1. (fulcrum)
in a different point of the bar
fulcrum
Give me a place to
stand and I will
move the earth
5.- FORCES IN MACHINES & MECHANISMS
Lever
a rigid bar
with a support point
bar
An object is placed on a point of the bar
A force F is applied
to overcome the resistance R the object exerts
Archimedes
2. aR aF
There are three different levers:
F R
aF = arm of
force
aR = arm of
resistance
intermediate fulcrum
F R
intermediate resistance
R F
intermediate force
arm = distance to the fulcrum
first-class second-class third-class
aF aR
5.- FORCES IN MACHINES & MECHANISMS
3. F
R
S
Scissors
S
R F
S
F
R
S
R
F
5.- FORCES IN MACHINES & MECHANISMS
Draw force /resistance / fulcrum to classify the following levers:
first-class
Nutcracker
second-class
Tweezers
third-class
Stapler
third-class
4. F
R
S
Clothes pin S
F
R
S
F
R
Bellows
S
F
R
5.- FORCES IN MACHINES & MECHANISMS
first-class
second-class
third-class
Mop / broom
Bottle opener
Cork screw second-class
Draw force /resistance / fulcrum to classify the following levers:
5. R
S
F
S
R
F
Wheel barrow
F
R
S
F
R
S
5.- FORCES IN MACHINES & MECHANISMS
Draw force /resistance / fulcrum to classify the following levers:
Oar
first-class
second-class
first-class
Nail hammer
first-class
Wheel barrow
6. 5.- FORCES IN MACHINES & MECHANISMS
Draw force /resistance / fulcrum to classify the following levers:
Pliers
Switch
Denture
Biceps Fishing
rod
Crane
Catapult
Shovel
7. F R = 12,5 N
I measure aR = 43,5 cm
I mesure aF = 16,5 cm
F · aF = R · aR
F · 16,5 cm = 12,5 N · 43,5 cm F = 12,5 N · 43,5 cm
16,5 cm
F = 32,95 N
It doesn’t spare
any effort!!
Calculations with levers:
5.- FORCES IN MACHINES & MECHANISMS
Law of the lever
1.- Calculate the force to lift a weight of 12,5 N
8. F R = 12,5 N
aR = 12,5 cm
aF = 43,5 cm
F · 43,5 cm = 12,5 N · 12,5 cm F = 12,5 N · 12,5 cm
43,5 cm
F = 3,59 N
5.- FORCES IN MACHINES & MECHANISMS
F · aF = R · aR
Calculations with levers: Law of the lever
1.- Calculate the force to lift a weight of 12,5 N If I swap the arms
It needs a force
4 times lower
than the resistance
9. R
I measure aR = 43,5 cm
I mesure aF = 59,5 cm
5 N · 59,5 cm = R · 43,5 cm R = 5 N · 59,5 cm
43,5 cm
R = 6,84 N
F = 5 N
5.- FORCES IN MACHINES & MECHANISMS
F · aF = R · aR
Calculations with levers: Law of the lever
2.- Weight you can lift applying a force of 5 N
Second-class
levers always
spare effort
10. aF = 2,08 m
aF
aR
aF + aR = 3,2 m
= 3,2 − aF
6,5 · aF = 12 · (3,2 − aF)
6,5 · aF = 38,4 − 12 · aF
aF =
18,5 · aF = 38,4 38,4
18,5
aR = 3,2 − aF aR = 1,12 m
= 3,2 − 2,08
F = 6,5 N R = 12 N
5.- FORCES IN MACHINES & MECHANISMS
F · aF = R · aR
Calculations with levers: Law of the lever
3.- With a bar of 3,2 m, calculate the arms to lift a weight of 12 N applying
a force of 6,5 N with a first-class lever.
