circle the forces below: gravityinertiafrictiondensityair resistancemass accelerationnormal...
TRANSCRIPT
• Circle the forces below:• gravity inertia friction density air resistance mass acceleration
normal force weight centripetal force tension• 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.• MORE FORCE DIAGRAMS • More examples & nonexamples of forces• Contact forces & at a distance forces• 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!• Make the cat thing into an article.
ScheduleMonday 4/7
LaunTuesdayReading Guide & Catapults
WednesdayForce notes
Thursday Friction Lab?
Friday Finish Friction Lab
Bell Work 4/11/14 – 4 min1. 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?
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)
Bell Work 11/12/14 – 4 min1. What is a force? Give 4 examples.2. What’s it measured in?3. 1.00 N = _____ lbs4. If you weigh 130. lbs, how many Newtons do
you weigh?
Force
A push or pull it is.What is the force, Yoda?
Measured in Newtonsor pounds it is.
Force
• Force – “a push or pull” between objects• Measured in pounds or Newtons• Examples of forces? (some on next page)
Forces
• Applied force – something actively pushes something• Friction • Normal Force• Air Resistance• Gravity (weight)• Tension Force• Spring Force• Magnetic Force• Electrical Force
Forces are vectors
• Forces have direction– Like displacement, velocity, & acceleration!
Newtons
• 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 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• Ball falling• Ball falling at terminal velocity• Ball on floor• Car going straight with cruse control on• Car keeping constant speed through turn
The Force of Gravity (weight)
• 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 lbsSPRING SCALE & 1 KG MASS
Calculating Weight• W = mg– 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
Calculating Weight• W = mg– 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:– Atoms compress together• Like a molecular trampoline
We’re normal!
Bell Work 4/10/14 – 3 minutesUse 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– Stronger– Objects aren’t moving (stationary)
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!
ImagineImagine 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
Friction • Helps us– 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– 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– Change types of surfaces in contact (gloves)– Increase force that’s pushing surfaces together
• 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, watch using cruse control, good 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 minPut your HW upsidedown on your desk! (study guide)
W = mg 1.00 N = 0.225 lbs1. 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 causes1. Must push air out of the way2. 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
Air resistance• Is good!– When skydiving! – Flying squirrels fly
• Is bad!– 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 surfaces2. Streamline object (design shape that minimizes
air resistance) – What is the most streamlined shape?
Streamlined!
Terminal Velocity• Speed at which a falling object stops accelerating.– (maximum falling speed for object)
• 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 minutesCopy 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 4/11/13 – 5 minutes
1. What is the terminal velocity for a raindrop?2. What is Vterminal for a penny?
3. 155 N = ? Lbs (use factor label)
ScheduleMonday
Notes on forces, normal force, & frictionTuesday
Complete & review study guide questionsAir resistance info
WednesdayReturn testTerminal velocity notesReview Motion & Force Study Guide
Thursday Review testCat graphStudy!WWVC
FridayQuiz on 10.3 Motion & ForceStudy 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 cross-sectional 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.
• 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
