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### Course: MCAT > Unit 8

Lesson 5: Work and energy- Work and energy questions
- Introduction to work and energy
- Work and energy (part 2)
- Work and the work-energy principle
- Work as the transfer of energy
- Work example problems
- Work can be negative!
- Conservation of energy
- Work/energy problem with friction
- Intro to springs and Hooke's law
- Potential energy stored in a spring
- Spring potential energy example (mistake in math)
- Conservative forces
- Power
- Introduction to mechanical advantage

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# Introduction to mechanical advantage

Explore the concept of mechanical advantage with simple machines like levers. Understand how input force can be multiplied to output a greater force, albeit over a shorter distance, using the law of conservation of energy. Discover the relationship between force, distance, and work, and get introduced to the concept of moments. Created by Sal Khan.

## Want to join the conversation?

- Isn't this a geometry mistake? I mean, when the swing moves down, its first position and the final position do not make a right triangle but rather an isosceles one. You could not use trigonometric equations there. Or am I somehow wrong?(12 votes)
- you are right, for larger values of the angle theta this would not work. but in this method we assume that theta is very (in fact infinitely) small. Thus the error disappears. This method is called "virtual work", idk if sal mentions it. you could do this with the correct geometry and sould get the same result for F.(14 votes)

- I am having a hard time explaining to a friend that mechanical advantage doesn't mean you are doing less work. Just spreading the work (energy used) over a longer distance. In your video above you do not emphasize the law of conservation of energy.

Do you have a video that emphasizes that.(5 votes)- OK, I have watched the video. Say I am riding my bike (lots of mechanical advantage) from point A to point B at 10 mph. I walk the same distance at 3 mph. I should be using the same amount of energy plus the energy it takes to pedal the mass of the bike. If there is less friction with the bike I would think it would still take more energy. ?(6 votes)

- So, I see that at6:00, Sal uses Newtons both as a unit of force and a unit of weight. Will this work for other units? e.g. "pounds of force" or "tonnes of force"

If so, can this theoretically be done with any weight unit?(6 votes)- yes it dosent matter what unit of weight you use, as long as that wait is being applied to the whole object.(1 vote)

- Could one say that we are determining a ratio of work equivalency?

ex. 10N*1m = 1N*10m

We arrange Forces and distances to match: Work in = Work Out

... setting up newton *meter or joule congruence..(4 votes) - Guys please help I am lost. Perhaps it is sound silly, but I don't understand why the weight STARTS to move. When it is moving and the work is done it is all clear - the energy and work conservation law. But why does the lever multiply the force when the system is stationary? I mean why does the machine START to move? Sorry I don't know how to explain better...(4 votes)
- I believe that you're asking why and how the lever multiplies the force even when the system is stationary. It is because of the following principle of levers: for two forces F1 and F2 on opposite sides of the lever, where the distances from each force to the fulcrum of the lever are D1 and D2 respectively, the lever balances when the product of F1 and D1 is equal to the product of F2 and D2. Once the lever is balanced, it is in a stable state, and remains in such a state until it is disturbed.(1 vote)

- so if im correct if the object is in the same place no matter of how it got there the work is the same(2 votes)
- Yes, at least in this video and in most other physics situations, because the force you are trying to work against is gravity, which is a
**conservative force**. As long as the force is conservative, then the only thing that matters is the initial and final position of the object. The most important non-conservative force is friction. You can learn more about this concept by watching this KA video:

https://www.khanacademy.org/science/physics/work-and-energy/work-and-energy-tutorial/v/conservative-forces(4 votes)

- Does Khan Academy offers some practice exercises on this topic (besides the videos)? If not, can someone tell me where to find them?(3 votes)
- when the 10N object goes up, how do we know it goes straight up and not going in a track that is curved to the left?(2 votes)
- It doesn't matter whether it goes straight up or in a curve-y track.(2 votes)

- In the example in this video about a fulcrum, why couldn't Sal have used the equation M1D1=M2D2 and we could have gotten the same answer?(2 votes)
- Try it and see. Good way to learn. Usually there's more than one way to do a problem .(2 votes)

- if the 10 newton force is acting in the direction opposite to the direction of movement, shouldn't it be taken as -10N ?(2 votes)
- If you define the direction of movement as positive, yes. But you don't have to do that (although it is usually more intuitive if you do)(2 votes)

## Video transcript

Welcome back. We'll now use a little bit of
what we've learned about work and energy and the conservation
of energy and apply it to simple machines. And we'll learn a little bit
about mechanical advantage. So I've drawn a simple
lever here. And you've probably
been exposed to simple levers before. They're really just kind
of like a seesaw. This place where the
lever pivots. This is called a fulcrum. Just really the pivot point. And you can kind of view this
as either a seesaw or a big plank of wood on top of a
triangle, which essentially is what I've drawn. So in this example, I have
the big plank of wood. At one end I have this 10
newton weight, and I've written 10 in there. And what we're going to figure
out is one, how much force-- well, we could figure out
a couple of things. How much force do I have
to apply here to just keep this level? Because this weight's going
to be pushing downwards. So it would naturally
want this whole lever to rotate clockwise. So what I want to figure out is,
how much force do I have to apply to either keep the
lever level or to actually rotate this lever
counterclockwise? And when I rotate the lever counterclockwise, what's happening? I'm pushing down on this
left-hand side, and I'm lifting this 10 newton block. So let's do a little thought
experiment and see what happens after I rotate this
lever a little bit. So let's say, what I've drawn
here in mauve, that's our starting position. And in yellow, I'm going to draw
the finishing position. So the finishing position
is going to look something like this. I'll try my best to draw it. The finishing position is
something like this. And also, one thing I want to
figure out, that I wanted to write, is let's say that the
distance, that this distance right here, from where I'm
applying the force to the fulcrum, let's say that
that distance is 2. And from the fulcrum to the
weight that I'm lifting, that distance is 1. Let's just say that, just for
the sake of argument. Let's say it's 2 meters and 1
meter, although it could be 2 kilometers and 1 kilometer,
we'll soon see. And what I did is I pressed down
with some force, and I rotated it through
an angle theta. So that's theta and this
is also theta. So my question to you, and
we'll have to take out a little bit of our trigonometry
skills, is how much did this object move up? So essentially, what
was this distance? What's its distance in the
vertical direction? How much did it go up? And also, for what distance did
I have to apply the force downwards here-- so that's this
distance-- in order for this weight to move up this
distance over here? So let's figure out
either one. So this distance is what? Well, we have theta. This is the opposite. This is a 90 degree
angle, because we started off at level. So this is opposite. And this is what? This is the adjacent angle. So what do we have there? Opposite over adjacent. Soh Cah Toa. Opposite over adjacent. Opposite over adjacent. That's Toa, or tangent. So in this situation, we know
that the tangent of theta is equal to-- let's call
this the distance that we move the weight. soon. So that equals opposite over
adjacent, the distance that we moved the weight over 1. And then if we go on
to this side, we can do the same thing. Tangent is opposite
over adjacent. So let's call this the distance
of the force. So here the opposite of the
distance of the force and the adjacent is this 2 meters. Because this is the hypotenuse
right here. So we also have the tangent of
theta-- now you're using this triangle-- is equal to
the opposite side. The distance of the force
over 2 meters. So this is interesting. They're both equal to
tangent of theta. We don't even have to
figure out what the tangent of theta is. We know that this quantity is
equal to this quantity. And we can write it here. We could write the distance of
the force, that's the distance that we had to push down on
the side of the lever downwards, over 2, is equal to
the distance of the weight. The distance the weight traveled
upwards is equal to the distance, the weight,
divided by 1. Or we could say-- this
1 we can ignore. Something divided
by 1 is just 1. Or we could say that the
distance of the force is equal to 2 times the distance
of the weight. And this is interesting, because
now we can apply what we just learned here to figure
out what the force was. And how do I do that? Well, when I'm applying a
force here, over some distance, I'm putting energy
into the system. I'm doing work. Work is just a transfer of
energy into this machine. And when I do that, that
machine is actually transferring that energy
to this block. It's actually doing work on the
block by lifting it up. So we know the law of
conservation of energy, and we're assuming that this is a
frictionless system, and that nothing is being lost to
heat or whatever else. So the work in has to be
equal to the work out. And so what's the work in? Well, it's the force that I'm
applying downward times the distance of the force. So this is the work in. Force times the distance
of the force. I'm going to switch colors
just to keep things interesting. And that has to be the same
thing as the work out. Well, what's the work out? It's the force of the weight
pulling downwards. So we have to-- it's essentially
the lifting force of the lever. It has to counteract the force
of the weight pulling downwards actually. Sorry I mis-said it
a little bit. But this lever is essentially
going to be pushing up on this weight. The weight ends up here. So it pushes up with the
force equal to the weight of the object. So that's the weight of the
object, which is -- I said it's a 10 newton object -- So
it's equal to 10 newtons. That's the force. The upward force here. And it does that for
a distance of what? We figured out this object, this
weight, moves up with a distance d sub w. And we know what the distance
of the force is in terms of the distance of w. So we could rewrite this as
force times, substitute here, 2 d w is equal to 10 d w. Divide both sides by 2 you d w
and you get force is equal to 10 d w 2 two d w, which is
equaled to, d w's cancel out, and you're just left with 5. So this is interesting. And I think you'll see where
this is going, and we did it little complicated this time. But hopefully you'll realize
a general theme. This was a 10 newton weight. And I only had to press down
with 5 newtons in order to lift it up. But at the same time, I pressed
down with 5 newtons, but I had to push down
for twice as long. So my force was half as much,
but my distance that I had to push was twice as much. And here the force is twice as
much but the distance it traveled is half as much. So what essentially just
happened here is, I multiplied my force. And because I multiplied
my force, I essentially lost some distance. But I multiplied my
force, because I inputted a 5 newton force. And I got a 10 newton force out,
although the 10 newton force traveled for
less distance. Because the work was constant. And this is called mechanical
advantage. If I have an input force of 5,
and I get an output force of 10, the mechanical
advantage is 2. So mechanical advantage is equal
to output force over input force, and that should
hopefully make a little intuitive sense to you. And another thing that maybe
you're starting to realize now, is that proportion of the
mechanical advantage was actually the ratio of this
length to this length. And we figured that out by
taking the tangent and doing these ratios. But in general, it makes sense,
because this force times this distance has
to be equal to this force times this distance. And we know that the distance
this goes up is proportional to the length of from the
fulcrum to the weight. And we know on this side the
distance that you're pushing down, is proportional to the
length from where you're applying the weight
to the fulcrum. And now I'll introduce you
to a concept of moments. In just a moment. So in general, if I have, and
this is really all you have to learn, that last thought
exercise was just to show it to you. If I have a fulcrum here, and
if we call this distance d 1 and we called this
distance d 2. And if I want to apply
an upward force here, let's call this f 1. And I have a downward force,
f 2, in this machine. f 2 times d 2 is equal
to d 1 times f 1. And this is really all
you need to know. And this just all falls
out of the work in is equal to the work out. Now, this quantity isn't
exactly the work in. The work in was this force--
sorry, F2-- is this force times this distance. But this distance is
proportional to this distance, and that's what you
need to realize. And this quantity right here is
actually called the moment. In the next video, which I'll
start very soon because this video is about to end. I'm running out of time. I will use these quantities to
solve a bunch of mechanical advantage problems. See