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# 3º ESO - MECHANISMS

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## Moisés Llorente Santos

on 4 February 2015

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#### Transcript of 3º ESO - MECHANISMS

UNIT 4: MECHANISMS
Which are the
points in common
between this four objects?
bike's gear
well's pulley
seesaw
corkscrew
so... could you define
why do we need a mechanism
to?
Now, it's time to open your book and look for the definition of mechanisms (page 106).
What's the difference between
transmit
and
convert
forces and motion?
Are mechanisms
useful
in our daily life? why?
Identify the
input element (driving force)
and the
output

element
in these objects:
bike's gear
well's pulley
seesaw
corkscrew
arm's force
kid's weight
hand's force
cork's resistance
leg's force
wheel's speed
1. MECHANISMS (PAGE 106)
2. LINEAR MOTION MECHANISMS (page 107)
What's the meaning of
"linear motion"
?
Which of the following mechanisms
is not linear
?
2.1 The lever (page 107)
In groups of 4-5 persons:
1. Try to guess the meaning of the following words: lever, fulcrum, load, bar, effort, resistance.

2. Identify the meaning of the letters R, r, F, d in a lever.

3. Using school material or any class material that you need, construct a class 1, 2 and 3 lever and try to lift some loads with them.
4. What's the force needed to lift a 150Kgs. load with a Class 1 lever if d=10cm. and r=30 cm.?
Solution: d=3m
Solution: F = 45N
In a lever, if I want to make the half of the force that I need to lift a load... how much do I need to enlarge or reduce the "d" dimension of the bar?
Clue: use the "law of the lever" equation
Solution:
to make the force the half of the initial one, you'd need to duplicate the "d" dimension of the bar, as follows:
The pulley (page 108)
Work in teams
Research and oral presentation:
1. What's the main difference between a fixed pulley and a movable pulley?

2. When is a fixed pulley balanced? And a movable pulley?

3. Draw a fixed and a movable pulley and identify the letters F and R on each drawing.

4. Compare the effort needed to lift a load with a movable pulley with the effort needed to lift the same load with a fixed one. Look for a reason.

5. Calculate the force needed to lift a weight of 100Kg with a fixed pulley. And with a movable pulley?
TRUE OR FALSE?
1. Is easier to lift a load with a fixed pulley than with a movable pulley...

2. A movable pulley is balanced when it satisfies this equation:

3. A Block pulley is balanced when it satisfies this equation:

4. A fixed pulley is a set of two pulleys - one is fixed while the other can move in a linear direction.
The pulley (page 108)
3. ROTARY MOTION MECHANISMS (page 109)
D1 = 10 cm
D2 = 20 cm
N1 = 6 rpm
N2 = ???
Calculate:
- The relative size of the wheels
- The velocity of the wheel 2 (N2)
Wheel 1
Wheel 2
Clue: use the "gear ratio" expression
Draw
a friction drive with the following features:

D1 = 20 mm D2 = 40 mm

If N1 = 30 rpm,
calculate N2

Draw
a pulley system with belt, with the following features:

D1 = 15 mm D2 = 45 mm

If N1 = 60 rpm,
calculate N2

G.R. = Z1/Z2 = 0,81
N2 = 250 x 0,81 = 202,5 rpm
EXERCISES:

- Page 126 ex. 4, 5.
- Page 113 ex. 14, 15.
Solution:
Data to calculate:
Solution:
Data to calculate: