Download 3 Types of friction

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BellRinger: September 18
Fg=ma
Fg is weight (force due to gravity)
m=mass of the object a=acceleration of
gravity 9.8 m/s2
1. What is the weight of a 6.7 kg box?
2. What’s the mass of a 76 kg crate on the
moon? On Jupiter?
3. What’s the mass of a 80N crate on earth?
Figure this out……….
You were kidnapped and knocked out.
Some time later, you wake up in a dark
warehouse. All the doors are locked and
there are no windows. You have a spring
scale and 5 kg mass. Weighing the mass,
you get a reading of 44.2N. Are you still on
earth? What’s your evidence?
What planet are you on?
Planet
g (m/s2)
Mercury
Venus
Earth
Earth's Moon
Mars
Jupiter
Jupiter's Moon Io
Saturn
Uranus
Neptune
Pluto
3.61
8.83
9.8
1.6
3.75
26
1.81
11.2
10.5
13.3
0.61
POST LAB DISCUSSION
Look at Part 1
When you changed mass what else
changed?
Only force should have, but your acceleration
should have stayed about the same> WHY??
The acceleration part of our equation F=ma
was the acceleration due to gravity…gravity
is constant….it doesn’t change!
Post Lab Discussion
Part 1
For #3 you were supposed to take
the highest mass you had from your
experiment and see what the force
would be like on the moon.
For example, if 0.500 kg was your
highest, and you had that on the
moon, your Force would be F=0.5 x
1.6 = 0.8 N (it is lower)
GRAVITY….
http://gravitymovie.warnerbros.com/#/video
s/main-trailer
Time to talk about that thing that
slows you down…that’s always
holding you back…
(not “the man”)
Friction is a force that opposes
motion, or “pushes back” when two
surfaces are in contact with each other
It’s always around and it affects
everything.
Without friction…
3 Types of friction:
1)Static Friction
2)Fluid Friction
3)Kinetic Friction
Types of friction:
1) Static friction: Friction that resists the
initiation (start) of sliding motion
between two objects that are in contact
and at rest.
Remember Newton’s 1st Law of Inertia…a body
at rest, stays at rest, unless an unbalanced
force acts on it? Usually the force keeping it at
rest is friction….
To make something move, you just overcome static friction
Types of friction:
2) Fluid friction: The friction between layers of a
fluid. Described by viscosity.
Viscosity – the “thickness” of a fluid.
Low viscosity=“thin;” high viscosity = “thick”
The less viscous a fluid is, the easier it moves or you
move through it.
http://www.youtube.com/watch?v=OikVWMMOTeE
http://www.youtube.com/watch?v=hvbGrfpCsjo
Which has a higher viscosity?
Air or Water???
How can you tell??
Types of friction:
3) Kinetic friction: The force that opposes the
movement of two surfaces in contact and
moving over each other.
To keep something moving, must apply a force larger than
kinetic friction.
Two types of Kinetic Friction:
a) Sliding – the objects are sliding
b) Rolling – the objects are rolling.
How did they build the Pyramids?
Do we need friction
to walk?
Why do you think yes
or no?
Friction depends upon 3 things:

1. surface area in contact
2. mass/gravity (weight) of object
3. coefficient of friction – “mu”
Surface Area:
Why does an ice skater wear thin
blades to skate with?
Of course! Less friction and they can go longer
Surface area affects friction (how
much of each object is in contact
with each other)
Weight:
What is harder to push from rest, a
bookshelf full of books or an empty
bookshelf?
Of course! Less friction and they can go longer
Mass x Gravity affects friction
The coefficient of friction
Mu – measured between two surfaces
Low mu – slippery
High mu – hard to move
Book friction…
I need 2 strong volunteers!!
Friction is cumulative
Small amounts of friction in a system add
up to become a large amount of friction.
And it can be measured. For
example:
The force pulling a truck is 2,000 N. What
is the approximate amount of static friction
acting on the truck if the truck does not
move?
Where is friction good?
Where is friction bad?
Homework
Explain two situations when you
want friction, and two when you
don’t.
They cannot be opposites.
Explain why you want friction or
don’t want it in each case.
Full sentences, good grammar.