**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?

don't look at your book!!!

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)

http://www.juntadeandalucia.es/averroes/recursos_informaticos/andared02/maquinas/

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

http://www.juntadeandalucia.es/averroes/recursos_informaticos/andared02/maquinas/

Draw

a friction drive with the following features:

D1 = 20 mm D2 = 40 mm

If N1 = 30 rpm,

calculate N2

http://www.juntadeandalucia.es/averroes/recursos_informaticos/andared02/maquinas/

Draw

a pulley system with belt, with the following features:

D1 = 15 mm D2 = 45 mm

If N1 = 60 rpm,

calculate N2

http://www.juntadeandalucia.es/averroes/recursos_informaticos/andared02/maquinas/

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.

Data I already know:

Solution:

Data to calculate:

Data I already know:

Solution:

Data to calculate:

Data I already know:

Data to calculate:

Solution:

Data I already know:

Data to calculate:

Diagram:

Solution: