Download centripetal force - Lawrence 6

Turning Forces
Circular, Satellite
& Planetary Motion
April 2012
Circular Motion
An object requires a
force for it to move
along a circular path.
If this force is removed
the object will continue
to move along a
straight line
tangentially to the
circle.
Centripetal Force
CENTRIPETAL FORCE is the general name given
to a centrally directed force that causes circular
motion.
Tension provides the
CENTRIPETAL FORCE
required by the
hammer thrower.
Other examples of centripetal force
Situation
Centripetal force
Earth orbiting the Sun
GRAVITY of the Sun
Car going around a bend.
FRICTION on the car’s tyres
Airplane banking (turning)
PUSH of air on the
airplane’s wings
ELECTROSTATIC attraction
due to opposite charges
Electron orbiting a nucleus
Factors affecting centripetal force
Centripetal force
INCREASES if:
- the object is moved FASTER
- the object’s mass is
INCREASED.
- the radius of the circle is
DECREASED.
Choose appropriate words to fill in the gaps below:
circular
An object will only move along a __________
path if it is
force
constantly acted on by a centripetal _________.
The force is
towards
always directed __________
the centre of the circular path.
increases
Centripetal force ___________
if the object moves in a smaller
radius path or at a __________
speed.
greater
centripetal force is the Moon orbiting the
An example of a _________
gravitational
Earth due to the Earth’s _____________
pull on the Moon.
WORD SELECTION:
gravitational force
greater circular
towards centripetal increases
Gravitational attraction
Gravity is a force exerted by all objects on
each other.
Gravitational force:
- is always attractive
- increases if the mass of the objects
is increased
- decreases if the distance between
the objects is increased
Gravitational field strength
Gravitational field strength is equal to the force exerted on
an object of mass 1kg.
On the Earth’s surface the gravitational field strength is
about 10 N/kg
Moon’s surface = 1.6 N/kg
Mars’ surface = 3.7 N/kg
Weight is the force of gravity on an object.
Complete
Answers
Surface
Field Strength
(N/kg)
Object mass
(kg)
Object weight
(N)
Earth
10
80
800
Moon
1.6
80
128
Mars
3.7
200
740
Jupiter
25
60
1500
Pluto
0.07
80
5.6
Choose appropriate words to fill in the gaps below:
objects on each other
Gravity is a force exerted by all ________
masses
because of their ________.
decreases if the distance between the
Gravitational force __________
increases if their masses are
objects is increased but __________
increased.
weight
_________
is the force of gravity on an object. On the Earth’s
newtons
surface an object of mass 1kg has a weight of 10 __________.
The Moon’s gravity is about one sixth the strength of the
mass
Earth’s because its _________
is much lower.
WORD SELECTION:
increases
newtons
masses
objects
mass
decreases
weight
Planetary orbits
The orbits of the planets
are slightly squashed
circles (ellipses) with the
Sun quite close to the
centre.
The Sun lies at a ‘focus’
of the ellipse
Planets move more quickly when they are closer
to the Sun.
faster
slower
The above diagram is exaggerated!
The time taken for
a planet to
complete one orbit
increases with its
distance from the
Sun.
Mercury
88 days
Venus
225 days
Earth
1 year
Mars
2 years
Jupiter
12 years
Saturn
29 years
Uranus
84 years
Neptune
165 years
Satellites
A satellite is a lower mass body
that orbits around a higher
mass body.
- The Moon is a natural
satellite of the Earth.
- The Hubble Space
Telescope is an artificial
(man-made) satellite of
the Earth.
- The Earth is a satellite of
the Sun.
How a satellite orbits
To stay in orbit at a satellite must move at a
particular speed.
too slow
too fast
correct speed
Communication satellites
These are usually placed in geostationary orbits
so that they always stay above the same place on
the Earth’s surface.
VIEW FROM
ABOVE THE
NORTH POLE
Geostationary satellites must have orbits that:
- take 24 hours to complete
- circle in the same direction as the Earth’s
spin
- are above the equator
- orbit at a height of about 36 000 km
Uses of communication satellites include satellite
TV and some weather satellites.
Monitoring satellites
They are used for weather, military,
and environmental monitoring.
They have relatively low orbital
heights (eg 500 km).
They take typically 2 hours to
complete one orbit.
They are considered to be in polar
orbits even though their orbits do
not always pass over the poles.
Question
What are the advantages / disadvantages of using a polar
orbiting rather than a geostationary satellite for monitoring?
ADVANTAGES
- it is nearer to the Earth allowing more detail to be seen and
- it is easier to place into orbit
- it eventually passes over all of the Earth’s surface
DISADVANTAGE
- unlike a geostationary satellite it is not always above the same point
on the Earth’s surface so continuous monitoring is not possible
GPS / SatNav
The satellites used for the
Global Positioning System
(GPS), as used in SatNav,
are in ‘polar’ orbits.
GPS makes use of
about 30 polar
orbiting satellites.
Choose appropriate words to fill in the gaps below:
lower
A satellite is a ________
mass object orbiting around a
higher mass body.
________
slowly
The larger the orbit of a satellite the more ________
it moves
longer it takes to complete one orbit.
and the ________
communications and have
Geostationary satellites are used for _____________
24 hours.
an orbital period of _____
monitoring
_____________
satellites normally use polar orbits.
WORD SELECTION:
monitoring
higher
longer
lower
communications
24
slowly