PDP Physics
Exoplanets
The three known
planets of the star
HR8799, as
imaged by the Hale
Telescope. The
light from the
central star was
blanked out by a
coronagraph..
Image: http://en.wikipedia.org/wiki/Exoplanet
ARE HUMANS ALONE IN THE
UNIVERSE?
Are there civilizations somewhere
else in the universe?
The Drake Equation








N = the number of civilizations in our galaxy with which
communication might be possible
R* = the average rate of star formation per year in our galaxy
fp = the fraction of those stars that have planets
ne = the average number of planets that can potentially support
life per star that has planets
fℓ = the fraction of the above that actually go on to develop life at
some point
fi = the fraction of the above that actually go on to develop
intelligent life
fc = the fraction of civilizations that develop a technology that
releases detectable signs of their existence into space
L = the length of time for which such civilizations release
detectable signals into space.
HOW DO WE KNOW WHAT’S OUT
THERE?
UNITS AND ORDERS OF
MAGNITUDE
One order of magnitude is a power of 10
peta
tera
giga
mega
kilo, k, 103
milli
micro
nano
pico
femto
One light year is the distance light
travels in one year
speed of light = 3.0×108ms-1
One astronomical unit is the average
distance between Earth and sun
1AU = 150×109m
WHAT IS IN OUR SOLAR SYSTEM?
What is in our own Solar System?
MVEMJSUN
Build a scale model of the solar system with a
model Sun 7cm in diameter.
Label your model with details about our solar
system.
ARE THERE PLANETS OUTSIDE OUR
SOLAR SYSTEM?
Exoplanets
Results reported in scientific journals
TED talk
What is the Kepler space telescope?
How does it discover exoplanets?
What features of a planet are scientists
interested in and why?
FORCES AND NEWTON’S LAWS
Voyager 1
launched 1977
 travelled 1.7 × 1010 km
 velocity 3.6 AU per year
 thrusters not firing

Newton’s 1st law
An object in motion remains at constant velocity
unless acted on by an unbalanced force
Objects dropped on the surface of
Jupiteraccelerate at 25ms-2
What makes an object
accelerate? A force

Unbalanced forces can:
– change velocity (speed or direction)
– change an object’s shape
Newton’s 2nd law: an object acted on by an
unbalanced force experiences an
acceleration proportional to the force and
inversely proportional to the mass.
How much force is needed?
Force = mass × acceleration
 F = ma

where F is the force in Newtons
 m is the mass in kg
 a is the acceleration in ms-2

Standing on the surface of Mercury
you will feel a force up through
your legs 37% of the force on Earth
Newton’s Third Law
If object A exerts a force on
object B,
then object B will exert an equal
but opposite force on object A.
or To every action there is an equal but
opposite reaction.
Velocity and Speed
velocity is the vector quantity, a direction is stated or implied
• speed = distance ÷ time
• velocity = distance ÷ time
• unit m/s or ms-1
Acceleration:
vector quantity measuring rate of change of velocity
“speed of the speed”
1g = 9.8ms-2
a=v
t
a = v-u
t
Draw graphs of motion for an
object thrown vertically upwards
with a velocity of 100m/s from a
planet
displacement
velocity
time
acceleration
time
time
GRAVITATION
Big and small numbers
• Which is larger 9×10-1 or 0.8?
• Calculate
9×105 × 3×108
9×10-5 × 3×108
9×105 ÷ 3×108
9×10-5 ÷ 3×108
• Check your answers using your TI calculator
(and the E button)
How do we know there is
gravity in the space?
4 fundamental forces
Strong and weak nuclear forces
 Electromagnetism
 Gravity

 Only gravity important over large
distances
Newton’s Law of Universal Gravitation
F = GMm
r2
G = 6.67×10-11 m3kg-1s-2
(universal gravitational constant)
M = mass of body 1
m = mass of body 2
r = distance between bodies
F = force between bodies
Are you attractive to the person next
to you?
Calculate the gravitational force on 1kg
at the surface of Earth.
Gravitational field strength
• 9.8N on 1kg
• g = 9.8Nkg-1
Image: http://en.wikipedia.org/wiki/Spring_scale
Why do astronauts feel
weightless?
http://www.youtube.com/watch?v=pk7Lc
ugO3zg&feature=related
 http://www.guardian.co.uk/science/video
/2011/mar/18/weightless-bremenmicrogravity-tower

FORCES IN 2D AND 3D
What is the difference between a
scalar and a vector?
•
•
•
•
•
•
•
•
force
energy
speed
length
displacement
momentum
acceleration
mass
Horizontal and vertical components
can be treated separately
Which direction does the bike accelerate?
How fast does the bike accelerate?
m = 80kg
210N
force
from
pedals
50N
800N
Draw a force diagram for the moon buggy.
g = 1.7N/kg
m = 210kg
driving force 760N, frictional force 260N
How fast does the moon buggy accelerate?
WHAT HAPPENS IF THE FORCES ARE NOT AT
RIGHT ANGLES TO EACH OTHER?
Components of a force
Fx = Fcosθ
 Fy = Fsinθ

F
Fy
θ
Fx
Calculating components
25N at 15º to horizontal
How fast does the bike accelerate
now?
θ = 30
50N
800N
How fast does the bike accelerate
now?
θ = 30
50N
210N
force
from
pedals
800N
ORBITS AND CIRCULAR MOTION
Why do astronauts feel weightless?
• Why are we weightless video?
• Solar system and orbits simulation
• The astronauts feel weightless because they
are falling at the same …
• The space station has a sideways velocity of
28000km/h, so …
• The space station is falling towards the Earth
but …
orbits
•
•
•
•
circular orbit (circle)
elliptical orbit (ellipse)
hyperbolic orbit (hyperbola)
escape velocity
When a planet orbits a star in a
circle, which of the following are
constant?
speed
 velocity
 acceleration

Centripetal force
v1
F
v2
v3
v4
Forces cause
accelerations,
which are
changes in
velocity
(speed or
direction)
Circular Motion
a=
F=
Which direction is the velocity?
What happens if the force is ’turned off’?
Estimate the force needed to turn
the bike around the ‘wall of death’
• Road Bike Party 2 (from 4:20)
• Glasses of water (from 2:10)
KEPLER’S LAWS OF PLANETARY
MOTION
Forces questions
5. In a test of strength, a horizontal rope has a 5
kg mass hung from the middle. A person pulls
the rope so that it makes an angle of 10º to the
horizontal on each side. Calculate the force from
each hand pulling on the rope.
Calculate the speed of a geostationary
satellite at 36 000km above Earth’s
surface.
Calculate the force needed to keep a
282kg satellite in orbit.
Gravity and orbits
simulation
Kepler’s Laws of Planetary
Motion
Planets move in elliptical orbits with the
sun as one focus
 A straight line from the sun to the planet
sweeps out equal areas in equal times
 The cube of the planet’s average
distance is proportional to the square of
the orbital period

Planets move in elliptical orbits with
the sun as one focus
A striaght line from the sun to the planet
sweeps out equal areas in equal times
The cube of the planet’s average distance is
proportional to the square of the orbital
period
Weighing the sun...
... can’t be done directly
But, we can calculate the sun’s mass from
observations of the planets
Build an Excel model of an object
accelerating in a uniform
gravitational field
http://oneworldscience.files.wordpress.com/2014/
02/build-an-excel-model-of-an-objectaccelerating-in-a-uniform-gravitational-field.docx
Exoplanet: a planet outside our
Solar System
The three known
planets of the star
HR8799, as
imaged by the Hale
Telescope. The
light from the
central star was
blanked out by a
coronagraph..
Image: http://en.wikipedia.org/wiki/Exoplanet
Detecting exoplanets
•
•
•
•
•
direct imaging method
radial velocity (Doppler) method
transit method
gravitational microlensing
transit timing variation method
Methods for detecting exoplanets
• What is the method and how does it work?
• How useful is it and how many planets has it
detected?
• Are there any problems with the method?
• Other information
radial velocity (dark blue)
transit (dark green)
timing (dark yellow)
direct imaging (dark red)
microlensing (dark orange
Number of extrasolar planet discoveries per year
through February 2014, with colors indicating method
of detection: http://en.wikipedia.org/wiki/Exoplanet
Investigating circular orbits
Investigating circular orbits
F=
2
mv
r
F = force in N
 m = mass in kg
 v = velocity in ms-1
 r = radius of circle in m

How does the centripetal force depend on:
the orbital speed at constant radius or
radius of orbit at constant frequency?
Force,
N
Time
for 10
orbit,
s
Time
for 1
orbit,
s
speed,
m/s
Force,
N
1
1
2
2
1
3
1
3
2
2
3
spin the bung, not the masses!
3
radius
of
orbit,
m
CRATERS
Meteor
Meteorite
Agpalilik meteorite
20 tonnes
Part of Cape York
meteorite, collided with
Earth in Greenland
10000 years ago
Meteorite
99.8% of
meteorites found
on Earth
originate in
asteroid belt
What affects the size of a crater?
IMPACT CRATER
Diameter of crater 180km
The impactor's estimated size
was about 10 km in diameter
Estimated to have released
4×1023 joules of energy,
equivalent to 100,000,000
megatons of TNT
(most powerful explosive
device ever detonated, the
Tsar Bomba had a yield of
only 50 megatons)
Momentum and impulse for craters
Acceleration g = 9.8m/s2
1
Impulse on ball 
change in momentum
Large negative
acceleration
2
3
Ball accelerates
Initial speed 0m/s
Ball moving just
before it hits
Ball carries
momentum
Ball stopped
Final speed 0m/s
STARS
Why do stars shine?
DVD
Star
Stuff