Transcript Orbits

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Forces and acceleration
An object will remain stationary or will move in the same
direction at a constant speed, unless the forces acting on
it are not balanced.
This will cause an acceleration in the direction of the
stronger force. This can make an object slow down or
speed up, or it can cause it to change direction.
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Acceleration in a circle
A motorbike drives round a corner at
a constant speed. Its direction
changes as it goes round the corner,
so even though its speed is constant,
it must be accelerating.
This acceleration must be at right angles (perpendicular) to
the direction of movement as it turns the corner, otherwise
its speed could not be constant.
Which way do you think the
motorbike is accelerating,
towards the inside of the
bend, or away from it?
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Forces causing circular motion
Any object that moves in a circle must be accelerating
towards the centre of that circle. What causes this?
What equation do you know that links force and acceleration?
F = m×a
Force and acceleration are both vector quantities, unlike mass,
so according to this equation, their directions must be equal.
All circular motion must therefore be caused by a force
acting towards the centre of the circle.
This type of force is known as a centripetal force.
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Centrifugal force or centripetal force?
Swing a mass around in a circle on the end of a string. Do you
feel a force pulling your hand outwards? This is often called a
‘centrifugal force’. You might have heard that centrifugal
forces cause circular motion, but this is not good physics!
Consider what is happening in this case. The mass on the end
of the string is the object that is performing circular motion, so
it is the forces on this object that are important:
centripetal force
The force on your hand is a reaction force, which can be
ignored when studying the motion of the mass.
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Thinking about circular motion
It is important to think of circular motion as an object being
continuously prevented from moving in a straight line, rather
than as if the object is being flung outwards from the centre.
A washing machine dries clothes by spinning them round
very fast:
The sides of the drum
provide the centripetal force
that keeps the clothes
moving in a circle, but water
is free to escape in straight
trajectories through the
holes in the sides.
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Examples of centripetal forces
Here are two more examples of circular motion caused by
centripetal forces:
Can you work out the direction of the force in each case,
and describe the type of force involved?
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Factors affecting centripetal forces
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Factors affecting centripetal forces
How does the centripetal force depend on mass?
F = ma, so force is proportional to mass.
The greater the mass, the larger the centripetal
force needed to maintain circular motion.
How does the centripetal force depend on speed and radius?
F = ma, so force is proportional to acceleration. If the truck
is going faster, or if its radius is smaller, then it is changing
direction more quickly, so its acceleration is greater.
The greater the speed, and the smaller
the radius, the larger the centripetal force
needed to maintain circular motion.
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Understanding centripetal forces
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What is gravity?
If a skydiver steps out of a plane, which way does he move?
What causes this effect?
Gravity is a universal force which
attracts any mass to every other
mass in the Universe.
Every mass has its own
gravitational field, like the one
surrounding Earth, but it takes two
objects to make a gravitational force.
Gravity is a very weak force, so small objects don’t stick
together, but if at least one mass is very large, the effect of
gravity is easy to see. Skydivers always fall back to Earth!
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Factors affecting gravity
The bigger the mass, the stronger its gravitational field, so
the Sun has a much stronger gravitational field than Earth.
But the further apart two objects are, the weaker the
gravitational forces between them. So when a skydiver
jumps out of a plane, he falls to Earth, not towards the Sun!
Gravitational fields are stronger:
 around larger masses
 at shorter distances.
The gravitational force between two objects can be increased:
 by increasing the size of either or both of the masses
 by decreasing the distance between them.
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Gravitational chaos!
Every mass in the universe attracts every other. That’s a lot
of forces to keep track of!
But gravity is a very weak
force, so most gravitational
forces at the Earth’s
surface can be ignored.
The gravitational field of a
pen, a person or even a large
mountain is too weak to have
a noticeable effect, so the
only gravitational field you
need to consider is Earth’s.
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Gravity at the Earth’s surface
Gravitational fields get weaker with increasing distance.
Do you feel any lighter on the top floor of your house than on
the ground floor?
The Earth is so large that small changes in height don’t affect
weight, so gravitational field strength is effectively constant:
weight = mass × gravitational field strength
= mass × 10 N/kg
This applies to objects at the Earth’s surface, at the top of a
mountain, or even in an aeroplane at 30000 feet…
…but be careful! This does not apply to satellites in orbit, or
to the forces between planets and stars.
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Understanding gravity
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Gravity as a centripetal force
Examples of centripetal forces can be found in many everyday
contexts, but what about circular motion on a large scale?
What is the centripetal force that
makes orbits possible?
Unlike a mass on a string, stars
and planets are not physically
connected to each other, but
they are attracted to each other
by gravity.
How does circular motion under
gravity compare to the types of
circular motion we are used to?
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Circular motion under gravity
The centripetal force required to keep a planet in circular
motion depends on mass, radius and speed. But the
gravitational force that a star actually provides only depends
on mass and radius. This means that for any specific radius,
a planet must move at one specific speed to stay in orbit.
 When a mass on a string is swung at an increasing speed,
the tension increases, while the radius remains constant:
 If a planet orbits a star at an increasing speed, the force
between them does not increase, so it moves out of that
orbit:
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Circular motion under gravity
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Elliptical orbits
In 1605 Johannes Kepler used his observations of the orbit of
Mars to predict that, rather than moving in perfectly circular
orbits, all the planets follow elliptical orbits around the Sun:
focus
Each orbit forms an ellipse with the Sun at one focus.
The two focuses of an ellipse are similar to the single centre
of a circle.
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Comets
Most of the planets travel around the Sun in near-circular
orbits. Comets also travel around the Sun but in highly
elliptical orbits.
gas tail
dust tail
The head of the comet
is a lump of ice and
dust a few kilometres
across. The tail only
appears when the
comet is near the Sun.
It consists of gas and
dust which are released
by the heat of the Sun.
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Data analysis
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What is a satellite?
A satellite is an object that orbits a planet. Satellites can be
natural or they can be artificial.
The largest satellite orbiting
Earth is the Moon. This is
Earth’s only natural satellite.
Artificial satellites are put into
orbit for a range of purposes, such
as mapping and surveillance.
The same physics applies to
satellites orbiting the Earth as to
planets orbiting the Sun.
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Types of orbit
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Uses for geostationary satellites
Geostationary satellites are particularly useful because they
stay fixed above a single point on Earth.
This makes them useful for
communications and satellite TV
broadcasting, because the satellite
never goes out of range.
Satellite dishes can be fixed to
face in the correct direction,
without the need to track the
movement of the satellite.
Geostationary satellites are also
used for weather forecasting.
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Problems with geostationary satellites
There are some disadvantages to geostationary satellites.
 All geostationary satellites must orbit over the equator at a
specific altitude of 36000 km. There are limited slots in this
orbit, which can lead to disputes when different countries
want a certain slot.
 A geostationary satellite can only ‘see’ a certain area of the
Earth’s surface – the rest is hidden from view.
 All geostationary satellites are a long way from Earth,
which causes delays in signals. This can be a
disadvantage during commercial or military
communications.
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Uses for polar orbit satellites
Polar satellites are particularly useful because they orbit at a
low altitude and high speed.
This makes them useful for mapping, as they can image the
Earth’s surface in higher resolution than more distant satellites.
It also makes them useful for
observation purposes, such as
military surveillance, or
weather monitoring, as they
can view the whole of the
Earth’s surface in one day.
However, polar satellites must be tracked from the ground,
and will be out of range for much of the time, causing delays
in data retrieval.
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Which type of satellite?
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Glossary
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Anagrams
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Multiple-choice quiz
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