Transcript Les Forces et Machines simples

Forces and machines
Forces surround us constantly. We need force to
play sports, to open a bottle and even to write!
Lets discuss the mechanics of the world
around us in this last chapter of the year.
Forces
 Discussion in class:
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If you are riding a bus and suddenly, the bus comes to a stop. What
do you feel?
If you take the elevator, how do you feel if you go up? Or down?
In a game of “tug-o-war”
Now think about your favorite sport. What kind of forces does it
entail?
There is also a push or a pull.
Force
 The symbol of force is “F” and the unit of
measurement is “newton” or “N” ex: 5 N ou 20 N
 A force is a mechanical action that sets an object in
motion.
 A force can change the speed and trajectory of an
object already in motion.
 It can also deform an object.
Ex: - Throw a baseball
- Skating
- Push (or pull) a drawer
- Compress a sponge
- Etc.
Types of forces (p 410)
We will discuss a few different types.
 Compression: it is a force applied to compress an object. It can
provoque or modify and object’s motion.
Ex: compressing a balloon!
Also, the muscles in your arm must
compress to lift objects.
Torsion: the action of compressing in rotation
direction
 Tension: the force applied to pull an object in one direction.
(especially on a rope)
Ex: Such as in the game of tug-o-war or like the acrobats.
 Flexion: When an object is pulled and pushed at the same
time.
 Shearing: When you pull on opposite direction to tear
something
 Friction: It is a force that slows down or stops the
movement or motion) between two surfaces.
Friction acts in the opposite direction of the motion.
Ex: Your shoes have rubber, so you dont stop and fall.
Also the tires from a car.
Air resistance is also a source of fricftion.
Observe:
What is the purpose of a zamboni?
To reduce friction between
the skates and the ice.
Air friction
What is the difference between these vehicles?
Aerodynamic cars and… cats! (cheetahs)
All have a slender body, which
makes for less air friction
http://www.youtube.com/watch?feature=pla
yer_detailpage&v=KIeXEiJuJUY
The consequences of forces
 Elastic deformation: ex trampoline (takes its original
shape).
 Permanent deformation: ex:can of soft drinks that
changes shape (it will take any shape, different from
the initial shape)
 Rupture: material breaks like the cables of the
Olympic stadium.
Types of motion
WHAT DO
THESE 3
SYSTEMS
HAVE IN
COMMON?
WHAT DO THESE 4
SYSTEMS HAVE IN
COMMON?
WHAT DO THESE 3 SYSTEMS HAVE IN COMMON?
Movement of translation
 Definition:
 The translational movement is the movement that an object or
part of an object does in a straight line.
Motion of rotation
 Definition:
 It is the type of movement that a piece of an object or the
entire object will do around an axis.
 Sometimes the rotation is partial:
 Part of the object is immobile, while the other part rotates.
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Ex: a door handle
Helicoidal motion
 Sometimes in some objects rotation and translation
are combined. Thus creating a movement that looks
like a spiral.
Newton’s laws of motion
Newton, is a british physist, born the year
Galileo died. He attended Trinity College at
University of Cambridge in 1661, he did his
own research and taught while he was still
a student!
His work "Philosophiae naturalis
principia mathematica", was published in
1687, has three sections and contains the
universal law of gravity and the famous
Newton’s laws of motion.
Do you like roller coasters?
Newton’s first law of motion
 The first law, also known as inertia, states that all
objects tends to remain in the same speed and
direction it was moving at (even at rest) unless an
outiside force acts upon it.
In a roller coaster, the train keeps
moving until the breaks are applied.
The first law… another example
The golf ball remains at rest
until an outside force acts
upon it.
Newton’s second law of motion
All acceleration of an object is proportional with the
forces applied on it. All acceleration is inversely
proportional to the mass.
Examples of acceleration:
Example 1: The more we pull on a rope at tug-o-war, the more we
accelerate.
Example 2: A person pushing a table, is not the same as three pushing a
table. The three together will accelerate way more!
Examples with mass:
If these same three people applied the same force on a car, they
won’t accelerate as much. Because it’s a lot heavier.
Newton’s third law
To every action, there is an equal and opposite reaction
The second and third laws applied on roller coasters…
2nd law
The force that we
feel from the
gravity as we free
fall from a very big
height is enormous
and can be
calculated:
Using F = mxa
m = mass of the
train and
passengers.
a = gravitational
acceleration (g)
9.8 m/s2
3d law
The roller coaster
undergoes curves that you
can feel. When the train
goes in a curve we feel
like the seats are pushing
us.
But you are also pushing
the seats with an equal
and opposite force. Which
helps you to hold on to
your seat!