what is a “force”? - kyrene school · pdf filea force is an interaction between 2...
TRANSCRIPT
A force is an interaction between 2 objects involving a push or a pull.
Forces are vectors - they are drawn as arrows (have size and direction).
The units of measure for force is: Newtons
Draw an object being pushed with a force vector!
5 Newtons
Unbalanced and Balanced ForcesUnbalanced forces cause objects to
accelerate (like your object being pushed).
Balanced forces means NO acceleration. Can you think of an example of when forces are
balanced and do NOT cause an object to accelerate?
Balanced Force = No acceleration
Unbalanced Force = Acceleration
Force or Action Force Vector Prediction
(draw in Force Vectors )
Real-World Example
(List an example of each
force or action in real-life)
Squeezing/Compression
Stretching/Tension
Bending
Sliding/Shearing
Twisting/Torsion
Crumple a paper
Pull on a rubber band
Bend a straw
Slide down a hill, earthquake
Open a soda bottle, ringing out a towel
It Matters….
Inertia is a property of matter
All objects are matter
• They are made of atoms
• Stuff you can see or touch!
The Greeks thought there were two types of motion:
Natural motion - what an object “naturally wants to do”
Violent motion - what an object has to be forced to do
The Greeks did not get the idea of friction!
Realized the role of friction by rolling marbles down ramps and sliding objects across tables.
If left to themselves,moving objects do NOT slow down.
Without friction objects keep the same velocity!
No Acceleration!
Tablecloth Video
Why do the items in the video
stay in the same place when
the tablecloth is pulled away?
Newton’s First Law says that objects do not
accelerate on their own – they need an
unbalanced force to accelerate.
This property of matter, which causes objects to
resist acceleration, has been named “inertia”.
The bigger the mass of an object the harder to
accelerate (boulder vs. rock)
So the bigger the mass, then the greater the inertia
Bigger Mass Greater Inertia Harder to accelerate
Inertia Activity
Pick 5 objects in the room and label
them from greatest inertia to least
inertia.
Explain in 10-20 words what inertia
means.
If an object is at rest or moving at a constant velocity it is at equilibrium(happy).
Static Equilibrium
Object at rest
Forces are balanced
(no acceleration)
Dynamic Equilibrium
Object moving at a
constant velocity
Forces are balanced
(no acceleration)
Static (At Rest) Equilibrium
Velocity is zero
Examples:
Weighing yourself on a set of scales
Computer setting on a table
Car parked on an incline
Table pushing up
Weight down
Weight downWeight down
Scales pushing up
Ramp pushing upFriction
Dynamic EquilibriumVelocity is constant
Examples:
Driving at constant velocity
Force from road
Weight down
Friction
Normal up
Weight downAir resistance
Terminal velocity in parachuting
Inertia Demo
What do you think will happen to the
cart and tennis ball when it gets to the
bottom of the ramp?
Make a prediction and explain your
reasoning.
Newton's 1st Law
and Mr. Ganey Driving
Because of inertia, objects (including
Mr. Ganey) resist changes in their
motion. If a car is going 40 mi/hr and is
stopped by a brick wall, Mr. Ganey
keeps moving at 40 mi/hr.
This is why we wear seat belts.
Extensions: Complete Chapter
2.1 Reading Section questions 1-3
page 47 (answer on listening guide)
Begin Chapter 2 Packet
Force Vector Fun!
Define Mass and Weight
Mass – the amount of stuff in an object
the quantity of matter (atoms) in an object
the measurement of the inertia
measured in kilograms (kg)
Weight – how hard an object pushes down
Is a measure of the force of gravity on an object Weight = Mass Acceleration of gravity
W = mg
F = ma
measured in Newtons (N)
Your Weight on Other Worlds
Acceleration due to gravity on the Moon is weaker than on Earth. Gravitational acceleration on Earth = 9.8 m/s2
Gravitational acceleration on Moon = 1.6 m/s2
So, you would weigh less on the Moon than on Earth.
But you would have the same mass on both the Earth and Moon! You are still you – you are still made up of the same stuff; the same amount of matter (atoms).
Weight depends on Gravity!
Your mass does NOT depend on where you are. (e.g. Earth, Moon, or space). Mass is universal – the same everywhere!
In space you are “weightless” but not “massless”.
W = mg
So if gravity is weak you are basically weightless and if gravity is strong you weigh a lot. Let’s try figuring out our weight on different worlds.
Calculate
Weight = mass x gravitational acceleration
Newtons = kg x little g
Earth g = 9.8 m/s2
Moon g = 1.6 m/s2
A soccer player has a mass of 45 kg.
Calculate the soccer player’s weight on
Earth and on the Moon.
Newton’s Second Law
Interactive
Check for Understanding
The same force is applied to each box.
If box 1 has a smaller mass than box 2,
which one will have the greater
acceleration?
Circle your answer.
F F
Box 1 Box 2
2nd Law (F = ma)
How much force is needed to accelerate a 1400 kilogram car 2 m/s2?
Write the formula
F = m x a
Fill in given numbers and units
F = 1400 kg x 2 m/s2
Solve for the unknown
2800 kg- m/s2 or 2800N
Extension Work
If not completed: Complete Chapter 2.1 Reading
Section questions 1-3 page 47
Record answers on listening guide
Complete Chapter 2.2 Reading Section questions 1-3 page 55
Record answers on listening guide
Chapter 2 Packet Complete 2.1, 2.2, Math Support, and Math Practice
Grab a laptop and try the -
Rocket Video
Which of Newton’s
Laws explains how
a rocket takes-off?
Who is considered
the father of the
modern-day rocket?
Action-Reaction Force Pairs Ever sat down on the floor?
Of course you have!
What do you do to stand back up?
You push down on the ground and
it pushes you back up.
The Earth has a lot of inertia (large
mass) so it does not accelerate
very much.
You have a less inertia (smaller
mass) so you accelerate upwards
quite easily.
Try out some
triceps push-ups
on your chair!
Action force
The Weight of your body
pushes down on the chair.
Gravity is pulling the mass of
your body downward!
We call this downward force
your weight.
Reaction force
The chair pushes back up
on your body.
That’s why it can hurt to sit in
a chair a long time!
Newton's Third Law
Identifying Force Pairs
Can you describe the
action and reaction
forces?
Action force:
Bat pushes ball
Reaction force:
Ball pushes bat
Ball moves forward and
bat moves backward.
Action force:
A car has wheels
which spin backwards.
As the wheels spin
backwards, they grip
the road and push on
the road backwards.
Reaction force:
The road then pushes
the car forward.
Identifying Force Pairs
Car moves forward as the
tires push backwards on
the road.
Identifying Force Pairs
Flying gracefully through the air, birds depend on Newton’s third law of motion.
Action force: Birds push down on
the air with their wings
Reaction force: The air pushes their
wings up and gives them lift
Identifying Force Pairs
Action force: A fish uses its fins to
push water backwards.
Reaction force: The water pushes the
fish forwards.
These action-reaction forces let the fish swim!
Identifying Force Pairs
3-2-1 Blast-off!Gases get pushed down and the
rocket gets pushed up!
Various fuels are burned in the engine, producing hot gases.
Action force: The hot gases slide
against the inside tube of the rocket and are pushed out of the bottom of the tube.
Reaction force:
As the gases move downward, the rocket the gases push the rocket upwards. Inertia and F = ma
As the rocket burns fuel (reduces
mass) it accelerates more easily!
Astronauts in Space
Remember everything is either matter, energy, or
nothingness!
The astronauts working on the space station have a
serious problem when they need to move around in
space: There is nothing to push on.
The solution is to throw something in the opposite
direction you want to move.
This works because all forces always come in pairs.
Challenge Question
How does a jellyfish move up and down in the water?
Illustrate your explanation with vectors. Hint: Look at page 57 in
your textbook.
Think about Newton’s Third Law! Make sure to draw action-
reaction force vectors!
Extension Work If not completed:
Complete Chapter 2.1 Reading Section questions 1-3 page 47
Record answers on listening guide
Complete Chapter 2.2 Reading Section questions 1-3 page 55
Record answers on listening guide
Complete Chapter 2.3 Reading Review questions 1-3 page 61
Record answer on listening guide
Chapter 2 Packet Complete 2.1, 2.2, Math Support, Math Practice, and 2.3.
Grab a laptop and try the -
Create a poster with an illustration and text describing the motion of an object
Top Posters will earn Bonus Points for your class period and be used as classroom
decoration!
• Can you explain Newton’s
Laws to your family using a
balloon?
• Think about Inertia, F=ma,
and action-reaction force
pairs.
So What’s Momentum ?
• Momentum is the measure of mass in motion
• Momentum = mass x velocity
• This can be abbreviated to : . momentum = mv
• Or, if direction is not an important factor : . . momentum = mass x speed
• So, a really slow moving truck and an extremely fast roller skate can have the same momentum.
Question :
• Under what circumstances would the roller skate
and the truck have the same momentum?
• If the roller skate moves really fast and the truck
moves really slow, then they could have the
same momentum. This is due to the fact that
momentum is the mass of an object times the
velocity it is travelling.
• A 1000 kg truck moving at 0.01 m/s has the same
momentum as a 1 kg skate moving at 10 m/s.
Both have a momentum of 10 kg m/s.
• (1000kg x .01m/s = 1kg x 10m/s = 10 kg x m/s )
1000 kg1 kg .01 m/sec10 m/sec
• ELASTIC COLLISIONS – objects bounce away
• INELASTIC COLLISIONS – objects stick together
Momentum transfers from one
object to another as they bounce
off each other
Collisions on an air track
Momentum Interactive
Copy the data table above into your notebook
Predict: When two objects collide, what will happen to their
motion?
Momentum Table
Mass - A (kg)Vi of 1 m/s
Collides with
Mass – B (kg)Vi of 0 m/s
Prediction Observed Result
1.0 kg 0.7 kg
1.0 kg 1.0 kg
1.0 kg 1.4 kg
Momentum Interactive
Conservation of MomentumMomentum is conserved if there are no outside forces when objects collide.
The only forces present during the collision are the action-reaction force pairs.
Extension Work Read as needed through Chapter 2.4
Complete the section questions 1-3 on
page 69 in your science notebook
Finish the Listening Guide and
Chapter 2 section questions
Complete Newton’s Laws Tree Map
Complete Chapter 2 Packet
All pages
Grab a laptop and try the
-
Create a poster with an illustration and text describing the motion of an object
Top Posters will earn Bonus Points for your class period and be used as classroom
decoration!
Independent Work Day
Complete Newton’s Laws Listening Guide and Chapter 2 sections questions
Newton’s Laws Tree Map
Complete Chapter 2 Packet Create a poster with an
illustration and text describing the motion of an object
Top Posters will earn Bonus Points for your
class period and be used as classroom decoration!
Go to the virtual lab and select some of the other tabs
Investigate these concepts
• Isaac Newton goes Skiing (Lab Book pages 222-225)
• Read, chunk, and create a SMART summary in your notebook.