What is a mechanism?
To
understand levers we must first understand the question: What is a mechanism? A
mechanism is a device which sends energy in the form of movement, and which
changes an input in motion, and force, into a desired output motion and force.
One example of a mechanism is a lever, such as a crowbar which can be used to
pick up a heavy object like a manhole cover. The multiplication of a force is
called mechanical advantage.
The
technological advances made by people through the ages have been closely linked
with their ability to harness energy and use it to perform mechanical work.
Although people are very clever creatures, our size, structure and muscles put
severe limitations on what we can and cannot do. Mechanical work is done by a
machine, which converts (changes) or transmits (passes on) energy. If we
connect mechanisms together we can build mechanical systems, which we call
machines that form a number of working parts (components) which we collectively
call mechanisms.
Simple
items such as scissors, bottle openers and tweezers are all mechanisms that we
use every day. Mechanisms are often used to allow a small effort to move a
large load. For example, a car jack allows us to lift a car to change a tyre.
The ability of mechanisms to make effort easier for us is known as mechanical
advantage. It is usually written as ratio, and is calculated by dividing the
load with the effort.
Mechanical
advantage= Load
Effort
Mechanisms used in this way are called force
multipliers. There are three different classes of lever, and each class
provides mechanical advantage. The levers are referred to as first-class,
second-class, and third-class, and will be discussed later.
What
is a lever?
A lever is made of a
stiff bar or rod that turns around a fixed point called the pivot or fulcrum on
which it rests, and is a simple machine or mechanism. The effort (pull or push)
is applied to the lever at one point in order to move a load at another point
of the lever. As mentioned earlier, the three types of lever are first-class,
second-class, and third-class. Each class has different positions for the
force, load and fulcrum (pivot point). The longer the rod (lever) the easier it
is to lift the weight. Normally the person would not be able to lift the weight
at all. The fulcrum is the place where the rod pivots (or rotates). The load is
the scientific name for the weight. The effort is quite simply the amount of
effort used to push down on the rod in order to move the weight
A lever consists of a rigid bar, which is free to turn about a
fixed point called a fulcrum. The fulcrum is a pivot point. The effort force
(push or pull) is exerted upon one lever arm, and the other lever arm will go
up or down in the opposite direction. The resisting weight is the load, which
is exerted upon the lever arm, which tends to move the lever in the opposite
direction of the applied force. The
force is applied at a different point from the load. The closer the fulcrum to
the load, the less force needed to lift the load. The force will move a greater
distance, and the load will move a shorter distance. The closer the fulcrum to
the force, the greater the force needed to lift the load. The force will move a
shorter distance, and the load will move a greater distance.
The definitions for the parts of a lever are as
follows:
v Effort: the pull or push force
v Pivot or fulcrum: the
point around which the load and effort turns
v Load: the object which
resists or works against the pulling or pushing force
First Class Lever
What Are First Class Levers?
In a first class lever, the fulcrum is placed
between the applied force and the load. A balance scale is one common example
of the first class lever. In this case, the fulcrum is located exactly between
the force and load. Note that the load in this example is not a resultant force
as it is with most lever applications. Balance scales can be used to compare
the weight of two objects or compare the weight of one object to a known
weight. If the force and load are equal, the lever arm will balance horizontally.
Common
examples of first-class levers include crowbars, seesaws, scissors, pliers, tin
snips (in pairs).
Mechanical
advantage
Levers
allow us to use a small effort to move a heavy load. This is known as
mechanical advantage and depends on:
v The length of the lever and the position of the fulcrum
v The positions of the effort (or force) and the load.
v The length of the lever and the position of the fulcrum
v The positions of the effort (or force) and the load.
The
distance between the effort and the fulcrum, and also the load and the fulcrum
results in the mechanical advantage and velocity ratio of the first-class
lever.
