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MORE FORCE DIAGRAMS (practice more frequently)
Contact forces & at a distance forces
look at the quiz & fix it!
Maybe make short activity designed to help students understand air
resistance too?
Make sure to discuss lab!!!
Make the cat thing into an article?
Swing a meter stick or better tennis racquet and see which way’s
easier (air resistance)
Have students look more at the design of objects, surface and
surface area, to see how it helps them fly or whatever.
Have kids do the friction lab before they talk about friction
This could be taught with more inquiry/group work/project type
stuff. That’s a plan yo!
That’s especially true with air resistance!
Schedule
Wednesday 11/11
Review quiz, update grades
Notes on Forces & Net Force
Thursday
Notes on Weight & Normal Force (color turkeys)
Monday 11/17
Review questions
Catapult project notes & info (plan due 11/23, project due 12/4)
Tuesday
Review questions & preview friction lab
Wednesday
Friction Lab
Thursday
Finish Friction Lab, Notes on Friction
Friday
Notes on air resistance
Monday 11/24
Bell Work 4/11/14 – 4 min
1. What is the difference between static and
kinetic friction?
2. What 2 things cause friction?
3. Geoff and Lizbeth decide to push a car. Lizbeth
pushes with a force of 60 lbs and Geoff pushes
with a force of 80 lbs. If they both push in the
same direction, what is the net force on the car?
4. Ian and Hilary decide to push a car. Ian pushes
with a force of 60 lbs and Hilary pushes with a
force of 80 lbs. If they push in the opposite
directions, what is the net force on the car?
Bell Work 11/11/15 – 4 min
1. All accelerations are caused by _______.
2. What is a force? Give 2 examples.
Motion & Force
Today you are going to…complete a reading guide & review
questions on forces.
So you can…truly understand what a force is, give examples,
and know how much force is a Newton.
You’ll know you’ve got it when you can
• Accurately answer the questions (we’ll review them along
with the notes)
Force
What is the force, Yoda?
A push or pull it is.
Measured in Newtons
or pounds it is.
Force
• Force – “a push or pull” between objects
– Measured in pounds or Newtons
– Examples of forces? (some on next page)
Forces
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Applied force – something actively pushes something
Normal Force
Friction
Air Resistance
Gravity (weight)
Tension Force
Spring Force
Magnetic Force
Electrical Force
Forces are vectors
• Forces have direction
– Like displacement, velocity, & acceleration!
Newtons
&
Pounds
• 1 Newton = 0.225 lbs (a little less than ¼ lb)
• 1 lb = 4.45 N
• 1 Newton is the force it takes to accelerate a 1
kg object at 1 m/s2 (1 kg object & apple)
– 1 Newton = 1 kg m/s2
– If starting from rest, after 1 second, its velocity is…
Net Force
• Total force on an object
• Fnet
• If net force is zero, acceleration is zero
– Forces are said to be “balanced”.
• Does that mean it’s not moving?
Net Force
If the object is not accelerating, that means it’s
not ___________________________________.
• It could still be __________________!
Net Force
• If Fnet is not zero, the object is ______________ in
the direction of the net force.
• This means its…
– Speeding up
– Slowing down
– Turning
Is the net force zero or not?
Car at red light
Car slowing to stop
Car going straight w/ cruise control on
Turning car keeping constant speed
Ball thrown into air
Ball stopped at top
Ball falling
Ball falling at terminal velocity
Magic Meter Stick!
Weight - The Force of Gravity
• Gravity – the attraction between any two
objects that have mass
• Weight – the force of attraction between a
massive object (Earth, moon, sun) and smaller
objects
Confusion of Mass & Weight
• Mass – the amount of matter in an object
– How much stuff there is
– Doesn’t change with location
– Measured in kg or g
• Weight – the force of gravity on the object
– How hard Earth (or other big thing) is pulling something
– Changes with location
– It’s a force!
– Measured in N or lbs
SPRING SCALE & 1 KG MASS
• W = mg
Calculating Weight
– g is acceleration due to gravity
– g = 9.81 m/s2 on Earth
• On Earth, 1.00 kg of matter weighs _____
• On Earth, 5.00 kg of matter weighs _____
SPRING SCALE & 1 KG MASS
• W = mg
Calculating Weight
– g is acceleration due to gravity
– g = 9.81 m/s2 on Earth
• If something weighs 10.0 N, what’s it’s mass?_____
• If something weighs 25.0 N, what’s it’s mass?_____
SPRING SCALE & 1 KG MASS
Normal Force
• the perpendicular support force exerted on an
object in contact with another stable object
• A table holding an object up
• Cause:
We’re
– Atoms compress together
• Like a molecular trampoline
normal!
Force Diagrams (bottom of front page of notes)
• Car at red light
• Ball falling
• Ball falling at terminal velocity
• Car going straight with cruse control on
Bell Work 4/10/14 – 3 minutes
Use the factor label method to solve the following.
1. How can you tell if the net force of an object is zero?
2. 160.0 lbs = ? N
3. 25.0 N = ? lbs
Is net force zero? Draw force diagram.
• Pulling block across table with constant velocity.
Friction!
• Force between two surfaces that are touching that
opposes motion between the surfaces
• If I push to the left, friction pushes to the ______
• Cause:
– All surfaces are rough & get caught on each other
• Fly on window
– Crazy intermolecular forces that aren’t truly understood
• Converts energy into heat!
Types of Friction
• Static friction
• Kinetic friction
– 2 types
• Sliding
• Rolling
Static Friction
• Static friction
– Objects aren’t moving (stationary)
– Strongest
Kinetic Friction
• Friction between 2 objects that are moving
relative to each other
• 2 types
– Sliding – one surface slides past another
• Sliding board!
– Rolling
• Object rolls on wheels or ball bearings
• Weakest!
Imagine
Imagine there’s no friction
It’s easy if you try
No opposing force to move you
No force to reply
Imagine all the people
Falling down today
• Helps us?
• Is harmful?
Friction
• Helps us
Friction
– We can walk!
– Tires grip road!
– Breaks stop car
– We can hold things
– Opening jars
– Things don’t slide all over the place
• Is harmful
– Slows things down
– Wears parts (socks, shoes, tires, engine parts)
– Heat – rope burn, engine can overheat
Question
• Where are most black holes found?
Friction
• Can reduce friction by
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Make surfaces smoother (or change surface)
Decrease force that’s pushing surfaces together
Lubricants (motor oil, grease, wax)
Ball bearings or wheels (change sliding to rolling)
• Can increase friction by
– Make surfaces rougher
– Increase force that’s pushing surfaces together
– Change types of surfaces in contact (gloves)
• Doesn’t really depend much on surface area!
Tires & Friction
• Worn/balding tires
• Ice, water, snow, & wet leaves
– _____ & changing _____
• Gravel
– Changes to ______ friction to _____ friction.
• What happens to stopping distance when it’s wet?
Why?
• 4 wheel drive is not 4 wheel stop :P
Hydroplane
• What to do!
http://www.youtube.com/watch?v=TdwuftIUwYY (4:39)
• Why do you need to slow down in the rain?
Hydroplane
• Hydroplaning – lose traction due to layer of water
• Prevent – drive slower in rain, don’t cruse control, good tire tread
• If hydroplaning – look for & steer to open space
– Antilock Breaks – keep gas lightly on
– No antilock breaks - ease off gas entirely
Bell Work 12/2/14 – 4 min
Put your HW upsidedown on your desk! (study guide)
W = mg
1.00 N = 0.225 lbs
1. How much does a 2.5 kilogram object weigh?
2. Convert the weight to lbs.
3. How much does a 200 g object weigh?
4. Convert to lbs.
Today you are going to…take notes & answer questions on air
resistance.
So you can…identify the cause and effects of air resistance.
You’ll know you’ve got it when you can
• Accurately answer the questions
Air resistance
• Force caused by air that opposes motion
• 2 causes
1. Must push air out of the way
2. Friction between air and surface of object
Air resistance
• Affected by 3 things:
– Speed
– Shape
– Cross-sectional area
• Force of air resistance increases as object
moves faster because it must push more air
out of the way in the same amount of time
• Is helpful!
Air resistance
– When skydiving!
– Flying squirrels fly
• Is harmful!
– When driving…hurts gas mileage above 55 mi/hr
– Trying to get plane to take off (called “drag”)
Air Resistance - Drafting
• Piece of paper demo!
• Change piece of paper to fall faster
• Drafting with two pieces of paper
• Mythbusters bike drafting big rig
– Skip to 1:30 in
– Tory will ride a bike at 20 mph & they’ll measure heartbeat
Air resistance
• To minimize air resistance:
1. Make smoother, aerodynamic surfaces
2. Streamline object (design shape that minimizes
air resistance)
1. Make surfaces smoother
2. Decrease cross-sectional area
– What is the most streamlined shape?
Streamlined!
Terminal Velocity
• Speed at which a falling object stops accelerating.
– (maximum falling speed for object)
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Force of air resistance = - weight
Raindrop - 10 m/s (22 mi/hr)
Penny – 29.5 m/s (65 mi/hr)
Human - 120 mi/hr
– mythbusters
Bell Work 10/2/14 – 3 minutes
Copy the following down on your bell work sheet.
1. terminal velocity for human - _____ mi/hr (___ m/s).
2. To achieve top speed, Frightful adopts a
______________ shape.
3. Frightful’s top speed - _______ mi/hr (____ m/s).
4. Mark Calland top speed - _______ mi/hr (____ m/s).
http://www.youtube.com/watch?v=1ukf2vntU44
Bell Work – 5 minutes
GET OUT ZE HOMEWORK!!
1. What is the terminal velocity for a raindrop?
2. What is Vterminal for a penny?
3. How much does a 6.0 kg raccoon weigh?
Schedule
Monday
Notes on forces, normal force, & friction
Tuesday
Complete & review study guide questions
Air resistance info
Wednesday
Return test
Terminal velocity notes
Review Motion & Force Study Guide
Thursday
Review test
Cat graph
Study!
WWVC
Friday
Quiz on 10.3 Motion & Force
Study 10.3 Motion & Force Reading Guide
Notes
Motion & Force STUDY Guide
Factor label N  lbs, lbs  N, kg  N, etc.
• Link of cat & tree incident (skip to 25 s in)
http://www.youtube.com/watch?v=_q_ULVhyfJg
• **This graph is taken from "How Cats Survive Falls from New
York Skyscrapers." It plots the percentage of adult humans
and cats killed by a fall versus the stories fallen.
• The adult human graph increases steadily to 100% while the
cat graph increases to about 10% and then decreases to about
5% for falls from 7 to 9 stories. Although the graph doesn't
show it, the survival rate holds steady at 95% from 9 to 32
stories.
• The greater an object's cross-sectional area and the less its mass,
the lower the terminal velocity and the sooner it's reached. A cat
reaches its terminal velocity of 60 mph within 5 stories of freefall.
• Person's terminal velocity is 120 mph.
• Once a cat reaches its terminal velocity, it then begins to slow
down. This is because the cat relaxes, changing its position from
back arched, head down, and legs pulled tightly underneath its
body, to resemble a spread eagle cat. This increases its crosssectional area and slows the cat down. The reason for this is that
our bodies are only sensitive to acceleration (this is why at times on
an airplane flight it feels as if you aren't moving at all). Relaxing also
causes the impact force to be spread out over more area when the
cat lands, resulting in a decrease in injuries to cats' limbs when they
fall seven or more stories.
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Forces 1
1 2 3 4 5 6 7 8
Forces 2
9 10 11 12 12.5
Forces 3
13 14 15
Friction 1
1 2 3 4 5
Friction 2
6 7 8 9 10 11
Air Resistance 1
1 2 3 4 5
Air resistance 2
6 7 8 9 10 11 12
F 14 Student 1 is pushing a desk across the floor.
Student 1 is pushing with a force of 60 N to the
right and the force of kinetic friction on the desk is
60 N.
Draw a force diagram, labelling all 4 forces.
What is the net force on the desk?
Will the desk accelerate? If so, in what
direction?