Levers and Levers in
human body
Lever
• Lever is a length of solid materials that is use
to apply force to another object
or
• Rigid structure that rotates around a fixed
point
• Levers has 3 parts
– Force (effort)
– Resistance (load)
– Axis (fulcrum)
• FA (Force arm
– Distance between force and axis
• RA (Resistance arm)
– Distance between resistance and axis
• Arrangement of Force arm & Resistance arm
determines type of lever
Types of Levers
• There are 3 types of levers
– First class levers
– Second class levers
– Third class levers
First class Lever
• Axis located between force and resistance
• If axis is closer to resistance, RA shorter and FA
will be longer.
• Easy to move
• If axis is closer to force opposite will occur
• Most efficient class of lever
• BALANCE
Examples for 1st class levers
• Seesaw
• Scissor
• Hammer removing a nail
Examples in human body
• Skull pivoting on the atlas vertebrae of the
spine, with weight of the head held by the
trapezius and sternocleidomastoid muscle of
the muscle of the neck.
Second class Lever
• Axis at one end, resistance in middle, force at
other end
• POWER / force
Examples for 2nd class levers
Examples in human body
• Action of the ball of the foot with the
gastrocnemius & soleus muscles of the
calf lifting the weight of the body,
which acting through foot during
plantar flexion
• Push ups
Third class levers
• Axis at one end, force in middle, resistance at
opposite end
• Most common in the body
• ROM / Speed
Examples for 3rd class levers
Base ball bat
Opening a door
Examples in human body
• Action of biceps muscles as it lifts a load in the
hand whilst pivoting elbow
• Action of hamstring muscle
MECHANICAL ADVANTAGE
•
•
•
•
•
Ratio between force arm and resistance arm
MA = FA/RA
Force Arm > Resistance arm= MA is >1
Force arm < Resistance Arm = MA is <1
The Greater the Mechanical Advantage, the
less force is needed to cause motion!
Calculate the Mechanical Advantage
• A man is lifting 4kg from his elbow.
Length of forearm up to the weight is
15cm and length of the biceps muscle is
30cm.
• Athlete is doing pushups. Distance from
his toes to COG is 90cm and toes to
hands on pushups are 150cm.
Mechanical disadvantage
• In third class levers due to the attachment of
muscle close to the fulcrum much more force
than load force should be provided to lift the
weight
Pulleys
Pulleys
• Definition
- a simple machine that consists of a rope that fits
into a groove in a wheel.
- can change size, direction of input force or both
• 3 Types of Pulleys
- fixed pulleys
- moveable pulleys
- pulley system
Single Fixed Pulley
• This pulley provides the user
Directional Advantage, allowing
someone to pull down to lift the load
up.
• Is Mechanical Advantage
provided with this system? No
• Effort needed to lift this load is equal
to the weight of the load.
• Load is supported by only 1 rope arm.
• Have to pull an amount of rope equal
to the height you wish to lift the load
Single Moveable Pulley
This pulley system provides Mechanical
Advantage, requiring only ½ the effort
needed to lift the weight of the load.
The load is supported by 2 rope arms under the pulley.
Each load arm supports half the weight of the load, so
the arm pulling up uses only 10 lbs of effort to lift this
load.
Length of pull twice as much rope as desired to lift the
load a certain height. Example: Pull 2 ft of rope for each
1 foot of height that want to lift the load.
Single Fixed/Single Moveable
Pulley
This pulley system combines the properties
of their single components into a system that
provides both Mechanical and Directional
Advantage.
This pulley provides Directional Advantage by allowing the
user to pull down to lift the load. It also provides
Mechanical Advantage by using 2 rope arms to support the
load, reducing the effort needed to lift the load by ½.
In this system length of pull 2 times as much rope for every
unit of height that you want to lift the load.
Pulley Power!
Single-Fixed Pulleys provide or Directional Advantage.
You can make use of Counter-Weights in order to reduce
your effort needed to lift the load. However, your effort
PLUS the counter-weight together must be equal to or be
more than the weight of the load being lifted.
Single-Moveable Pulleys provide Mechanical
Advantage. You must pull 2 times the distance in rope
that you wish to lift the load because the load is
supported by 2 rope arms. You gain a Mechanical
Advantage of 2 because you are using only ½ as much
effort to lift the load.
.
Combining Single-Fixed and Single-Moveable into
Compound Pulley Systems allows the user to gain
both Mechanical and Directional Advantage. The
more rope arms supporting the load the greater the
Mechanical Advantage