Simple Machines


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:

00:05
Hi. It’s Mr. Andersen and today I’m going to talk about simple machines. Before
00:09
we talk about simple machines however we should define what a machine is. A machine is any
00:14
device that uses energy to do work. One of my favorite machines is the bicycle. It allows
00:19
me to move around town very quickly when there’s not too much snow on the ground. Now that
00:24
we’ve defined a machine as anything that can use energy to do work, we should probably
00:29
define what work is. Work is, in science, defined as a force exerted over a distance.
00:36
And so let’s say I lift and an apple. An apple has a weight of 1 newton and I lift it 1 meter.
00:43
Well then the work is the force times the distance or 1 newton times 1 meter or 1 joule
00:50
of work is done. And I could use a simple machine to do such a task. So if we look at
00:56
the bicycle again, that machine, and we tear it down into its individual parts, what we’ll
01:01
find is that this complex machine is actually made up much simpler parts. And if we pair
01:07
that down to the simplest of simple parts, we call those simple machines. And there’s
01:12
some debate on what actually makes up a simple machine. The general list of six simple machines
01:18
kind of grew right of the Renaissance and the work of da Vinci on some of his earliest
01:23
drawings of machines. And so in general they’re defined as the lever, the wheel and axel,
01:30
the pulley, the inclined plane, the wedge and the screw. And a definition for what a
01:35
simple machine is is any mechanical device that changes the direction or the magnitude
01:40
of a force. Now you’ll find that some of the things were left off that list that you might
01:45
think of as simple machines. A gear is simple a detailed example of a wheel and axel. And
01:52
hydraulics is interesting. Hydraulics allows us to magnify our forces. And so we maybe
01:57
should include that in our list of simple machines. Now the more in detail you look
02:02
at simple machines, what you’ll find is that all simple machines are actually one of two
02:07
different types. They are either a lever and a wheel and axel and a pulley are examples
02:12
of a lever. Or they are an inclined plane. And a wedge is simply two inclined planes.
02:17
And a screw is an inclined plane wrapped about a cylinder. And so the easiest way to look
02:23
at simple machines is to look at these two types. And so first let’s talk about the lever.
02:28
You’ve learned about levers probably your whole life. The parts of a lever are going
02:31
to be the arm and then the fulcrum. And so there’s an old quote that says if I had a
02:37
lever long enough and a place to rest it I could lift the world. And so let’s look at
02:41
that lever. On one side you have the world or the earth. And on the other side you have
02:46
you. The fulcrum then sits right in the middle. And so you input a force on one side of the
02:52
lever. We call that the input force or F sub i. And you get an output force on the other
02:57
side. And that’s called F sub o. Or output force. And so we can look at any kind of a
03:04
lever and we can measure how well it does at magnifying your input force. And we call
03:10
that mechanical advantage. So mechanical advantage is defined as the ratio between the output
03:16
force and the input force. And so when you push down on one side of a lever, for example
03:22
with an input force of 2 and you’re able to magnify that and get an output force of 10,
03:29
that would be a mechanical advantage of 10 divided by 2 or 5. Now you probably know that
03:34
in science you don’t get anything for free. And so what’s the magic of a simple machine?
03:39
Well there’s not really any magic at all. Remember we defined work as force times a
03:44
distance. And so if you look at a lever, you may be applying a less force, let’s say a
03:49
2 newton force. But you have to apply that over a great distance. And so on the other
03:54
side, on the the output side, you may get a greater force but you’re going to only get
03:58
that over a less distance. And so our mechanical advantage can be greater than 1 if we’re ever
04:04
magnifying our force. Or it can be less that 1 if we’re trying to increase our distance.
04:09
So your arm for example has a mechanical advantage much less than one. What does that allow you
04:15
to do? It allows you to have a greater length of mobility. The other type of simple machine
04:20
is the inclined plane. And so the inclined plane allows us to talk about a term called
04:25
efficiency. So let’s say that we’re lifting a 2 newton force, or a 2 newton weight. And
04:31
we want to lift that 1 meter. But it’s too heavy for us to lift. And so instead of doing
04:38
that we use a inclined plane. Let’s say using a inclined plane, instead of applying a 2
04:43
newton force we’re able to apply a force of 0.2 newtons. The problem with an inclined
04:50
plane is that we have to actually drag it a great distance. Let’s say we have to drag
04:54
that 12 meters instead of 1 meter. Well we don’t have to pull as hard, and that’s the
05:01
reason that going up stairs is easier than climbing up a ladder. But we’re actually putting
05:06
more work in. And we’re losing some of that energy to the friction of the inclined plane
05:11
itself. And so there’s, when you say that a simple machine allows you to do more work,
05:18
it’s probably not true. What you’re doing is you’re actually doing more work by adding
05:23
a simple machine. But you’re able to change the direction or the magnitude of the force.
05:28
And so we use a term called efficiency. Efficiency is when the input work and the output work
05:35
are identical. And 100 percent efficient simple machine simply does not exist. We usually
05:41
have to put way more work into a machine than we’re able to get out of it. And so that’s
05:47
simple machines. And I hope that’s helpful.


This post was previously published on YouTube.

Photo credit: Screenshot from video.