Transcript Electricity

18/07/2015
Mechanics
W Richards
The Weald School
Uncertainty
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Consider a ruler:
It has an uncertainty of ±0.5mm
Now consider the time taken for a ball to drop:
Drop no.
Drop 1
Drop 2
Drop 3
Uncertainty
Time taken
to fall/s
Percentage =
uncertainty
Uncertainty
Average value
X 100%
Summary
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Take appropriate measurements and complete the table:
Thing to
measure
Width
of book
Width
of table
leg
Diam. of
hair
Depth of
beaker
What
Meas. 1 Meas. 2 Meas. 3 Ave.
device?
Uncert- %
ainty
uncertainty
Density
Density =
Mass
Volume
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ρ=
m
V
1) What is the density of a piece of wood of volume 2m3 and
mass 1200kg?
2) Air only has a density of 1.3kg/m3. What is the mass of
0.2m3 of air?
3) Carbon dioxide is more dense and the same volume would
have a mass of 0.38kg. What is its density?
4) The mercury in a thermometer has a volume of 5x10-5m3.
If the density of the mercury is 13600kg/m3 what mass of
mercury is in the thermometer?
Density
Object
Mass/kg
Volume/m3
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Density/
kg/m3
Standard Form and prefixes
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Prefix
Giga
Symbol
G
Multiplier
109
Mega
Kilo
Milli
M
K
m
106
103
10-3
Micro
Nano
Pico
μ
n
p
10-6
10-9
10-12
Try these hard questions…
• What is 1mm2 in m2?
•
•
•
What is 1μm2 in m2?
What is 10mm3 in m3?
How many pm3 fit in a cubic kilometre?
International System of Units
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There are six basic quantities we need to know about. Their
units are called S.I. units:
Base quantity
Base unit
Symbol
Length
metre
m
Mass
kilogram
kg
Time
second
s
Current
ampere
A
Temperature
Kelvin
K
Amount of
substance
mole
mol
Derived Units
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Derived units are units that are made up out of base units.
For example, the unit for speed (metre per second) comes
from the base units for distance and time.
The following units are derived.
Use suitable equations to express
each unit in terms of base units:
1) Newton (force)
2) Joule (energy)
3) Pascal (pressure)
4) Watt (power)
5) Coulomb (electric charge)
Homogeneity
Calculate the following:
10kg + 5m = ??
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It doesn’t make
sense! You can’t add
kilograms to metres.
That’s just silly.
For an equation to be correct it has to be “homogenous”. In
other words, it has to add and equal the same type of units.
Prove, by considering base units or otherwise, that the following equations
are homogenous with respect to units:
1) Volume of cylinder = πr2h
2) Acceleration = change in speed / time taken
3) Kinetic energy = ½mv2
4) Power = force x velocity
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Distance, Speed
and Time revision
Speed = distance (in metres)
time (in seconds)
D
S
T
1) Simon walks 200 metres in 40 seconds. What is his speed?
2) Howard covers 2km in 1,000 seconds. What is his speed?
3) How long would it take Ryan to run 100 metres if he could
run at 12m/s?
4) Ben throws a book at Dan and it travels at 50m/s for 0.2s.
How far away was Dan?
5) Chris is learning to drive. He drives his car at 85mph
(about 40m/s). How long does it take him to drive 20km?
Some subtle differences…
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“Distance” is how far you have gone, “displacement” is how far
you are and can be positive or negative:
Distance =
Distance =
Displacement =
Displacement =
Start
-1 metre
1 metre
Distance
Distance
= =
Displacement
Displacement
= =
Some subtle differences…
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“Distance” is how far you have gone, “displacement” is how far
you are and can be positive or negative:
Speed =
Speed =
Velocity =
Velocity =
Start
-1 metre
1 metre
Speed
Speed
= =
Velocity
Velocity
= =
“Speed” is how fast you go. “Velocity” is how fast in a given
direction.
Vector vs. scalar
Scalar quantities have size only and no direction.
Vector quantities have both size and direction.
Scalar or vector???
Scalar
Vector
8. Power
2. Distance12. Acceleration
1. Mass
6. Energy
7. Time
3. Displacement
4. Speed
11. Force
10. Current
5. Velocity
9. Momentum
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40
Distance
(metres)
30
20
10
0
Time/s
20
40
60
80
100
1) What is the velocity during the first 20 seconds?
2) What is the displacement after 60 seconds?
3) What is the velocity during the last 40 seconds?
4) What is the displacement after 100 seconds?
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20
10
Displacement
(metres)
0
-10
-20
Time/s
20
40
60
80
100
1) What was the displacement after 20 seconds?
2) What was the velocity between 20 and 40 seconds?
3) When was this person travelling the fastest?
4) What was the average speed for the first 40 seconds?
Understanding Velocity
1) Is this car travelling at constant speed?
2) Is this car travelling at constant velocity?
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Understanding Velocity
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40
30
Displacement
(metres)
20
10
0
Time/s
20
40
1) What’s the average velocity?
2) What’s the velocity at 60s?
60
80
100
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Acceleration
V-U
Acceleration = change in velocity (in m/s)
(in m/s2)
time taken (in s)
A
T
1) Ryan accelerates on his bike from 0 to 10ms-1 in 5 seconds.
What is his acceleration?
2) Harry drops a ball and it accelerates downwards at a rate
of 10ms-2 for 12 seconds. What speed did it reach?
3) A car accelerates from 10 to 20ms-1 with an acceleration of
2ms-2. How long did this take?
4) A rocket accelerates from 1,000ms-1 at a rate of 20ms-2
for 2 minutes. What speed did it reach?
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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?
2) What is the acceleration from 20 to 30 seconds?
3) What was the acceleration from 30 to 50s?
4) How far did the object travel altogether?
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20
10
Velocity
(metres)
0
-10
-20
Time/s
20
40
60
80
100
1) When did the object have zero acceleration?
2) What is the average acceleration from 0 to 40s?
3) What was the acceleration from 40 to 60s?
4) How far did the object go between 50 and 100s?
A closer look at motion graphs
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Consider a bouncing ball:
Displacement
Time
A closer look at motion graphs
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Consider a bouncing ball:
Velocity
Time
A closer look at motion graphs
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Consider a bouncing ball:
Acceleration
Time
Equations of Motion
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If we’re talking about any object travelling in a straight line with constant
acceleration then we can use these 4 “golden equations”…
Golden equation
equation #1
#1
Golden
Goldenequation
equation#2
#1
Golden
Vel
Vel
v
v
Ave
v-u
u
u
T
Ave. velocity = (u + v) / 2
Therefore x =
u+v
t
2
T
Acc = (v – u) / t
Therefore v = u + at
Equations of Motion
Goldenequation
equation#3
#1
Golden
Goldenequation
equation#4
#1
Golden
Vel
From eqn #2 t = (v-u) / a
v
u
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v-u
t(v-u)/2
ut
T
From equation #2 (v-u) = at
Therefore x = ut + t/2 x at
Therefore x = ut + ½at2
From eqn #1 x = t(u+v) / 2
So x = (v-u) (v+u)
2a
2ax = v2 – u2
Therefore v2 = u2 + 2ax
Equations of Motion
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u+v
x=
2 t
v = u + at
x = ut + ½at2
v2 = u2 + 2ax
They’re
golden!
Example questions
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1) Ben drops a ball on Dan’s foot. How long does the ball take
to fall 1m? 2m? Why is the second answer not twice the
first?
2) Ryan flies to Belgium. His aeroplane has a maximum
acceleration on the ground of 3.4ms-2. What is the
minimum length of runway needed to reach its take off
speed of 110ms-1 and how long will this take?
3) Luke likes watching kangaroos. A kangaroo jumps to a
vertical height of 2.8m. For how long was it in the air?
4) Tom likes baseball. A baseball pitcher can release a ball at
40ms-1 after accelerating through a distance of 2.5m.
Calculate the average acceleration of the ball.
Example questions
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5) Andrew wants to play with the air track. The air track is
slightly tilted. He pushes a trolley up the track with a
speed of 1ms-1 and its acceleration due to the tilt is 0.5ms-2
down the track. How long does it take to drop 1m below
the starting point?
6) Howard travels in a rocket powered sledge and accelerates
from rest to 284ms-1 in 5s and then comes to a rest in 1.5s.
Calculate his acceleration in both stages.
7) Harry has a good chance of surviving a car crash with a
seatbelt on if his deceleration does not exceed 30g.
Calculate the distance by which the front end of the car
must collapse in if a crash occurs at 70mph.
Vertical Projection
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If I throw this ball upwards with a speed of 40ms1 how high will it go?
Use v2 = u2 + 2ax
0 = 402 + (2 x -9.81 x x)
0 = 1600 – 19.62x
1600 = 19.62x
x = 1600/19.62
x = 81.5m
Practice Questions
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1) How far will a cricket ball go if it is thrown upwards with an
initial velocity of 10ms-1?
2) How far will a table tennis ball go if it is thrown upwards
with an initial velocity of 5ms-1?
3) A human cannonball is projected vertically upwards and she
reaches a vertical height of 20m before coming back down.
How fast was she going when she left the cannon?
4) A test tube falls off the table. If the table is 1m high how
fast was the test tube going when it hit the floor?
Measuring g
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Consider the equation x = ut + ½at2…
If we consider a ball being dropped then u=0, so x = ½at2
We also know that a = g, therefore…
x = ½gt2
x
x
x = ½ g t2
x
x
y=mx+c
Gradient = g
x
½t2
Projectile Motion
Aha! If I let go of the branch when
he fires his gun I’ll be safe because
the bullet will go above me…
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Projectile Motion
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Question – how long did this take
and how fast was the bullet?
1.5m
50m
1) Use x = ut + ½at2 vertically to find the time
2) Then use speed = distance / time horizontally to get the
speed
Example questions
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1) Ben throws a bowling ball at Tom and it lands on his foot.
If the ball started 1.2m above Tom’s foot and the distance
between them was 2m calculate both the time taken and
the initial velocity of the ball.
2) Rob fires a gun and the bullet leaves the barrel at a speed
of 200ms-1. If it landed on the ground 500m away calculate
how long the journey took and how high up Rob was holding
the gun from ground level.
3) Andrew likes knocking test tubes off the table. If he hits
one with an initial velocity of 2ms-1 and the table is 1m high
calculate the time taken for the test tube to hit the floor
and how far away from the table it landed.
Recap questions
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1) Andrew Murray hits a tennis ball and it passes horizontally
over the net and lands just inside the baseline of the court.
The net has a height of 1.07m and is 11.9m from the
baseline. Find the horizontal speed of the ball.
2) David Beckham takes a free kick and it flies into the top
corner horizontally. If the corner is 2.4m above the
ground and the goal is 18m away calculate the time taken
for the ball to reach the goal.
Newton’s
st
1
Law of Motion
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Basically, a body will remain at rest or
continue to move with constant velocity as
long as the forces acting on it are balanced.
…and an unbalanced
Newton 1642-1727
backwards force will make
me slow down…
An unbalanced forwards
force will make me
accelerate…
Newton’s
nd
2
Law of Motion
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The acceleration of a body is proportional to
the resultant force causing its acceleration
and is in the same direction.
Newton 1642-1727
In other words…
F
force = mass x acceleration
M
A
Revision questions
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 1.5ms-2. How
heavy is the car?
3) A car accelerates at a rate of 5ms-2.
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?
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M
A
Testing Newton’s
nd
2
Law
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For each version of the experiment:
1) Draw a diagram of how you set it up
2) Describe your method
3) Describe what equipment you used to get the results and
how you analysed them (you only need to do this once as
they’re both the same).
Newton’s
rd
3
Law of Motion
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When body A exerts a force on body B, body
B exerts an equal and opposite force on body
A.
My third law says
that if I push to
the right I will
move backwards
as well.
Newton 1642-1727
Newton’s
rd
3
Law of Motion
What will happen if I push
this satellite away from me?
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Types of Forces
Gravitational
(W=mg)
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Electromagnetic/
electrostatic
+++
+ +
Nuclear (2 types)
+++
+ +
Describe each force,
including a comment on the
distance it works over,
whether it repels or attracts
and other important points
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.
This is a Newton III pair of
forces
Free body force diagrams 2
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Consider a man standing on a table on the Earth:
Newton I vs. Newton III
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These two forces are acting on the same body,
they’re two different types of force and the man
is in equilibrium as long as the forces balance –
this is a “Newton I pair of forces”.
These two forces are acting on
different bodies, they’re both the
same type and they are always equal
and opposite – this is a “Newton III
pair of forces”.
Summary
Newton I
A law about the forces on _
_____ ____
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Newton III
A law about the forces on
____ _______ _____
Concerns any _____ of forces Always concerns ____ forces
only
The forces can be ______
types
Both forces are ___ ______
type
If there are two forces and
the body is in equilibrium the
forces are _____ and
________
The two forces are ALWAYS
______ and ________
Newton I only applies when
the body is in ________
Newton III ______ applies
Random recap questions
18/07/2015
1) Nick runs the last 100m of a 200m race over 15s. If he
was accelerating at a rate of 1ms-2 during those 15s how
fast was he running when he passed the 100m mark?
2) Ben throws a ball from the 1st floor at Ryan below. If the
ball travels for 1.5s before hitting Ryan how far above Ryan
is Ben? If Ryan is 20m away from the building how fast did
Ben throw it?
3) Dan is swinging a conker around on a piece of string. Draw
a free body force diagram for each object (you may find it
easier to draw both on the same diagram).
4) For each of the forces in the previous two diagrams
identify the Newton III pair and describe what the force
is, what is acts on and its direction.
Vectors
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10km
10km
14.1km
100ms-1
5ms-1
100.1ms-1
Resolving Vectors
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Consider a diagonal push:
This force is given by:
F1 = F sin θ
θ
F1
F2
This force is given by:
F2 = F cos θ
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Resolving Vectors – example questions
Calculate the horizontal and vertical components of the
following:
1)
2)
10N
20N
35O
50O
Work out the size and direction of the resultant force:
3)
4)
8N
10N
50O
80O
20N
15N
45O
30O
Free body force diagrams 3
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Consider a man on a sloping table:
Reaction (a contact force) is
perpendicular to the surface.
Friction (a tangential contact
force) goes up the slope. Let’s
combine the forces…
Resultant force is zero,
so no acceleration
Free body force diagrams
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1) Draw a free body force diagram
for a ladder against a wall.
2) A car pulls a caravan along the
M25. Draw a free body force
diagram for the caravan.
3) Draw a free body force diagram
for a 4-wheel drive car driving up
the M1.
4) Draw a free body force diagram
for a 2-wheel drive (engine at the
front) car driving up the M1 as well.
Moments revision
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A moment is a “turning force”, e.g. trying to open or close a
door or using a spanner. The size of the moment is given by:
Moment (in Nm) = force (in N) x PERPENDICULAR distance
from pivot (in m)
Calculate the following turning moments:
5 metres
100 Newtons
2 metres
200 Newtons
Turning Moments revision
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2 metres
200 Newtons
Total ANTI-CLOCKWISE
turning moment = 200x2 =
400Nm
2 metres
100 Newtons
Total CLOCKWISE turning
moment = 100x2 = 200Nm
The anti-clockwise moment is bigger so the seesaw will
turn anti-clockwise
Balanced or unbalanced?
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Turning Moments
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Consider a man walking along a plank of wood on a cliff.
How far can he walk over the cliff before the plank tips over?
Aaarrgghh
Man’s weight =
800N
1m
3m
Plank’s weight =
200N
Another example
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Consider a car on a suspension bridge:
How much weight does each support take?
20m
3m
Weight of car =
10,000N
Weight of bridge
= 500,000N
A recap question
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1) State the principle of moments
2) Calculate the mass of man in the example given below:
30kg
0.4m
1.2m
Momentum
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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 kgms-1)
(in kg)
(in ms-1)
M
What is the momentum of the following?
1) A 1kg football travelling at 10ms-1
2) A 1000kg Ford Capri travelling at 30ms-1
3) A 20g pen being thrown across the room at 5ms-1
4) A 70kg bungi-jumper falling at 40ms-1
V
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 = 50ms-1
Mass = 1000kg
Speed = 20ms-1
Mass = 800kg
Mass = 1800kg
Speed = ??ms-1
Momentum before = momentum after…
…so 1000 x 50 + 800 x 20 = 1800 x V…
…V = 36.7ms-1
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Momentum in different directions
What happens if the bodies are moving in opposite directions?
Speed = 50ms-1
Mass = 1000kg
Speed = 20ms-1
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 kgms-1
Speed after collision = 34000 kgms-1 / 1800 = 18.9ms-1
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 ms-1 what was the speed
of the alpha particle?
4
2
α
More questions…
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1) 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?
2) 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?
3) 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?
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?
Newton’s
nd
2
Law and Impulse
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Instead of F=ma Newton actually said that the force acting on
an object is that object’s rate of change of momentum. In
other words…
mv
Force = Change in momentum (in kgm/s)
(in N)
Time (in s)
Also called “impulse”
F
T
For example, David Beckham 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) Ben 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.
2) Nick thinks it’s funny to hit tennis balls at Tom. He 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.
3) Dan 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.
4) Simon strikes a 200g golf ball away at 50m/s. If he
applied a force of 50N calculate how long his club was in
contact with the ball for.
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Another way to ask the same question…
Here’s a situation we looked at earlier…
Speed = 50ms-1
Mass = 1000kg
Speed = 20ms-1
Mass = 800kg
What’s the impulse of the car on the left if the cars stick
together?
Energy loss in collisions
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In the “Forces” module we looked at how to calculate an object’s kinetic
energy:
Kinetic energy = ½ x mass x velocity squared
in J
in kg
in m/s
We’ve also said that in a collision momentum is conserved (unless an
external force acts). The same cannot usually be said for kinetic energy…
For example, consider the following collision. How much kinetic energy is
lost?
Before
Speed = 50m/s
Speed = 20m/s
Mass = 1000kg
Mass = 800kg
After
Mass = 1000kg
Speed = 20m/s
Mass = 800kg
Speed = 30m/s
Energy loss in collisions
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Consider a head-on collision where the cars stick together. How much
kinetic energy is lost in this example? Where does all the energy go?
Before
Speed = 50m/s
m=800Kg
Speed = 30m/s
m=3000Kg
After
Speed = 10m/s
In this example more kinetic energy was lost. We say
it was a “less elastic collision”. An “elastic collision” is
one where the kinetic energy is conserved.
Work done
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Work done (in joules) is simply the force needed to move an
object multiplied by the distance moved in the direction of the
force:
ΔW
ΔW = FΔx
F
Δx
Power
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Power (in watts) is “the rate of doing work”:
ΔW
P = ΔW
Δt
P
Also, using our “work done” equation:
P = ΔW = FΔx
Δt
Δt
Δt
ΔW = FΔx
…therefore
P = Fv
Random questions on work and power
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1) Luke pushes Ben in the direction of a cliff. If he uses a
force of 40N and he moves Ben 10m in 4s calculate the
work done and Luke’s power rating.
2) Dan runs up some stairs and has a power rating of 600W
while he does so. If he does it in 5 seconds and his weight
is 750N calculate how high the stairs are.
3) A man pulls a block of wood at an angle of 200 to the
horizontal and uses a force of 50N. If the distance
travelled horizontally is 5m calculate the work done by the
man and his power if the journey lasted 10 seconds.
200
50N
Conservation of Energy
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Consider a bouncing ball:
Gravitational
Potential Energy
Time
Conservation of Energy
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Consider a bouncing ball:
Kinetic Energy
Time
Conservation of Energy
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Now put these graphs together:
Kinetic Energy
Total
energy of
the ball
Time
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Radioactivity
W Richards
The Weald School
Structure of the atom
A hundred years ago people thought
that the atom looked like a “plum
pudding” – a sphere of positive
charge with negatively charged
electrons spread through it…
Ernest Rutherford, British scientist:
I did an experiment (with my colleagues
Geiger and Marsden) that proved this
idea was wrong. I called it the
“Scattering Experiment”
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The Rutherford Scattering Experiment
Alpha
particles
(positive
charge, part
of helium
atom)
Most particles passed
through, 1/8000 were
deflected by more than
900
Conclusion – atom is made up of a small, positively
charged nucleus surrounded by electrons orbiting
in a “cloud”.
Thin gold
foil
The structure of the atom
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ELECTRON –
negative, mass
nearly nothing
NEUTRON –
neutral, same
mass as
proton (“1”)
PROTON –
positive, same
mass as
neutron (“1”)
Atoms are roughly 10-10m in diameter,
while the nucleus is 10-15 – 10-14m
The structure of the atom
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Particle
Relative Mass
Relative Charge
Proton
1u (1.7x10-27kg)
+1.6x10-19C
Neutron
1u (1.7x10-27kg)
0
Electron
0
-1.6x10-19C
MASS NUMBER (A) = number of
protons + number of neutrons
SYMBOL
PROTON NUMBER (Z) = number of
protons (obviously)
No. of neutrons N = A - Z
Isotopes
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An isotope is an atom with a different number of neutrons:
Notice that the mass number is different. How many
neutrons does each isotope have?
Each isotope has 8 protons – if it didn’t then it just
wouldn’t be oxygen any more.
A “radioisotope” is simply an isotope that is radioactive –
e.g. carbon 14, which is used in carbon dating.
Quarks
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We can investigate the structure of protons by bombarding
them with electrons:
Low energy scattering
e-
P
Elastic collision. Electrons and protons behave as expected.
High energy scattering
e-
P
Inelastic collision. Energy is “absorbed” by the proton and
increases its internal energy. This is Deep Inelastic
Scattering and suggests that the proton is made of smaller
particles called quarks.
Introduction to Radioactivity
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Some substances are classed as “radioactive” – this means that
they are unstable and continuously give out radiation:
Radiation
The nucleus is more stable after emitting some radiation – this
is called “radioactive decay”.
Ionisation
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Radiation is dangerous because it “ionises” atoms – in other
words, it turns them into ions by giving them enough energy to
“knock off” electrons:
Alpha radiation is the most ionising (although short range).
Ionisation causes cells in living tissue to mutate, usually
causing cancer.
The Geiger-Muller Tube
Metallic case (cathode)
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Mixture of argon
and halogen
Mica end window
Central anode
Types of radiation
Unstable
nucleus
New
nucleus
Alpha
particle
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1) Alpha () – an atom decays into a new
atom and emits an alpha particle (2
protons and 2 ______ – the nucleus of a
______ atom)
2) Beta () – an atom decays into a new
atom by changing a neutron into a
_______ and electron. The fast moving,
Beta high energy electron is called a _____
particle particle.
Unstable
nucleus
New
nucleus
Unstable
nucleus
New
nucleus
3) Gamma – after  or  decay surplus
______ is sometimes emitted. This is
called gamma radiation and has a very
high ______ with short wavelength.
The atom is not changed.
Gamma
radiation
Words – frequency, proton,
energy, neutrons, helium, beta
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Changes in Mass and Proton Number
Alpha decay:
241
Am
95
237
Np
93
+
4
+
0
2
α
Beta - decay:
90
Sr
38
90
Y
39
“positron”
Beta + decay:
11
6
C
β
-1
11
B
5
+
0
+1
β
Blocking Radiation
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Each type of radiation can be blocked by different materials:



Sheet of
paper (or
6cm of air
will do)
Few mm of
aluminium
Few cm of
lead
Summary
Property
Charge
Rest mass
Penetration
What is it?
Ionising
ability
Alpha
Beta -
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Beta +
Gamma
Deflection by Magnetic Fields
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Alpha and beta particles
have a charge:
+
2 protons, 2 neutrons,
therefore charge = +2
+
1 electron, therefore
charge = -1
-
Because of this charge, they will be deflected by electric and
magnetic fields:
+
-
Background Radiation
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13% are
man-made
Radon gas
Food
Cosmic rays
Gamma rays
Medical
Nuclear power
Nuclear fission
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More
neutrons
Neutron
Uranium
nucleus
Unstable
nucleus
New nuclei
(e.g. barium
and krypton)
Chain reactions
Each fission reaction releases
neutrons that are used in
further reactions.
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Radioactive Decay
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Radioactivity is a random process. The number of
radioisotopes that will decay clearly depends on the number of
radioisotopes present at that point in time:
Activity (in Bq) = λN
λ = “The decay constant” and has units of s-1. It is constant
for a particular radioisotope.
Half Life
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The decay of radioisotopes can be used to measure the
material’s age. The HALF-LIFE of an atom is the time
taken for HALF of the radioisotopes in a sample to decay…
= radioisotope
At start
there are 16
radioisotopes
After 1 half
life half have
decayed
(that’s 8)
= new atom formed
After 2 half
lives another
half have
decayed (12
altogether)
After 3 half
lives another
2 have
decayed (14
altogether)
A radioactive decay graph
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Count
1 half
life
1 half
life
1 half
life
Time
Half Life
To calculate half life there are a few methods:
1) Read from a graph
2) Calculate using an equation
t½ = ln2
λ
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Half Life questions
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1) The graph shows the activity of
a radioisotope. Determine the
half life and decay constant.
2) If there are 106 atoms present
right now calculate how many
will decay over the next second.
100s
3) What percentage of a sample of radioactive material will
exist after 200 years if the half life is 50 years?
4) Uranium decays into lead. The half life of uranium is
4,000,000,000 years. A sample of radioactive rock
contains 7 times as much lead as it does uranium. Calculate
the age of the sample.