Download Forces and Motion

22/05/2017
Forces and Motion
What is motion?
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• In Physics, motion
• Write down 5
is referred to as
examples of motion
the movement of an
object in relation
to time and its
reference point
Physical Challenge!!!
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Teams of 3-4.
Build a structure using newspaper and tape that can
do the following:
•Allows a marble to have the slowest average speed
while in the structure
AND
•Allows a marble to have the fastest average speed
over 3m when it leaves the structure
AND
•Hits a target 3m away (a coke can)
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Distance, Speed
and Time
d
V
Speed = distance (in metres)
time (in seconds)
V= d
t
Speed is
measured in meters per second
(ms-1)
t
What is speed?
• How fast an object is moving over a
distance over time.
• What two variables are needed?
• Distance
• Time
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Measuring speed
In a group of 3-4
designate:
• A runner
• A timer
• A recorder
• A organiser
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• What to do:
• 1. Get your runner to run
50m. Record the time.
• 2. Run a set distance (set
by me) at the same pace
they ran the 50m. Record
the time taken
• 3. Run a set distance at the
same pace as above. Record
the distance
What is speed?
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• Speed (v) is calculated using the
distance an object travels divided by
the time it takes to travel that
distance.
• Instantaneous speed is the actual
speed at any moment.
• The average speed is the speed over
part or all of a journey.
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Distance, Speed
and Time
d
V
Speed = distance (in metres)
time (in seconds)
V= d
t
Speed is
measured in meters per second
(ms-1)
t
Speed vs. Velocity
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Speed is simply how fast you are travelling…
This car is travelling at a
speed of 20m/s
Velocity is “speed in a given direction”…
This car is travelling at a
velocity of 20m/s east
An example to copy…
A car travels 600 m in 20 s,
• what is its average speed?
• ALWAYS
•
•
•
•
1. give the formula
V = d/t
2. show the working
= 600 m / 20 s
3. Give the answer and the units
4. Round if necessary to 2DP
= 30 ms-1
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Try these
• 1. A car travels 100 meters in 5
seconds. Calculate the cars speed.
• 2. A boy runs 200 meters in 20
seconds. Calculate the boys speed.
• 3. A plane travels 3000 meters in 6
seconds.
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Speed units- they can change
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• The way we measure speed can
depend on how fast the object is
moving. What units are distance and
time being recorded in below?
• 112 km/h
10000 cm/s
• 65 m/h
12 m/min
• 76 km/s
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• Speed units can be decieving….
• Which object is travelling faster?
• A. 100 m/s
• B. 36 km/h
Some problems to try…..
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1.
Alexandr Vinokurov (KAZ) won the mens road
race (249.5Km) in 5.45.57 Hours. Find his
average speed in Km/h
2. Linda Melanie Villumsen (NZ) came fourth in
the women road race covering 29Km in
37:59.18 minutes. Calculate her
average speed.
3. The Kiwi’s Cohen and Sullivan won the rowing
double skulls final covering 2000m in 6.31.67min.
Calculate their average speed.
4. Compare their speeds over the 3rd 500m
(1.41.22min) with the final 500m ( 1.33.90min).
5. Allison Schmitt (USA) won the 200m freestyle in
1.53.61min. Calculate her average speed.
Think about these scenarios
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What will change in the following
situations (either speed, distance or
time will increase or decrease)
• 1. a car takes longer to travel 100m
• 2. a car moved at a faster speed
• 3. a car travelled at 50 km/h over
500m then 1000m- which one will take
longer to cover?
Distance-time graphs
2) Horizontal line =
40
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4) Diagonal line
downwards =
30
Distance
(metres)
20
10
0
Time/s
20
1) Diagonal line =
40
60
80
100
3) Steeper diagonal line =
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40
Distance
(metres)
30
20
10
0
Time/s
20
40
60
80
100
1) What is the speed during the first 20 seconds?
2) How far is the object from the start after 60 seconds?
3) What is the speed during the last 40 seconds?
4) When was the object travelling the fastest?
How would you show
acceleration?
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Keywords to know:
•
•
•
•
Constant speed
Acceleration
Speed
Stationary
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• Rate of distance
over time
• Change in speed
over time
• Not moving
• Covering the same
distance over time
Making darts
• Task: to design two
darts.
• 1. a dart that can
cover 5m with the
fastest speed.
• 2. a dart that can
cover 3m with the
slowest speed.
• You dart must only be
made out of paper,
paper clips and tape.
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• You must make two
results table to
record each trial
and final speed.
Distance
Time
Average
speed
Review Questions
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• 1. Calculate the speed of a car covering 120m in 5 seconds.
• 2. Calculate the distance a bike travels if it takes 12 seconds
and moves at a speed of 4m/s.
• 3. Calculate the time it takes a person to run 50m at a speed
of 3 m/s
• 4. Draw a simple distance vs time graph to show the journey
below:
• a. A car accelerates to a speed of 20m/s over 5 seconds.
• B. the car travels at a constant speed of 20 m/s for 10
seconds.
• C. The car accelerates for 3 seconds to 35 m/s
Acceleration
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• 1. Get out your car app
• 2. In your car start from a stationary
position and accelerate for 10
seconds.
• 3. record down your speed.
• 4. how could you calculate
acceleration (what variables do you
have?).
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Acceleration
ΔV
Acceleration = change in velocity (in ms-1)
(in ms-2)
time taken (in s)
A
ΔT
Acceleration measures the rate that speed changes over time.
e.g When a car starts increasing in speed from a stop sign, we say that
it is accelerating.
Acceleration is an increase in speed over time.
Deceleration is a decrease in speed over time.
The unit for acceleration is ms-2. This means meter per second per
second! E.g a car accelerating at 2 ms-2 is increasing its speed by 2
meters per second every second it travels.
Examples
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Velocity-time graphs
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1) Upwards line =
80
Velocity
ms-1
4) Downward line =
60
40
20
0
10
2) Horizontal line =
20
30
40
50
3) Upwards line =
T/s
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80
60
Velocity
ms-1
40
20
0
T/s
10
20
30
40
1) How fast was the object going after 10 seconds?
2) What is the acceleration from 20 to 30 seconds?
3) What was the deceleration from 30 to 50s?
4) How far did the object travel altogether?
50
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Distance-time graph for non-uniform motion
40
Distance
(metres)
Object is
accelerating
up to here
30
Object is now
decelerating
20
10
0
Time/s
20
40
60
80
100
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Velocity-time graph for non-uniform motion
40
Speed
(m/s)
Object’s
acceleration
is increasing
30
Object’s
acceleration
is decreasing
20
10
0
Time/s
20
40
60
80
100
Calculating acceleration
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• Collect data so you can calculate two
different rates of acceleration from your
object.
• Spend 5 minutes discussing how you will do
this.
• You must take some evidence (photo, video)
to show your data gathering.
• You must describe and explain several
variables you controlled to make your
results accurate.
• You must show you working to calculate the
two acceleration values.
Car journey app
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• 1. Copy the simple journey graphs into
your book
• 2. Complete the journeys on the
board using your car app.
• 3. Underneath each graph describe
your motion through the journey.
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Calculating acceleration of a point in
your journey
To be done in pairs:
•Timer/ odometer reader
•Racer
What to do: calculate the rates of acceleration below:
1.From a stationary position accelerate (at max) for 5 seconds.
Record the odometer reading and calculate.
2.From a speed of 40km/h accelerate slowly for 10 seconds
3.From a speed of 80km/h accelerate fast for 5 seconds
4.Make your own one up
What units will you use for acceleration?
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What is causing the car to
accelerate?
What is a force?
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A force is a “push” or a “pull” that can act on an object. Some
common examples:
________ – pulls
things downwards
_____ – acts against
anything moving
___ ________ (drag) – acts
against anything moving through air
______ – keeps things afloat
Words – upthrust, air resistance, friction, weight
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What happens to an object
when forces are applied?
• Accelerate (speed up or slow down)
• Change direction
• Change shape
Balanced and unbalanced forces
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Consider a camel standing on a road.
What forces are acting on it?
support
These two forces would be equal –
we say that they are BALANCED.
The camel doesn’t move anywhere.
Weight
Balanced and unbalanced forces
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support
What would happen if we took the
road away?
Weight
Balanced and unbalanced forces
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What would happen if we took the
road away?
The camel’s weight is no longer
balanced by anything, so the camel
falls downwards…
Weight
Balanced and unbalanced forces
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What would happen if we took the
road away?
The camel’s weight is no longer
balanced by anything, so the camel
falls downwards…
Balanced and unbalanced forces
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1) This animal is either
________ or moving
with _____ _____…
3) This animal is getting
_______….
2) This animal is getting
_________…
Balanced and unbalanced forces
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Acceleration due to gravity
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• When an object is in free fall it will accelerate
due to gravity at 10ms-2.
• When objects fall from a large height they do not
continue to accelerate but eventually reach a
constant speed. This speed is called terminal
velocity.
• This occurs because eventually air resistance will
be evenly balanced with gravity.
What will happen in the following scenarios?
• A. a coin and a feather are dropped, they have the
same mass.
• B. two coins are dropped, one is heavier but both
have the same surface area.
Newton’s laws
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• 1. When forces acting on an object
are balanced the object will either be
stationary or moving at a constant
speed.
• 2. The larger the mass the more
force is needed for the object to
accelerate.
• 3. For every force there is a reaction
force.
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What different forces are
there?
•
•
•
•
•
•
•
•
•
Frictional Force
Weight Force
Tension Force
Electrical Force
Support Force
Magnetic Force
Air Resistance Force
Applied Force
Spring Force
• Write down examples where you might see these forces
being applied.
Drawing force diagrams
•
•
•
•
•
•
Forces must be drawn in pairs
Arrows must be touching the
object
Arrow size must be a
reflection of the force being
applied e.g.
If forces are balanced the
arrows must be the same
length
If the forces are unbalanced
the arrows must be different
sizes
Named examples of forces
must be given
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Net Force
• If all the paired
forces were added
up then the sum or
total force is called
the Net force
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Force and acceleration
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If the forces acting on an object
are unbalanced then the object will
accelerate, like these wrestlers:
Force (in N) = Mass (in kg) x Acceleration (in m/s2)
F
M
a
Force, mass and acceleration
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1) A force of 1000N is applied to push
a mass of 500kg. How quickly does
it accelerate?
F
2) A force of 3000N acts on a car to
make it accelerate by 1.5m/s2. How
heavy is the car?
3) A car accelerates at a rate of
5m/s2. If it weighs 500kg how
much driving force is the engine
applying?
4) A force of 10N is applied by a boy
while lifting a 20kg mass. How
much does it accelerate by?
M
a
To discuss properties of forces
Unistructual
Multistructual
Relational
Extended
Abstract
I can
communicate
one idea about
forces
I can
communicate
several ideas
about forces
I can
communicate
several ideas
about forces
I can
communicate
several ideas
about forces
AND
AND
Explain how
they are linked
together
Explain how
they are linked
together
AND
I can make a
generalisation
about forces
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Discussing the journey of an object
• 1. Name all forces acting on the
object (name them in pairs)
• 2. describe the strength of the
forces and the direction that they
occur
• 3. describe if the forces are balanced
or unbalanced
• 4. Explain the net force
• 5. Describe the motion of the object
as a result of the forces and the
direction
Mass vs Weight
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• Mass is the amount of matter an object
has. This is measured in Kg’s
• Weight is the force of gravity acting on a
mass. This is measured in Newtons!
• This can be calculated by:
• Weight= F gravity= mass X gravity
» Where gravity is 10N Kg -1
» What does that mean?
Why is g= 10Nkg-1?
• Gravity will apply 10N of
force on an object for
every Kg of mass it
contains.
Why?
• Gravity is a force
• When an object is dropped
only gravity is acting on the
object
• The force(s) are
unbalanced (what do
objects do?)
• Gravity causes objects to
accelerate at 10ms-2
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• Find the force of
these objects:
• 1. an object falls with
a mass of 12 Kg.
• 2. an objects falls with
a mass of 26Kg
So all you need to do is
multiply the mass by 10
Helicopter investigation
Design an experiment to
investigate acceleration
due to gravity.
You have access to:
•1m ruler
•Paper clips
•Helicopter
•Stop watch (on your
phone)
•Electronic Scales
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• You must record
some data
• You must change an
independent
variable with a
range of 3
• You must repeat 3
times
Review Questions
•
•
•
•
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1. Define the term weight
2. Define the term mass
3. Compare the units they are measure in.
4. g=10NKg-1. Write a sentence to describe what
this means.
• 5. two objects are dropped. One has a mass of 2kg
and the other a mass of 42Kg. Calculate the force
of each.
• 6. Ignoring drag/air resistance. Which object
would hit the ground first? Why?
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Aussie Science competitionTomorrow start of P3
•
•
•
•
•
-allocated one hour
-only use pencils
-eraser
-calculator
-ruler
Mass and Weight
MASS
Mass is the amount of matter in an
object.
Mass is measured in kilograms (kg).
Mass is constant anywhere in the
universe.
For the mass of an object to change it
must lose or gain matter.
weight
Weight is the force of gravity acting on
a mass.
Weight is measured in Newtons (N).
Larger gravity means larger weight
force.
On earth a one kg mass has a weight of
10N on Earth.
Alien Gravity
Use the boxes to help you work out the weight of the alien.
My mass is 50kg. If
gravity on Earth is
10m/s2, what is my
weight?
Weight = mass x gravity
= _______kg x ______m/s2
= ________ N
Weight = mass x gravity
= 50 kg x 10 m/s2
= 500 N
My mass is
still 50kg. But
how much do I
weigh here?
Gravity = 30N/kg
= 30Nkg-1
Weight = mass x gravity
= _______kg x _____N/kg
= ________ N
Weight = mass x gravity
= 50 kg x 30 N/kg
= 1500 N
More Alien Gravity
On this small
moon I only
weigh 75N. I
wonder what
the gravity
is?
g = 12 m/s2
Weight = mass x gravity
= 50 kg x 12 m/s2
= 600 N
Gravity
= _______
/ ________
Gravity
=
weight
/ mass
= _______N / ______kg
2
= 75
/ 50 kg
= ________
m/sN
= 1.5 m/s2
More Alien Gravity
On this planet,
I weigh 750N.
My brother
weighs 600N.
What’s my
mass?
Gravity = weight / mass
= 750 N / 50 kg
= 15 m/s2
Mass = weight / gravity
= 600 N / 15 m/s2
= 40kg
Hint :- Use the weight and mass of the
first alien to find the gravity of the
planet. Then use that to find the mass
of his brother.
Fun with Eggs
22/05/2017
Oh nooooooooooo!!!!!
• An egg has been dropped from a height of 3m.
Design a safety parachute for your own egg.
Specs:
• Must have a parachute
• At least half of the egg must be visible at the end
of the drop.
Winning criteria:
Non-cracked egg
Lowest WEIGHT
Slowest time to fall 3m.
Balloon Experiment
• 1. Why can 1 person lie down on several
balloons but couldn’t stand?
• 2. What happened to the balloon shape
when a person was lying on them? What
about when more people were added?
• 3. Why was it harder to sit on balloons
when the cardboard was removed?
• 4. Where did the balloons need to be
placed (relative to the body)? Why?
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Pressure
• Pressure is a measure of how
much force is pushing on a
certain area.
P
• It can be calculated by:
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F
Pressure (in Pa) = Force (in N) / Area (in m2)
• P=F/A
• The unit for pressure is Pascals (Pa),
but the measurements are often large
so kPa are common (KiloPascal)
• 1 pascal (Pa) is a pressure of 1 newton
per square metre
A
Find the pressure on your feet
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•
•
•
•
•
•
1. Trace one of your feet.
2. Count the number of cm2.
3. Convert into m2.
4. Find your mass.
5. Convert to weight force.
6. Calculate pressure. Remember you
have two feet!
Explaining pressure
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• Use the variables in the formula for
pressure to explain what is happening.
• E.g if a person stands on two feet and tries
to balance on one foot how would the
pressure on the one foot change?
• 1. Has the area changed?
• 2. Has the force changed?
• 3. Has the pressure changed as a result?
Friction
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• Friction is the force which opposes
one surface moving (or trying to) over
another.
• Heat is often produced as a result of
friction (due to energy conservation).
Uses of friction
• Where friction is
useful:
• Tyres
• Brakes
• Shoes (sprigs)
• Snow chains
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• Where friction is
not useful:
• Car engines
• Car transmission
• Ice skates
Stopping a car…
Tiredness
Too many
drugs
Thinking
distance
Too much
alcohol
Poor
visibility
Wet roads
Icy roads
Tyres/brakes
worn out
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Braking
distance
Driving too
fast