16. Gravity and Space - Mr. Brick's Web Page

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Transcript 16. Gravity and Space - Mr. Brick's Web Page

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Confused about weight and mass?
WARNING: This lesson may alter your weight!
Have you ever heard anyone
saying that “the scales don’t tell
the truth about their weight”?
You may be surprised to hear
that they are right!
Scales give a reading in kilograms,
which are the units of mass, not
the units of weight.
The confusion arises because most
people use the word ‘weight’ when
scientists would use the word ‘mass’.
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What are mass and weight?
Mass and weight are not the same!
Mass is the amount of matter in an object and is
measured in kilograms.
Mass is not a force and has the same value anywhere
in the Universe, including outer space.
Weight is a force and is caused by the pull of gravity
acting on a mass. Like other forces, weight is measured
in newtons and has both magnitude and direction.
Weight has different values depending on where the
object is in the Universe.
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Weight and mass on different planets
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Why does weight vary?
Gravity is the force that
attracts objects with mass
towards each other.
The bigger the mass of the
object, the stronger the
force of gravity.
For example, an apple will
have the same mass on
Earth as on the Moon, but
its weight will be different.
The Earth has a bigger mass
than the Moon and so exerts
a stronger gravitational pull
on the apple.
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Weight and mass on the Moon
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How do we calculate an objects weight?
The weight of an object depends on its mass and the
gravitational field strength:
weight = mass x gravitational field strength
The units for these quantities are as follows:
 Weight is measured in newtons (N).
 Mass is measured in kilograms (kg).
 Gravitational field strength is measured in
newtons per kilogram (N/kg).
The gravitational field strength depends on the force of gravity.
On Earth it is 10 N/kg, but it varies depending on planet size.
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Calculating the weight of a car
A car has a mass of 10,000 kg.
What is the weight of the car?
(Use 10 N/kg as the value of
the gravitational field strength.)
weight = mass x gravitational
field strength
weight = 10,000 kg x 10 N/kg
weight = 100,000 N
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Calculating the weight of an astronaut
An astronaut and his equipment
have a mass of 150 kg.
What is the weight when he is
standing on the Moon?
(Use 1.6 N/kg as the value of
the gravitational field strength.)
weight = mass x gravitational
field strength
weight = 150 kg x 1.6 N/kg
weight = 240 N
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Calculating weight and mass
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Why does the Earth move around the Sun?
What force can be large enough to change the direction
of a planet? gravity
The size of gravitational attraction depends on the size of
an object’s mass.
Small everyday objects have such
a relatively tiny mass that we do not
notice their gravitational attraction.
However, with objects as big as
moons, planets and stars, the
gravitational attraction is much
more noticeable.
So why don’t the planets clump together?
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Newton’s thought experiment
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What orbits the Earth?
A satellite is an object that orbits
another object.
What is the largest satellite to orbit
Earth? Clue: it is natural.
The Moon.
The Moon rotates about its own axis once every 27.5 days,
which is the same time it takes for the Moon to orbit the Earth.
This means that from Earth we always see the same side of
the Moon.
Humans have also launched many artificial satellites into
Earth’s orbit. These are used for communications, imaging
and scientific monitoring.
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How do planets orbit the Sun?
The planets travel in elliptical
orbits around the Sun. This
is due to the force of gravity.
The pull of the Sun’s gravity
causes the planets to speed
up when they move towards
the Sun and slow down when
they move away from it.
The changing speeds of the planets as they orbit the Sun
make it very difficult for the orbits to form a perfect circle.
All the planets, except Pluto, orbit the Sun in the same plane.
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Threats to Earth
Threats to Earth are often
covered in the media.
Can you think of any
potential threats to life
on Earth from outer space?
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Meteors, meteorites and asteroids
As the Earth moves through
space, it collides with lumps
of material which burn up in
the atmosphere.
These are called meteors
and can be seen as streaks
of light in the night sky.
A large meteor may not burn up completely and hits the
Earth with a lot of energy. This is called a meteorite.
Asteroids are very large rocks that normally orbit the Sun.
Sometimes, they break free and collide with planets.
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Effects of an asteroid impact
The effect of a large asteroid hitting our planet would be
catastrophic, it could destroy many species, including the
human race.
The damage that
an asteroid might
cause can be
estimated by
examining crater
sites created by
earlier asteroid
impacts, both on
Earth and other
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Did an asteroid wipe out the dinosaurs?
There is evidence that an asteroid impact may have wiped
out the dinosaurs 65 million years ago.
In 1980, a layer of iridium,
an element commonly found
in asteroids, was discovered
in rocks in Italy.
This layer was subsequently
found in rocks all over the
world. This suggested that
asteroid material had spread
out all over the world at the
same time in history.
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Dinosaurs and asteroid impact
The theory of mass extinction from an asteroid impact
lacked one major piece of evidence: a crater large enough
to be the impact site.
Then, in 1990, a scientist
called Alan Hildebrand
was looking at some old
geological data.
He found evidence of a
crater with a diameter of
100 km, at Chicxulub,
There is still debate about whether this impact wiped out
the dinosaurs, but it is one of the most popular theories.
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What are comets?
Most of the planets travel around the Sun in slightly elliptical
orbits that are almost circular. Comets also travel around the
Sun but in very elliptical orbits.
For most of its orbit, a comet is a
long way from the Sun.
The head of a comet head is a
lump of ice and dust.
The tail only appears when the
comet’s orbit takes it nearest to
the Sun.
A comet’s tail consists of gas and
dust released by the Sun’s heat.
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What is a Near-Earth object?
Near-Earth objects (NEOs) are asteroids or comets that
pass within a third of the distance of the Earth from the Sun.
About 50,000 small NEOs fall on the Earth as meteorites
each year without causing much damage. However, it is
estimated that around 1000 NEOs are bigger than 1 km.
The paths of NEOs are
monitored by telescopes
around the world and the
risk of a collision assessed.
So far, observations have
not found any large NEOs
that will collide with Earth.
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Objects in space – matching activity
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How is space observed?
Humans have observed space for the last 5,000 years.
Astronomy was revolutionised
400 years ago when the optical
telescope was invented.
The optical telescope allowed
scientists to see more of the
Universe than ever before.
The view from optical telescopes
is affected by the Earth’s
atmosphere and light pollution.
They are often located on high
ground in very remote areas
to minimise these effects.
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What is the Hubble Space Telescope?
The Hubble Space Telescope (HST) is a satellite in orbit
around the Earth. It was deployed from the Space Shuttle
Discovery on 24 April 1990.
The HST is outside the
Earth’s atmosphere and
does not experience the
same interference as
Earth-based telescopes.
This means that its
images of the Universe
are much more detailed
than images observed
from Earth.
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What is a radio telescope?
Astronomers do not only observe the Universe using optical
telescopes that detect visible light. They also observe other
wavelengths of electromagnetic radiation.
Radio telescopes are
used to observe radio
waves from other parts
of the Universe, which
are able to pass through
the Earth’s atmosphere.
Radio telescopes are
also used to look for
alien life in the Universe.
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Search for extra-terrestrial intelligence
The Search for Extra-Terrestrial Intelligence (SETI) is
the world’s attempt to communicate with other life forms
in the Universe.
Humans cannot yet travel to other solar systems, so we
rely on radio telescopes and transmitters to listen for
communications from other planets.
Several major SETI projects have
been established since the 1960s,
some of which use participant's
home computers to help analyse
the radio waves they receive.
However, an authentic extra-terrestrial
signal has yet to be detected.
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Is there life out there?
Although many people believe that there is intelligent life on
other planets, it is a great challenge to find this.
Should we spend lots of money looking for aliens when there
are still huge numbers of plants and animals on our planet
that we know nothing about?
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Are we alone?
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Why do we explore space?
Space exploration helps us to find out
more about the Solar System and
answer questions such as ‘Is there
life on any of the other planets?’
As well as furthering our knowledge
about space, research into space
has also lead to a number of
technological developments that
may not have occurred otherwise.
Ski boots and smoke alarms
are based on technology from
the space programme.
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What about crewed space missions?
Since humans first travelled
into space in 1961, crewed
space missions have played
an important part in space
However, atmospheric pressure, distance and temperature
all limit the places that humans can visit.
Crewed missions are very expensive and put lives at risk.
The Challenger and Columbia space shuttle disasters are
tragic reminders of the dangers involved.
In total, 15 astronauts and 4 cosmonauts have been killed
in space missions.
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What about uncrewed space missions?
Astronauts need constant supplies of food and air on space
missions. Imagine the size of the storage space that would
be needed to take a human far out into space.
Uncrewed space probes can
travel further and endure more
intense conditions than
humans can.
Some, like the Martian Rovers,
have been so successful that
they have had their missions
Despite this, many people believe that a human geologist
would still be more effective than any number of robots.
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Messages to other worlds
In 1977, NASA launched the Voyager 1 spacecraft to
photograph the gas-giant planets.
On board was the Voyager Golden Record, a record that
contains sounds and images from our culture and a map of
the Solar System.
Although it will take 40,000
years for Voyager 1 to even
come close to another star,
the record is an important
symbol of our willingness to
communicate with aliens.
Imagine that you were making your own ‘Golden Record’
to send into space. What information would you include?
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Advantages of space probes
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The future of space exploration
In 2005, new goals were set
out for NASA:
 complete the International
Space Station by 2010
 develop a new spacecraft
 return to the Moon by 2020.
As a result of these goals
crewed missions to Mars may
become a reality. However, in
order to fund these plans,
projects like the Hubble Space
Telescope have to be cut back.
What do you think the future of space exploration should be?
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Opinions on space exploration
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How can humans survive in space?
Outer space is a very harsh
environment for humans to visit.
Without protection, humans would
rapidly fall unconscious due to
lack of oxygen.
Blood would be boiled or frozen
due to the lack of air pressure and
extreme temperature.
DNA would also be destroyed by
cosmic radiation.
Spacesuits allow astronauts to control their oxygen supply,
air pressure and temperature, which enables them to move
around in space in relative safety.
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Could humans live in space colonies?
In order for humans to survive long term in space,
sustainable artificial environments need to be built.
The International Space
Station is home to a crew
of at least two astronauts.
They conduct research
and perform experiments
about the long-term
effects of living in space.
The results of these experiments may one day prepare the
way for more complex structures such as permanent Moon
bases, or maybe even space colonies.
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 asteroid – A lumps of rock up to 1,000 kilometres in size.
 comet – A lump of rock and ice, which has a very elliptical
orbit around the Sun.
gravity – The force of attraction between all objects.
mass – The amount of matter in an object. It is measured
in kilograms.
meteor – A lump of material from space that burns up in
the Earth’s atmosphere.
meteorite – A large meteor that does not completely burn
up in the Earth’s atmosphere and crashes to Earth.
near-Earth object – An asteroid or comet whose orbit
brings it close to the Earth’s orbit.
orbit – The path made by one body moving around another.
weight – The force caused by the pull of gravity acting on
a mass. It is measured in newtons.
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Multiple-choice quiz
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