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1ºESO - MECHANISMS

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by

Moisés Llorente Santos

on 22 April 2015

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Transcript of 1ºESO - MECHANISMS

DICTIONARY GAME (in groups)
RULES:
UNIT 6 - MECHANISMS (pages 102 - 112)
1. MAKE UP A DEFINITION FOR A GIVEN WORD, AND WRITE IT ON A PAPER (5 MINUTES).
2. THE TEACHERS WILL MIX ALL YOUR DEFINITIONS WITH THE CORRECT ONE AND, THEN, WE'LL READ OUT ALL OF THEM.
3. NEXT, EACH GROUP WILL DEBATE FOR 1 MINUTE TO MEET AN AGREEMENT ABOUT THE DEFINITION THEY THINK IS THE CORRECT ONE.
4. YOU'LL GET 1 POINT PER EACH GROUP VOTING YOUR DEFINITION.
5. IF YOUR GROUP GUESSES THE CORRECT ONE, YOU'LL GET 2 POINTS.
6. IF NOBODY GUESS THE CORRECT
DEFINITION, TEACHERS WILL GET
1 POINT.
GROUP 1
GROUP 2
GROUP 3
GROUP 4
GROUP 5
CLAIRE &
MOISÉS
MECHANISM
TO BALANCE
EFFORT
GEAR
LEVER
FULCRUM
TOTAL
THE WINNER GROUP WILL
HAVE A POSITIVE!!!
MECHANISM:
A mechanism is something that transmit and convert forces to help us to carry out some tasks easily.
TO BALANCE:
To mantain a body or object in a state of equilibrium.
EFFORT:
Is the use of physical or mental power to do something.
GEAR:
Is a wheel with teeth used in some mechanisms.
LEVER:
Is a rigid bar used to lift something heavy.
FULCRUM:
Is the point that supports a lever.
1ºA - winner group 2
WHY HUMANS NEED MECHANISMS?
MECHANISMS

are devices that transmit and convert forces and motion, helping us to carry out some tasks with less effort.
PARTS OF A MECHANISM:
Mechanism
Driving
force
Output
element
The effort you apply
The result you get
Identify the two parts in this pulley
Mechanisms that
transmit motion and
forces
Linear motion
Rotary motion
Levers
Pulleys
Friction drives
Pulley systems
Gear systems
Mechanisms that
transform motion
and forces
From rotary into
linear
From rotary into
reciprocating
Rack and
pinion
Crank-link-slider
Mechanisms
classification
1.1 Linear motion mechanisms (page 102)
LEVERS: A lever is a rigid bar that is used with a point of support or a fulcrum to allow us to lift heavy objects easily.
PARTS OF A LEVER:
1. The rigid bar.
2. The fulcrum.
3. The force you apply (effort, F)
4. The resistance you want to lift (R).
F
R
5. The distance between the force and the
fulcrum (d)
6. The distance between the resistance and the
fulcrum (r)
d
r
WHAT'S THE NEEDED FORCE TO GET THIS LEVER BALANCED?
F
R
d
r
F x d = R x r
Law of the lever
Calculate F to balance this lever:
F = ???
R = 50Kgs.
d = 2m
r = 1m
EXAMPLES OF LEVERS
corkscrew
paper cutter
scissors
wheelbarrow
CLASSES OF LEVERS
Class 1:
fulcrum between F and R.
Class 2:
R between F and fulcrum.
Class 3:
F between R and fulcrum.
Example: seesaw, scissors.
Examples: wheelbarrow, corkscrew.
Examples: fishing rod, tweezer.
F? R? Fulcrum?
It's time to practise!
http://phet.colorado.edu/sims/html/balancing-act/latest/balancing-act_en.html

Exercise 1
: Calculate the needed effort (F) to lift a weight (R) of 50Kgs using a class 1 lever if the length of the effort arm (d) is 8m. and the length of the resistance arm (r) is 3m. Draw the lever.
Exercise 2
: What might be the length of the resistance arm (r) in a class 1 lever to lift a 75Kgs. resistance (R) using a 5 metres length effort arm (d) and a force of 45 kgs.. Draw the lever.
Sol.: F = 18,75 Kgs.
Sol.: r = 3 m.
Reinforcement exercises
1. Draw in your notebook a class 1, class 2 and class 3 lever, indicating F, R, d and r on each drawing.
2. Calculate the needed length of the effort arm (d) in a class 2 lever if you want to lift a 100Kg resistance (R) using a force (F) of 25 Kg. The length of the resistance arm (r) is 2 m. Draw the lever.
1.1 Linear motion mechanisms (page 102)
PULLEYS: A pulley is a wheel with a groove for a rope, chain or belt to go round it, with the purpose of reducing the effort needed to lift a load.
FIXED PULLEY
MOVEABLE PULLEY
A fixed pulley consists of an only wheel with a groove for a rope or chain. The effort (F) needed to lift a load (R) is...
F = R
F
R
A moveable pulley has two pulleys, one is fixed while the other can move. The effort (F) needed to lift a load (R) is...
F =
R
2
F
R
Reinforcement exercises
1. Calculate the needed effort to lift a 250Kg load (R) if you use:

a) A fixed pulley.

b) A moveable pulley.
Fixed and moveable pulleys
2. Rotary motion mechanisms (page 104)
FRICTION DRIVE
A friction drive consists of two or more wheels that are in contact. This mechanism is used in industry, to make metal sheets or rolls of paper.
In a rotary motion mechanism, input and output elements trace a circular trajectory.
D1
N1
D2
N2
D1: diameter of wheel nº1 (input wheel)
D2: diameter of wheel nº2 (output wheel)
N1: velocity of wheel nº1 (input wheel)
N2: velocity of wheel nº2 (output wheel)
Gear ratio equation (must fulfill):
N1 x D1
=
N2 x D2
Units of N1 and N2???
r.p.m.
The input wheel (wheel nº1) of a friction drive system rotates at 250 rpm. The diameter (D1) of this wheel is 15 cm. The diameter of the output wheel (D2) is 45 cm.

a) Draw the mechanism, indicating the input and output wheels and their rotation direction.




b) Calculate the velocity of the output wheel (N2).
EXAMPLE EXERCISE:
Example of a real friction drive system:
a paper press
PULLEY SYSTEMS
These are two pulleys that are a certain distance apart, rotating simultaneously because of a belt.
D1
N1
D2
N2
Gear ratio equation (must fulfill):
N1 x D1
=
N2 x D2
Example of a real pulley system: a washing machine engine.
Calculate the diameter of the output wheel (wheel nº 2) in a pulley system if it rotates at 120 rpm. The diameter of the drive wheel (wheel nº1) is 30 cm. and it rotates at 80 rpm. Draw the system.
EXAMPLE EXERCISE:
GEAR MECHANISMS
Gear mechanisms are sets of wheels with teeth called "cogs" that are in contact.
Z1
N1
Z2
N2
Z1: number of teeth of the drive wheel.
Z2: number of teeth of the output wheel.
N1: velocity of the drive wheel.
N2: number of teeth of the output wheel.
Gear ratio equation (must fulfill):
N1 x Z1
=
N2 x Z2
Example of a real gear mechanism: a clockwork.
Look at the following gear mechanism and calculate the velocity of the drive gear if the output gear rotates at 300 rpm.
EXAMPLE EXERCISE:
GEAR MECHANISMS WITH A CHAIN
These are two gears that are a certain distance apart, rotating simultaneously by means of a metal chain.
N1 x Z1
=
N2 x Z2
Gear ratio equation (must fulfill):
It's time to practise!:

http://www.juntadeandalucia.es/averroes/recursos_informaticos/andared02/maquinas/
Moisés Llorente
Colegio Madrigal (Fuenlabrada)
Technologies 1º ESO

Gear ratio:
is the quotient between the velocity of the output element and the velocity of the input element.

N1
x D1 =
N2
x D2

N2
N1
D1
=
D2
Why is gear ratio important?
The
gear ratio
gives us an idea about how many times the output velocity in a mechanism is bigger or smaller than the input velocity.
For instance:
In the following mechanism, the gear ratio is...
D1 = 10 cm.
D2 = 30 cm.
G.R.
=
N2
N1
=
D1
D2
=
10
30
=
1
3
N2
So...
N1
=
1
3
N2 =
N1
3
The output velocity (N2) is 3 times smaller than the input velocity (N1)
If the input wheel is smaller than the output wheel, we have a
reducing system:
N2 < N1
This kind of systems are designed to increase the force of the driving element (a motor, etc.)
If the input wheel is bigger than the output wheel, we have a
multiplying system:
N2 > N1
This kind of systems are designed to increase the velocity of the driving element (a motor, etc.)
With gears (teeth) it's just the same!!!
G.Ratio =
N2
N1
=
Z1
Z2
Imagine that you are using the biggest
front gear
of your bike (55 teeth) and the smallest
rear cog
(11 teeth)... which would be the gear ratio of your bike's gear system?
front gear
rear cog
G.Ratio =
Z1
Z2
=
55
11
=

5
How many times is N2 bigger or smaller than N1???
N2 = 5 x N1
INTERESTING VIDEOS
Manual Gear Box
Mechanical watch
Full transcript