some
of the things on this picture and and we
can use different colors so this is
going to be the output force and let me
get that to be a little bit thicker so
you can actually see it maybe in blue so
you can see it better so that's my
that's my output force right that's the
weight of the object that he's actually
lifting up so that we would label the
output force the output distance is this
right it's how high he has raised that
Barrel up off the ground within the pool
then we've got to label
his um input force which would be how
hard he's pulling on
here and the input distance and this
one's kind of weird um to think about
but it's actually how much rope he's
pulled through um if he started with the
very beginning of the Rope so we have
all those regions and if you think about
let's just do a quick little thing of
the incline plane it's kind of the same
thing right if I raise the Box up an
incline plane the Box itself is the
force output the distance that the Box's
going to be moved is the input distance
right over which my input force which is
how hard I'm pushing on the box it's
going to be raised some height
D okay distance out so you can kind of
see some of the similarities the barrel
is the force out that's the load that's
the same as the Box on the incline the
input distance is the distance over
which you apply your Force which would
be pretty much the the length of the
rope that you pull through the
pulley now let's take a look at this
pulley and see what is it actually being
used for so let me move
um actually that's okay we'll just look
at it here how hard is he having to pull
well actually in this picture he's
actually having to pull the exact same
weight as the barrel so the the pulley
here is not actually making his input
force any less cuz remember that's what
simple machines want to do they want to
make your input force less by increasing
your input distance right and that's
going to keep the work done the same so
basically you have to pull a whole lot
of string through the pulley but you're
going to have to pull with less effort
in this picture that's not happening the
only thing that the pulley is useful for
here and this is actually one of the
uses of a pulley and there are two uses
that you need to know one of the uses is
it just redirects the force so instead
of him having to stand up here somewhere
on a little platform and pull up he can
pull down just like blinds right and you
can end up redirect directing the force
to make it more convenient but it's not
making his input force less so let's
look on the next slide is how can the
pulley actually make something a job
easier to
do here's an example uh picture here we
have a fixed pulley which is that one
and then here we have a movable pulley
which is this one okay and you can see
the one on the left is not actually
being used for anything except to
redirect force and that's not considered
mechanical advantage um by sort of
numerical mean so it's not actually
making the input force less by numerical
amount so it's not making like say that
paint bucket is 10 lb you would still be
pulling with 10 lbs it's just you're
pulling in a different direction so it
makes it you know sort of more
convenient but it's not making it any
easier whereas the pulley system on the
right is actually making it easier which
is pretty cool and you can see that if
you really analyze the situation so let
me get a different color here and you
can see that the wall the the ceiling or
whatever this wooden thing is is
actually holding half of the the weight
the PT the paint can's uh weight so if
that's a 10 lb paint can this side is
holding 5 lb of
bit um you might remember some of our
Dynamics problems where we had like a
painting hanging on the wall and that
was about how it worked now this would
be exactly 5 lbs cuz it's a bit of an
angle but we're not going to consider
that so I'm having to pull up on this
side
and I'm only pulling with 5
lb even though I'm lifting a 10 lb
object so that's kind of interesting so
let's take a look at a more complicated
pulleys where this one's you you still
have to pull upwards and that may not be
that convenient so let's see if we can
even redirect it again to make the
pulling
less so here's a pretty crazy picture
just look at each one consecutively and
then we'll figure out how is the pulley
actually helpful so the first one on the
far left left this pulley you would
actually have to pull on this with 100
Newtons all it's doing is redirecting
the force however the next one the way
we know is we'll count these two
supporting strings now notice the pulley
on the
bottom this
one that one's movable so the box is
attached to the bottom of it and it's
not attached to the Rope the rope's
going through it and the way that I
would string that pulley is I'd actually
start by tying a knot and I'm going to
you can tra it with me I'm going to use
this green color we'd start by tying a
knot right here we'd Loop the string
around this
pulley we Loop it again around that
pulley and I'm going to pull straight
down like that now if you think of it
before the way that the last picture was
in the last slide you would have you
would have had what I'm about to trace
in yellow you would have had this and
you'd be pulling straight up but we
redirected it one more time so we could
pull and so here you're actually going
to be pulling with whatever force is on
this outside rope over here so that
would be the one
uh me draw this in this one's holding uh
50 Newtons and this one's holding 50
Newtons and you're pulling on that one
that's still 50 Newtons so that's you're
only having to exert Now 50 Newtons of
force which is pretty cool let's do the
next one here's even makes it even
better even more useful use a green
again to trace the string so now I have
a movable pulley on the bottom and I
have a double pulley at the top there's
you can get those where the the one this
pulley right here is actually U mounted
right next to that one to make it take
up less room but basically we're going
to trace we're going to trace from here
this is the string it goes around that
so it's attached it's fixed it's
attached right there this one goes
around that pulley this the string goes
around here We're looping it through
kind of like a yo-yo and then around the
top one and I'm pulling down like that
let's count how many strings we have
supporting we've got one that's going to
take some of the weight that's going to
take some of the weight and then that's
the weight you're pulling with right now
it's pulled redirected round so each of
those would be how much do you think if
this one was 100 divid 2 which is 50
each this one's 100 divid 3 which is
going to be about what 33 Newtons each
and that's how hard you'd have to pull
with 33 Newtons the last string is the
one you're pulling on it's just
redirected down similarly let's do the
next one Trace that one
out uh use this blue now you see we
flipped to attach right here now we've
got two double pulley the bottom one's
movable the top one's fixed on the
ceiling Trace out the Rope goes around
the
inside and then goes around the outside
and I'm pulling down like that so let's
count 1 2 2 3 4 so if I'm pulling on
this outside one right here right this
one on the outside that means that I'm
pulling with only you guess to 25
Newtons so if I have more pulleys
here more Loops of string I'm having to
do a lot less effort the very last one
we could even do it without tracing the
the string even though you can trace it
it starts in the middle goes to the
second ones goes to the outer ones we've
got one
1 2 3 4 five 6 we're pulling on the very
outside one right here that's redirected
so you take 100 and divide by six and
that's what you're pulling with so that
makes pulley easy so here's a little
trick let's go to the new a new slide
actually we can do it on this slide I'll
just move the picture up out of the way
a
bit I'll just use these as example to
find the mechanical advantage or I guess
I should say the ideal mechanical
advantage
you could do distance in over distance
out um and get it that way by measuring
how much string you pulled through over
how high it was lifted but there's an
easier way all you have to do
is say that the IMA is the
number of
supporting Loops or strings and they're
not separate strings that's why I say
Loops so on this first pulley over here
it would be 1 2 that's how many many
you're pulling with now or how many is
supporting it so that mechanical
advantage is two so it makes sense it
mechanical advantage is the number you'd
multiply your input force by okay so and
that would mean that would be right I'm
pulling with 50 Newtons multiply by two
that means I can lift 100 Newton
box so you don't count the string you're
pulling on because it's already counted
as this one right here right this one
that wraps around so on this one I'm
going to have 1 2 3 so that mechanical
advantage would be number supporting
Loops three very good and let's move
along this one would be one uh let me
see actually count that it's kind of
colored over the whole thing 1 2 3 4
it's a mechanical advantage of four so
pulley is pretty easy in that regard you
can actually combine pulley with other
simple machines like um if you have an
inclined plane and you have a pulley at
the top here you have a box you have a
pulley attached to the box and then you
do the little Loop pattern right you
pull around and like this you're
actually having to pull with a lot less
because the incline planes helping you
and the pulley helping you at the same
time so that's the video on pulley hope
that helps when you make your uh
complete your worksheet and uh good luck
with coming up with some designs in the
next installment