Transcript Physics Unit 2a - The Connected Hub

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Physics Unit
2a
P2.1.1 – Resultant Forces
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Balanced and unbalanced forces
Consider a camel standing on a road.
What forces are acting on it?
Reaction
These two forces would be equal –
we say that they are BALANCED.
The camel doesn’t move anywhere.
Weight
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Balanced and unbalanced forces
Reaction
What would happen if we took the
road away?
Weight
Air Resistance
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Air resistance is a force that opposes motion through air. The
quicker you travel, the bigger the air resistance:
The same applies to a body falling through a liquid (called
“drag” or “upthrust”).
Balanced and unbalanced forces
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Balanced and unbalanced forces
1) This animal is either
________ or moving
with _______ _____…
2) This animal is getting
________…
3) This animal is getting
_______….
4) This animal is also
either _______ or moving
with ________ ______..
Words - Stationary, faster, slower or constant speed?
Summary
Complete these sentences…
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If an object is stationary and has NO resultant force on it the object
will…
If an object is stationary and a resultant force acts on it the object will…
If an object is already moving and NO resultant force acts on it the
object will…
If an object is already moving and a resultant force acts on it the object
will…
…accelerate in the direction of the
resultant force
…continue to move at the same
speed and the same direction
…continue to stay stationary
…accelerate in the direction of the
resultant force
Resultant Force
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Calculate the resultant force of the following:
500N
100N
700N
600N
50N
700N
700N
200N
800N
800N
100N
P2.1.2 – Forces and Motion
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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
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Force, mass and acceleration
1) A force of 1000N is applied to push
a mass of 500kg. How quickly does
it accelerate?
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 has a mass of 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?
F
M
A
2m/s2
2000kg
2500N
0.5m/s2
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Distance, Speed
and Time
D
Speed = distance (in metres)
time (in seconds)
S
T
1) Freddie walks 200 metres in 40 seconds. What is his
speed?
5m/s
2) Hayley covers 2km in 1,000 seconds. What is her
speed?
2m/s
3) How long would it take Lauren to run 100 metres if she
runs at 10m/s?
4) Jake travels at 50m/s for 20s. How far does he go?
5) Izzy drives her car at 85mph (about 40m/s). How long
does it take her to drive 20km?
10s
1000m
500s
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Distance, Speed
and Time
D
Speed = distance (in metres)
time (in seconds)
S
t
1) Sarah walks 2000m in 50 minutes. What is her speed in
m/s?
0.67m/s
2) Jack tries to walk the same distance at a speed of 5m/s.
How long does he take?
400s
3) James drives at 60mph (about 100km/h) for 3 hours. How
far has he gone?
4) The speed of sound in air is 330m/s. Molly shouts at a
mountain and hears the echo 3 seconds later. How far
away is the mountain? (Careful!)
300km
495m
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Distance-time graphs
2) Horizontal line =
40
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
1) What is the speed during the first 20 seconds?
100
0.5m/s
2) How far is the object from the start after 60 seconds?
40m
3) What is the speed during the last 40 seconds?
1m/s
4) When was the object travelling the fastest?
40-60s
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
Circular Motion
1) Is this car travelling at constant speed?
2) Is this car travelling at constant velocity?
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Acceleration
V-U
Acceleration = change in velocity (in m/s)
(in m/s2)
time taken (in s)
A
1) A cyclist accelerates from 0 to 10m/s in 5 seconds.
What is her acceleration?
T
2m/s2
2) A ball is dropped and accelerates downwards at a rate of
10m/s2 for 12 seconds. How much will the ball’s velocity
increase by?
120m/s
3) A car accelerates from 10 to 20m/s with an acceleration
of 2m/s2. How long did this take?
5s
4) A rocket accelerates from 1,000m/s to 5,000m/s in 2
seconds. What is its acceleration?
2000m/s2
40
Distance
(metres)
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30
20
10
0
Time/s
20
40
60
80
100
1) What was the velocity in the first 20 seconds?
1.5m/s
2) What was the velocity between 20 and 40 seconds?
0.5m/s
3) When was this person travelling the fastest?
80-100s
4) What was the average speed for the first 40 seconds?
1m/s
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Acceleration
V-U
Acceleration = change in velocity (in m/s)
(in m/s2)
time taken (in s)
A
T
1) Will accelerates from standstill to 50m/s in 25 seconds.
What is his acceleration?
2m/s2
2) Pierre accelerates at 5m/s2 for 5 seconds. He started at
10m/s. What is his new speed?
35m/s
3) Elliott is in trouble with the police. He is driving up the A29
and sees a police car and brakes from 50m/s to a standstill.
His deceleration was 10m/s2. How long did he brake for?
5s
4) Another boy racer brakes at the same deceleration but only
for 3 seconds. What speed did he slow down to?
20m/s
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Velocity-time graphs
1) Upwards line =
80
Velocity
m/s
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
m/s
40
20
0
T/s
10
20
30
40
50
1) How fast was the object going after 10 seconds?
40m/s
2) What is the acceleration from 20 to 30 seconds?
2m/s2
3) What was the deceleration from 30 to 50s?
3m/s2
4) How far did the object travel altogether?
1700m
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80
60
Velocity
m/s
40
20
0
T/s
10
20
30
40
50
1) How fast was the object going after 10 seconds?
10m/s
2) What is the acceleration from 20 to 30 seconds?
4m/s2
3) What was the deceleration from 40 to 50s?
6m/s2
4) How far did the object travel altogether?
1500m
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80
60
Velocity
m/s
40
20
0
T/s
10
20
30
40
50
This velocity-time graph shows Coryn’s journey to school.
How far away does she live?
2500m
P2.1.3 – Forces and Braking
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Stopping a car…
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What two things must the driver of the car do in order to stop
in time?
Tiredness
Stopping a car…
Thinking
distance
Too many
drugs
(reaction time)
Too much
alcohol
Poor
visibility
Wet roads
Icy roads
Tyres/brakes
worn out
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Braking
distance
Driving too
fast
Total Stopping Distance = Thinking Distance + Braking Distance
Stopping a car…
What happens inside the car when it stops?
In order to stop this car the
brakes must “do work”. This work
is used to reduce the kinetic
energy of the vehicle and the
brakes will warm up.
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P2.1.4 – Forces and Terminal
Velocity
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Terminal Velocity
Consider a ball falling through a liquid:
Some questions to consider:
1) What forces are acting on
the ball?
2) How do those forces
change when the ball gets
faster?
3) Will the ball keep getting
faster? Explain your
answer in terms of forces
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Terminal Velocity
Consider a skydiver:
1) At the start of his jump the air
resistance is _______ so he
_______ downwards.
2) As his speed increases his air
resistance will _______
3) Eventually the air resistance will be
big enough to _______ the
skydiver’s weight. At this point
the forces are balanced so his
speed becomes ________ - this is
called TERMINAL VELOCITY
Words – increase, small,
constant, balance, accelerates
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Terminal Velocity
Consider a skydiver:
4) When he opens his parachute the
air resistance suddenly ________,
causing him to start _____ ____.
5) Because he is slowing down his air
resistance will _______ again until
it balances his _________. The
skydiver has now reached a new,
lower ________ _______.
Words – slowing down, decrease,
increases, terminal velocity, weight
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Velocity-time graph for terminal velocity…
Parachute opens –
diver slows down
Velocity
Speed
increases…
Terminal
velocity
reached…
Time
New, lower terminal
velocity reached
Diver hits the ground
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Weight vs. Mass
Earth’s Gravitational Field Strength is 10N/kg. In other
words, a 1kg mass is pulled downwards by a force of 10N.
W
Weight = Mass x Gravitational Field Strength
(in N)
(in kg)
(in N/kg)
M
1) What is the weight on Earth of a book with mass 2kg?
2) What is the weight on Earth of an apple with mass 100g?
g
20N
1N
3) Lee weighs 700N on the Earth. What is his mass?
70kg
4) On the moon the gravitational field strength is 1.6N/kg.
What will Lee weigh if he stands on the moon?
112N
P2.1.5 – Forces and Elasticity
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Force and Extension
Consider a mass on a spring:
What happens when a mass is
added?
When a force is applied to
this spring it will change
shape and extend. The
spring will have “stored
elastic potential energy”
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Elastic Potential Energy
Elastic potential energy is the
energy stored in a system
when work is done to change
its shape, e.g:
Describe the energy changes
when the mass is:
1) At the top of it’s
movement
2) In the middle
3) At the bottom
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Force and Extension
Task: Find an expression that
relates extension to the
amount of weight added.
Weight added
(N)
Extension (mm)
1
2
3
4
5
6
Force = Spring constant x extension
F = ke
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Force-Extension Graph for a spring
Force/N
The “limit of
proportionality”
Force is proportional to
extension as long as you don’t
go past the “limit of
proportionality”
Extension/mm
P2.2.1 – Forces and Energy
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Work done
When any object is moved around work will need to be
done on it to get it to move (obviously).
We can work out the amount of work done in moving an
object using the formula:
Work done = Force x distance moved
in J
in N
W
in m
F
D
Example questions
1.
Hannah pushes a book 5m along the table with a force of
5N. She gets tired and decides to call it a day. How much
work did he do?
2. Courtney lifts a laptop 2m into the air with a force of 10N.
How much work does she do? What type of energy did the
book gain?
3. Tom does 200J of work by pushing a wheelbarrow with a
force of 50N. How far did he push it? What type of
energy did the wheelbarrow gain?
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25J
20J,
GPE
4m, KE
4. Dan cuddles his cat and lifts it 1.5m in the air. If he did
75J of work how much force did he use?
50N
5. Simon drives his car 1000m. If the engine was producing a
driving force of 2000N how much work did the car do?
2MJ
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A Practical Example of Doing Work
Consider a rocket re-entering the Earth’s atmosphere:
The rocket would initially have
a very high _______ energy.
This energy would then _____
due to friction caused by
collisions with _______ in the
atmosphere. These collisions
would cause the rocket to ____
up (_____ is “being done” on
the rocket). To help deal with
this, rockets have special
materials that are designed to
lose heat quickly.
Words – work, kinetic,
particles, heat, decrease
Recap questions on Weight
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1) Jack weighs 600N on the Earth. What is his mass in kg?
60kg
2) Mike pushes Alistair with a force of 20N. If Alistair
moves 2m how much work did Mike do on him?
40J
3) Josh weighs 120N on the moon, where g=1.6N/Kg. What
is his mass and what would he weigh on the Earth?
75kg,
750N
4) Rose does 100J of work by pushing her pencil case across
the table. If she applied a force of 5N how far did she
push it?
20m
Energy and Power
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The POWER RATING of an appliance is simply how much
energy it uses every second.
In other words, 1 Watt = 1 Joule per second
E
E = Energy (in joules)
P = Power (in watts)
T = Time (in seconds)
P
T
Some example questions
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1) What is the power rating of a light bulb that transfers
120 joules of energy in 2 seconds?
60W
2) What is the power of an electric fire that transfers
10,000J of energy in 5 seconds?
2KW
3) Rob runs up the stairs in 5 seconds. If he transfers
1,000,000J of energy in this time what is his power
rating?
0.2MW
4) How much energy does a 150W light bulb transfer in a)
one second, b) one minute?
150J,
9KJ
5) Jonny’s brain needs energy supplied to it at a rate of
40W. How much energy does it need during a 50 minute
physics lesson?
120KJ
6) Lloyd’s brain, being more intelligent, only needs energy at
a rate of about 20W. How much energy would his brain
use in a normal day?
1.73MJ
Gravitational Potential Energy
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To work out how much gravitational potential energy
(GPE) an object gains when it is lifted up we would use
the simple equation…
GPE
(Joules)
=
Weight
(newtons)
x
Change in height
(metres)
GPE
(Remember - W=mg)
mg
H
Some example questions…
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How much gravitational potential energy have the following
objects gained?:
1.
A brick that weighs 10N lifted to the top of a house
(10m),
100J
2. A 1,000kg car lifted by a ramp up to a height of 2m,
20KJ
3. A 70kg person lifted up 50cm by a friend.
350J
How much GPE have the following objects lost?:
1.
A 2N football dropping out of the air after being kicked
up 30m,
60J
2. A 0.5N egg falling 10m out of a bird nest,
5J
3. A 1,000kg car falling off its 200cm ramp.
20KJ
Kinetic energy
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Any object that moves will have kinetic energy.
The amount of kinetic energy an object has can be found using
the formula:
Kinetic energy = ½ x mass x velocity squared
in J
in kg
KE =
½
in m/s
mv2
Example questions
1) Bex drives her car at a speed of 30m/s. If
the combined mass of her and the car is
1000kg what is her kinetic energy?
2) Emma rides her bike at a speed of 10m/s. If
the combined mass of Emma and her bike is
80kg what is her kinetic energy?
3) Rob is running and has a kinetic energy of
750J. If his mass is 60kg how fast is he
running?
4) Josh is walking to town. If he has a kinetic
energy of 150J and he’s walking at a pace of
2m/s what is his mass?
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450,000J
4000J
5m/s
75kg
Test questions…
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1)
Julia tries to run 100m in 12 seconds and succeeds. How fast did
she run?
8.3m/s
2)
Isabelle accelerates at a rate of 2m/s2 for 3 seconds. If she
started at 10m/s what was her final speed?
16m/s
3)
Jake decides to lift his book up into the air. His book has a mass
of 100g and he lifts it 50cm. Calculate the work done.
0.5J
4)
Jamie accelerates from 0 to 10m/s in 5 seconds. If her mass is
60kg how much force did her legs apply?
120N
5)
Lily rides 1km at a speed of 20m/s. How long did the journey
take?
6)
Rob thinks it’s funny to push Jack with a force of 140N. If Jack
has a mass of 70kg calculate his acceleration.
2m/s2
7)
Vicky slams on the brakes on her bike and her brakes do
20,000J of work. If the combined mass is 100kg what speed
was she travelling at?
20m/s
8)
Paddy has a mass of 75kg. If he accelerates from 10 to 20m/s
in 2s how much force did he apply?
375N
50s
9)
Test questions…
Bex amuses herself by throwing things at Kit. If she throws a ball
with a speed of 20m/s and the distance between her and Kit is 5m
how long will it take to reach him?
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0.25s
10) Dave throws calculators around the room with a force of 20N. If
each calculator has a mass of 200g calculate the acceleration.
100m/s2
11) Max has a mass of 70kg. What is his weight on Earth, where the
gravitational field strength is 10N/kg?
700N
12) Kathryn does some work by pushing a box around with a force of
1N. She does 5J of work and decides to call it a day. How far did
she push it?
5m
13) On the moon Jake might weigh 112N. If the gravitational field
strength on the moon is 1.6N/kg what is his mass?
70kg
14) Heather likes bird watching. She sees a bird fly 100m in 20s. How
fast was it flying?
5m/s
15) How much kinetic energy would Simon have if he travelled at a
speed of 5m/s and has a mass of 70kg?
875J
P2.2.2 – Momentum
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Momentum
Any object that has both mass and
velocity has MOMENTUM. Momentum
(symbol “p”) is simply given by the formula:
P
Momentum = Mass x Velocity
(in kgm/s)
(in kg)
(in m/s)
M
V
What is the momentum of the following?
1) A 1kg football travelling at 10m/s
2) A 1000kg Ford Capri travelling at 30m/s
3) A 20g pen being thrown across the room at 5m/s
4) A 70kg bungi-jumper falling at 40m/s
10kgm/s
30,000kgm/s
0.1kgm/s
2800kgm/s
Conservation of Momentum
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In any collision or explosion momentum is conserved (provided that there
are no external forces have an effect). Example question:
Two cars are racing around the M25. Car A collides with the back of car B
and the cars stick together. What speed do they move at after the
collision?
Speed = 50m/s
Mass = 1000kg
Speed = 20m/s
Mass = 800kg
Mass = 1800kg
Speed = ??m/s
Momentum before = momentum after…
…so 1000 x 50 + 800 x 20 = 1800 x V…
…V = 36.7m/s
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Momentum in different directions
What happens if the bodies are moving in opposite directions?
Speed = 50m/s
Mass = 1000kg
Speed = 20m/s
Mass = 800kg
Momentum is a VECTOR quantity, so the momentum of the
second car is negative…
Total momentum = 1000 x 50 – 800 x 20 = 34000 kgm/s
Speed after collision = 34000 kgm/s / 1800 = 18.9m/s
Another example
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Consider the nuclear decay of Americium-241:
237
93
Np
241
95
Am
If the new neptunium atom moves away at
a speed of 5x105 m/s what was the speed
of the alpha particle?
2.96x107 m/s
4
2
α
More questions…
1.
A car of mass 1000kg heading up the M1 at 50m/s collides
with a stationary truck of mass 8000kg and sticks to it.
What velocity does the wreckage move forward at?
2. A defender running away from a goalkeeper at 5m/s is hit
in the back of his head by the goal kick. The ball stops
dead and the player’s speed increases to 5.5m/s. If the
ball had a mass of 500g and the player had a mass of 70kg
how fast was the ball moving?
3. A white snooker ball moving at 5m/s strikes a red ball and
pots it. Both balls have a mass of 1kg. If the white ball
continued in the same direction at 2m/s what was the
velocity of the red ball?
4. A gun has a recoil speed of 2m/s when firing. If the gun
has a mass of 2kg and the bullet has a mass of 10g what
speed does the bullet come out at?
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5.6m/s
70m/s
3m/s
400m/s
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Recap question on momentum
1. Matt and Dan are racing against each other over 400m at
Sports Day. Matt is running at 8m/s and catches up with
Dan who is running at 6m/s. After the collision Matt stops
and Dan moves slightly faster. If Matt’s mass is 60kg and
Dan’s is 70kg calculate how fast Dan moves after the
collision.
12.9m/s
2. Bobbie is driving her 5kg toy car around. It is travelling at
10m/s when it hits the back of Heather’s (stationary) leg
and sticks to it. Assuming Heather’s leg can move freely
and has a mass of 10kg calculate how fast it will move after
the collision.
3.3m/s
Safety features
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How do air bags and crumple zones work?
Basically:
1) The change in momentum is the same with or without an
airbag
2) But having an airbag increases the time of the collision
3) Therefore the force is reduced