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Overview
Crusader Education’s Wedge is crafted from durable hardwood to provide a long-lasting piece of
laboratory equipment that lets students explore
the basic concept of this simple machine. The
hinged boards and the narrow wedge allow students to clearly experience the force amplification that this simple machine makes possible. The
addition of elastic bands allows for quantitative
measurements.
The Wedge: A Simple Machine
Most simple machines have one basic goal: to allow you to perform a task using less force than you would otherwise need. The
wedge is one of the oldest of the simple machines. It is really a
stone age tool that has been used for thousands of years for stripping bark from trees and splitting logs.
And the wedge is a simple machine that you see very, very frequently, though you might not think of it! Our office space has
wedges by every door, that we use to hold the doors open! The
force that we apply to the
end of the wedge turns
into a very large force
between the wedge and
the floor that keeps it
from sliding.
Of course, wedges are also used the way they always have
been: to split wood. Of all of the simple machines, the
wedge is the one that is the least changed over the years
and the easiest to recognize!
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Physics Principles
Suppose you raise a maul (a heavy wedge on a handle) over
your head, and bring it down on a piece of wood that you want
to split. Two things happen:
1) The wedge turns the vertical motion of the wedge into a horizontal force. The wedge, like other simple machines, changes
the direction of a force.
2) The wedge works a bit like a ramp, or an inclined plane. The
wedge needs to move a long way in order to split the wood just
a small amount. This means that the force is amplified—a small
force on the wedge turns into a large force on the wood. You
can’t split a log with your bare hands, but the wedge amplifies
the force so that the force of your body is all that is needed!
Experiments
Part I: The Basic Principle
Force Amplification
Step 1: Put several rubber bands on the outer of
the two sets of pegs. Now, lift up the top board,
and feel the resistance. Now, force the boards
apart by sliding the wedge between them. This
is easier—and if you oil the wood so that the
wedge slides more easily, it’s even easier! (You
can use lemon oil for this—the kind meant for
finishing furniture!)
Ask your students to note the motion. The
wedge moves a long way to raise the top board
by a small amount; that’s why it gives as much
force as it does. A small force over a long distance is turned into a large force over a small
distance, just as for other simple machines. A
skinnier wedge would move even farther, and so
would provide even more force amplification.
So: The force is amplified, and it is directed in
another direction, both of the key elements of a
simple machine.
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Step 2: Next, do the same experiment as above,
but place the rubber bands on the inner pegs.
Will it be easier or harder to force the boards
apart? As you can see, the rubber bands don’t
stretch as much, so it will be easier to force the
boards apart than in step 1.
1. Increase the number of rubber bands. How
does this change the force necessary to move
Here are some other variations you could make the wedge?
on this basic experiment:
2. Rather than use the rubber bands to make a
force, place a weight on the top board. Now
you can lift the weight with the wedge!
Part II: A Bit More Detail
Other Uses of the Wedge
Step 1: Rather than pushing the wedge, tap it
with your hand. Now you are changing the impulsive force of your blow into a much larger
force moving the boards apart. An even gentle
tap will make a remarkably large force! It’s
easy to force the boards apart even with all of
the rubber bands in place.
Step 2: Use several rubber bands on the outer
set of pegs, and drive the wedge in between the
two boards. Now, let it go. It will stay in place!
This is how a door wedge works. The friction
between the wedge and the boards is enough to
keep it from sliding, so you can use a wedge to
prop a door open. A shallower angle makes a
more effective prop. Can you see why?
1. How could you increase the force of the
wedge? (You can make a skinnier, longer
wedge. This will amplify the force even
Here are some questions to ask your students
more.)
about this exercise:
2. Can you think of practical examples of
wedges? (There are many—one example is
on the next page.)
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Extensions
Where do you see wedges? We’ve seen a few examples; here’s one more to get you thinking!
Wedges in the Kitchen: The Knife
A Wedge Makes an Opening
You often see wedges used to make an opening.
A knife is a wedge: it is thicker at the top than at
the blade. As it moves through a banana, it forces
it apart. Scissors are just two wedges that work
together.
What other examples can you think of in which
you use something that is skinny on one side and
thicker on the other that you use to make an opening by forcing in the skinny side?
And you use this principle in less obvious ways as
well. Think about how you hold your arms when
you dive into a swimming pool...
This guide written and illustrated by Brian Jones,
Little Shop of Physics, Colorado State University
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