SCIE 4001
3.5 Simple Machines
3.5 Levers
3.5 Wheel & Axle
3.5 Inclined Plane
3.5 Pulley
3.5 Screw
3.5 Wedge
 

Unit 3 STRUCTURE AND MECHANISMS

3.5 Simple Machines

What is a simple machine?

— People have developed a number of devices to make work easier.
— Often called the "six simple machines" they are the wheel and axle, the lever, the inclined plane.
— The pulley, screw and the wedge are actually just modifications, extensions and/or combinations of the first three.

How do we make work easier?

Recall: Work is done when a force moves an object in the direction of the force. This is accomplished by having these 'simple machines' perform one or more of the following functions:

— increasing the magnitude of a force (force multipliers).
— changing the direction of a force.
— increasing the distance or speed of a force (distance multipliers).
— transfer a force from one place to another.

— Sometimes we require a larger force than our muscles can generate. Some simple machines are able to magnify the forces produced by our muscles. These are called force multipliers.
crobar
Crowbar used to lift a heavy object
gclamp
G-Clamp to generate greater squeeze an object.
wedge
Wedge used to split a block.
 
— Sometimes it is more convenient to change the direction of a force..
crobar
Turning the wheel in a circular manner causes the force to become vertical to lift the bucket
gclamp
Using the pulley - pulling down a rope causes the load to rise up.
wedge
Two gears can change the direction of rotation
 
— Sometimes we want to increase the distance or speed of a force (distance multipliers).
crobar
Wheel and Axle combined with gears - The pedals move through a much shorter distance than the wheels, so we are able to ride longer distances than we can run.
gclamp
The inclined plane makes loading this barrel easier work.
wedge
Notice the smallest gear spins fastest.

All of our devices transfer force from one point to another.

So do we violate the law of conservation of Energy when we use these simple machines?

No! Work done is Force X distance in the direction of the force.
Ignoring friction we find that if we increase the force, we decrease the distance and vice versa.
In mathematical terms: Effort X distance moved by Effort = Load X distance moved by Load
Fe X de = FL X dL

E X de
=
L X dL

L  
dE
————
=
————
E  
dL

From this we can see that in the force multipliers (L > E) then the distance moved by Effort > distance moved by Load and vice versa.
In everyday language if you want a large force it can only move through a small distance. Probably a good example is the vehicle jack. Your small force (on the jack handle) moves by a much greater distance than the load (the weight of the vehicle).

How do we measure the ability of a simple machine to be a force multiplier?
Force multiplication is probably one of the more important uses of simple machines so we measure it by a quantity called Mechanical Advantage.

  L  
dE
Mechanical Advantage = ————
=
————
  E  
dL

What are the efficiencies of simple machines?
Friction is a fact of life. As long as surfaces slide past each other energy will be lost as heat due to friction. While friction may be cut down by using lubrication and wheels, no real device can be friction free. We measure efficiency of the simple machine as the ratio of output energy to input energy.

  OUTPUT
Efficiency = ——————
  INPUT
Concept by Kishore Lal. Programmed by Kishore Lal... Copyright © 2015 Kishore Lal. All rights reserved.