#### Transcript Unit P2 - Physics for your Future 2

```Speed vs. Velocity
1) Is this car travelling at constant speed?
2) Is this car travelling at constant velocity?
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Vector vs. scalar
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Scalar quantities have size (“magnitude”) only and no direction.
Vector quantities have both size and direction.
Scalar or vector???
Scalar
Vector
2. Distance10. Acceleration
1. Mass
6. Energy
7. Time
3. Displacement
4. Speed
9. Force
8. Current
5. Velocity
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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
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
20
40
60
80
1) What is the speed during the first 20 seconds?
100
Time/s
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
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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
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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
Introduction to Forces
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A force is a “push” or a “pull”. Some common examples:
Weight (mg) – pulls
things towards the
centre of the Earth
Friction – a contact force
that acts against anything
moving
Air resistance/drag – a contact
force that acts against anything
moving through air or liquid
Upthrust – keeps things afloat
Free body force diagrams
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The Earth pulls Newton down with a
gravitational force of 700N.
direction
what
on
what
type
size
Newton pulls the Earth up with a
gravitational force of 700N.
Action and reaction are equal
and opposite!!
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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
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
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 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?
F
M
A
2m/s2
2000kg
2500N
0.5m/s2
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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) James weighs 700N on the Earth. What is his mass?
70kg
4) On the moon the gravitational field strength is 1.6N/kg.
What will James weigh if he stands on the moon?
112N
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
Topic 4 – Momentum, Energy,
Work and Power
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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
Heavy
vehicle
Tyres/brakes
worn out
(reaction time)
Braking
distance
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Too much
alcohol
Poor
visibility
Wet/icy
Driving too
fast
Total Stopping Distance = Thinking Distance + Braking Distance
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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 and
therefore reduces the “rate of change of momentum”
3) Therefore the force is reduced
Force and momentum
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Newton’s second law of motion says that the force acting on an
object is that object’s rate of change of momentum. In other
words…
Force = Change in momentum (in kgm/s)
(in N)
mv-mu
Time (in s)
Also called “impulse”
F
T
For example, Ronaldo takes a free kick by kicking a stationary football with
a force of 40N. If the ball has a mass of 0.5kg and his foot is in
contact with the ball for 0.1s calculate:
1) The change in momentum of the ball (its impulse),
2) The speed the ball moves away with
Example questions
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1) Paddy likes playing golf. He strikes a golf ball with a force
of 80N. If the ball has a mass of 200g and the club is in
contact with it for 0.2s calculate a) the change in
momentum of the golf ball, b) its speed.
16Kgm/s,
80m/s
2) Courtney thinks it’s funny to hit tennis balls at Kit. She
strikes a serve with a force of 30N. If the ball has a
mass of 250g and the racket is in contact with it for 0.15s
calculate the ball’s change in momentum and its speed.
4.5Kgm/s,
18m/s
3) Tom takes a dropkick by kicking a 0.4kg rugby ball away at
10m/s. If his foot was in contact with the ball for 0.1
seconds calculate the force he applied to the ball.
40N
4) Jenny strikes a 200g golf ball away at 50m/s. If she
applied a force of 50N calculate how long her club was in
contact with the ball for.
0.2s
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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
E
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
Stopping a car…
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Recall our earlier situation regarding stopping distances…
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
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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