Mr. Andersen explains the simple principles behind simple machines. He shows how the mechanical advantage of a simple machine can increase the input force. A brief discussion of work is also included.
Transcript Provided by YouTube:
Hi. It’s Mr. Andersen and today I’m going to talk about simple machines. Before
we talk about simple machines however we should define what a machine is. A machine is any
device that uses energy to do work. One of my favorite machines is the bicycle. It allows
me to move around town very quickly when there’s not too much snow on the ground. Now that
we’ve defined a machine as anything that can use energy to do work, we should probably
define what work is. Work is, in science, defined as a force exerted over a distance.
And so let’s say I lift and an apple. An apple has a weight of 1 newton and I lift it 1 meter.
Well then the work is the force times the distance or 1 newton times 1 meter or 1 joule
of work is done. And I could use a simple machine to do such a task. So if we look at
the bicycle again, that machine, and we tear it down into its individual parts, what we’ll
find is that this complex machine is actually made up much simpler parts. And if we pair
that down to the simplest of simple parts, we call those simple machines. And there’s
some debate on what actually makes up a simple machine. The general list of six simple machines
kind of grew right of the Renaissance and the work of da Vinci on some of his earliest
drawings of machines. And so in general they’re defined as the lever, the wheel and axel,
the pulley, the inclined plane, the wedge and the screw. And a definition for what a
simple machine is is any mechanical device that changes the direction or the magnitude
of a force. Now you’ll find that some of the things were left off that list that you might
think of as simple machines. A gear is simple a detailed example of a wheel and axel. And
hydraulics is interesting. Hydraulics allows us to magnify our forces. And so we maybe
should include that in our list of simple machines. Now the more in detail you look
at simple machines, what you’ll find is that all simple machines are actually one of two
different types. They are either a lever and a wheel and axel and a pulley are examples
of a lever. Or they are an inclined plane. And a wedge is simply two inclined planes.
And a screw is an inclined plane wrapped about a cylinder. And so the easiest way to look
at simple machines is to look at these two types. And so first let’s talk about the lever.
You’ve learned about levers probably your whole life. The parts of a lever are going
to be the arm and then the fulcrum. And so there’s an old quote that says if I had a
lever long enough and a place to rest it I could lift the world. And so let’s look at
that lever. On one side you have the world or the earth. And on the other side you have
you. The fulcrum then sits right in the middle. And so you input a force on one side of the
lever. We call that the input force or F sub i. And you get an output force on the other
side. And that’s called F sub o. Or output force. And so we can look at any kind of a
lever and we can measure how well it does at magnifying your input force. And we call
that mechanical advantage. So mechanical advantage is defined as the ratio between the output
force and the input force. And so when you push down on one side of a lever, for example
with an input force of 2 and you’re able to magnify that and get an output force of 10,
that would be a mechanical advantage of 10 divided by 2 or 5. Now you probably know that
in science you don’t get anything for free. And so what’s the magic of a simple machine?
Well there’s not really any magic at all. Remember we defined work as force times a
distance. And so if you look at a lever, you may be applying a less force, let’s say a
2 newton force. But you have to apply that over a great distance. And so on the other
side, on the the output side, you may get a greater force but you’re going to only get
that over a less distance. And so our mechanical advantage can be greater than 1 if we’re ever
magnifying our force. Or it can be less that 1 if we’re trying to increase our distance.
So your arm for example has a mechanical advantage much less than one. What does that allow you
to do? It allows you to have a greater length of mobility. The other type of simple machine
is the inclined plane. And so the inclined plane allows us to talk about a term called
efficiency. So let’s say that we’re lifting a 2 newton force, or a 2 newton weight. And
we want to lift that 1 meter. But it’s too heavy for us to lift. And so instead of doing
that we use a inclined plane. Let’s say using a inclined plane, instead of applying a 2
newton force we’re able to apply a force of 0.2 newtons. The problem with an inclined
plane is that we have to actually drag it a great distance. Let’s say we have to drag
that 12 meters instead of 1 meter. Well we don’t have to pull as hard, and that’s the
reason that going up stairs is easier than climbing up a ladder. But we’re actually putting
more work in. And we’re losing some of that energy to the friction of the inclined plane
itself. And so there’s, when you say that a simple machine allows you to do more work,
it’s probably not true. What you’re doing is you’re actually doing more work by adding
a simple machine. But you’re able to change the direction or the magnitude of the force.
And so we use a term called efficiency. Efficiency is when the input work and the output work
are identical. And 100 percent efficient simple machine simply does not exist. We usually
have to put way more work into a machine than we’re able to get out of it. And so that’s
simple machines. And I hope that’s helpful.
This post was previously published on YouTube.
Photo credit: Screenshot from video.