WARMUP SEPT 17TH - THINK
You were kidnapped and knocked out.
Some time later, you wake up in a dark
warehouse. All there is in the room is a
scale and a light, so you decide to weigh
yourself. WOW!!! You gained a whole lot
of weight last night…what happened?
You remember seeing alien figures in a
dream-state, and realize you must be on
another planet. Is the planet smaller or
more massive than Earth?
WEIGHT AND MASS…
Fg = mg
(formula for weight)
Fg (weight)
is the force an object has
due to gravity Gravity on
2
the Earth 9.8m/s
WEIGHT AND MASS…
What’s the mass of my 893N ex-husband
that just happens to be in my basement?
WEIGHT PROBLEMS!
FG=MG
What does a 60kg tool
weigh?
What’s the mass of a crate of tools
that weighs 234 N?
WEIGHT AND MASS:
What does a case of the new
Chipotle Gatorade with a mass of
54.8kg weigh?
WHAT WOULD A 60KG TOOL WEIGH:
On Mars (accel of gravity 3.7m/s2 )?
On Jupiter (accel of gravity 24.8m/s2 )?
Gravity : (m/s
Planet
a=
Mercury
3.61
WHAT PLANET
Venus
8.83
ARE YOU ON?
Earth
9.8
1.6
YOU HAVE A 4KG Earth's Moon
3.75
CAT THAT WEIGHS Mars
Jupiter
26
14.4N
Jupiter's Moon Io 1.81
Saturn
11.2
Uranus
10.5
Neptune
13.3
Pluto
0.61
NOW IT’S TIME FOR NEWTON’S 2ND
LAW
Newton’s 1st: Inertia. At rest, stay at
rest; in motion, stay in motion.
Newton’s 2nd: What happens when a
force acts on an object?
Acceleration.
Newton’s Second Law:
Force = mass x acceleration
Force equals mass times acceleration.
F=ma
NEWTON’S SECOND LAW:
• The acceleration of an object is directly
proportional to the force applied to the object,
and inversely proportional to the mass.
• -or –
F
then A
if M
then A
if
CALCULATE THE FORCE HOW DO WE?
HOW? LIKE THIS:
My 4 kg cat, Jeter, ran at our kitten Shadow
with an acceleration of 3.0 m/s2. He didn’t
stop. Poor little kitten! After suffering a mild
concussion, we wondered….how much force
did Jeter hit him with?
Force = mass x acceleration, or…
4 x 3 = 12
12 WHAT?
Remember Force has its own special
unit…
1kg x 1m/s2 = 1 newton (1N)
F = ma
Learn it. Live it. Love it.
FORCE = MASS(ACCELERATION)
1. How much force is needed to push
a lawnmower of 30 kg at 2.5 m/s2?
2. How much force must be applied
to a bowling ball that has a mass of
5kg to achieve an acceleration of
25m/s2?
3. What is the mass of a baseball that
has an acceleration of 125 m/s2 after a
force of 44 Newtons has hit it?
4. What is the acceleration of a 22kg
wolf that hits attacks a bear with a
force of 400 Newtons?
NOW YOU PRACTICE!
WITHOUT FRICTION…
Friction - a force that opposes
motion, or “pushes back” when
two surfaces are touching.
It’s always around and
it affects everything.
WITHOUT FRICTION…
FRICTION- WORKS AGAINST YOU …..BUT IT
CAN BE HELPFUL!
WHEN DO I WANT…
A lot of friction?
 http://www.youtube.com/watch?v=yTKwS-J7qVE
A little bit of friction?

http://digg.com/video/the-science-of-figure-skating
 http://www.nbclearn.com/olympics
FRICTION TYPES AND PARTS…
There are three Basic Types of
Friction
Static
Kinetic
Fluid
TYPES OF FRICTION:
1) Static friction: Friction that resists
the start of sliding motion between two
objects at rest.
Keeps things stopped
TYPES OF FRICTION:
2) Kinetic friction: The force that
opposes the movement of two surfaces
that are in contact and moving over
each other.
To make something move, you just overcome static friction by pulling
with a force larger than kinetic friction.
KINETIC FRICTION:
Sliding or Rolling
TYPES OF FRICTION:
3) Fluid friction: The friction
between layers of a fluid.
2 types- Liquid and Gas
Liquid- water, oil etc.
Gas - Air
TYPES OF FRICTION:
Described by viscosity.
Viscosity – the “thickness” of a fluid.
Low viscosity = “thin;”
High viscosity = “thick” or “viscous.”
The less viscous a fluid is, the easier it
moves.
Friction DOES depend upon:
1. surface area in contact
2. mass/gravity (weight) of
object
3. the texture of the surface
FRICTION IS CUMULATIVE
Small amounts of friction in a
system add up to become a large
amount of friction.
TEXTBOOK FRICTION…CHALLENGE