AQA GCSE Physics 3-1b
Turning Forces
Circular, Satellite
& Planetary Motion
GCSE Physics pages 222 to 233
April 10th 2010
AQA GCSE Specification
CIRCULAR MOTION
13.2 What keeps bodies moving in a circle?
Using skills, knowledge and understanding of how science works:
• to identify which force(s) provide(s) the centripetal force in a given
situation
• to interpret data on bodies moving in circular paths.
Skills, knowledge and understanding of how science works set in the
context of:
• When a body moves in a circle it continuously accelerates towards
the centre of the circle. This acceleration changes the direction
of motion of the body, not its speed.
• The resultant force causing this acceleration is called the
centripetal force.
• The direction of the centripetal force is always towards the centre
of the circle.
• The centripetal force needed to make a body perform circular
motion increases as:
– the mass of the body increases;
– the speed of the body increases;
– the radius of the circle decreases.
SATELLITE AND PLANETARY MOTION
13.3 What provides the centripetal force for
planets and satellites?
Using skills, knowledge and understanding of how science works:
• to interpret data on planets and satellites moving in orbits that
approximate to circular paths.
Skills, knowledge and understanding of how science works set in the
context of:
• The Earth, Sun, Moon and all other bodies attract each other with
a force called gravity.
• The bigger the masses of the bodies the bigger the force of gravity
between them.
• As the distance between two bodies increases the force of gravity
between them decreases.
• The orbits of the planets are slightly squashed circles (ellipses)
with the Sun quite close to the centre.
• Gravitational force provides the centripetal force that allows
planets and satellites to maintain their circular orbits.
• The further away an orbiting body is the longer it takes to make a
complete orbit.
• To stay in orbit at a particular distance, smaller bodies, including
planets and satellites, must move at a particular speed around
larger bodies.
• Communications satellites are usually put into a geostationary
orbit above the equator.
• Monitoring satellites are usually put into a low polar orbit.
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
Circular motion
Notes questions from pages 222 & 223
1.
2.
3.
4.
5.
6.
7.
8.
Explain why an object moving in a circle has a centrally directed
acceleration by copying the bullet points below Figure 2 on page
222.
Copy and answer question (a) on page 222.
What is a ‘centripetal force’? Give two examples of centripetal
force.
Copy and answer question (b) on page 223.
What factors affect the size of the centripetal force acting on an
object moving in a circle?
Copy and answer question (c) on page 223.
Copy the ‘Key points’ table on page 223.
Answer the summary questions on page 223.
Circular motion
ANSWERS
In text questions:
(a) It would fly off at a
tangent.
(b) The force of gravity due
to the Earth on it.
(c) There is much less
friction so the
centripetal force is less
and the car must go
slower.
Summary questions:
1. (a) D, C.
(b) C, B.
2. (a) Friction.
(b) Pull (tension).
(c) Gravity.
(d) Electrostatic force.
Circular Motion Simulations
Ladybug Revolution - PhET - Join the ladybug in an exploration of rotational
motion. Rotate the merry-go-round to change its angle, or choose a
constant angular velocity or angular acceleration. Explore how circular
motion relates to the bug's x,y position, velocity, and acceleration using
vectors or graphs.
Motion in 2D - PhET - Learn about velocity and acceleration vectors. Move
the ball with the mouse or let the simulation move the ball in four types of
motion (2 types of linear, simple harmonic, circle). See the velocity and
acceleration vectors change as the ball moves.
Motion produced by a force - linear & circular cases - netfirms
Uniform circular motion - Fendt
Carousel - centripetal force - Fendt
Relation between speed and centripetal force - NTNU
Vertical circle & force vectors - NTNU
Circular Motion & Centripetal Force - NTNU
Inertia of a lead brick & Circular motion of a water glass - 'Whys Guy' Video
Clip (3 mins) (2nd of 2 clips)
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
Gravitational attraction
Notes questions from pages 224 & 225
1.
2.
3.
4.
5.
6.
7.
8.
State Newton’s rules on gravity.
Copy and answer questions (a) and (b) on page 224.
Describe how the force of gravity on a space vehicle
changes as it travels from the Earth to the Moon.
Copy and answer question (c) on page 224.
Define what is meant by ‘gravitational field strength’.
Show that a mass of 200 kg weighs approximately 32 N on
the Moon.
Copy and answer question (d) on page 225.
Copy the ‘Key points’ table on page 225.
Answer the summary questions on page 225.
Gravitational attraction
ANSWERS
In text questions:
(a) The force of gravity on it due
to the Sun.
(b) Their mass is too small.
(c) The force of gravity on the
Moon is less, so less energy
would be needed to escape
from the Moon.
(d) The force of gravity due to
the Earth.
Summary questions:
1. (a) Increases
(b) Stays the same
(c) Decreases.
2. (a) The force of gravity is less
on the Moon so it is easier for
the astronaut to move up and
down.
(b) The force of gravity is less
on the Moon so the ball can
go higher for the same
change of gravitational
potential energy.
Gravity Simulations
Free-fall Lab - Explore Science
Galileo Time of Fall Demonstration - 'Whys Guy' Video Clip (3 mins) - Time
of fall independent of mass - Leads slug and feather with and without air
resistance. (1st of 2 clips)
Distance Proportional to Time of Fall Squared Demonstration - 'Whys Guy'
Video Clip (3:30 mins) - Falling distance proportional to the time of fall
squared. (2nd of 2 clips some microphone problems)
Lunar Lander - PhET - Can you avoid the boulder field and land safely, just
before your fuel runs out, as Neil Armstrong did in 1969? Our version of this
classic video game accurately simulates the real motion of the lunar lander
with the correct mass, thrust, fuel consumption rate, and lunar gravity. The
real lunar lander is very hard to control.
Moonlander Use your thrusters to overcome the effects of gravity and bring
the moonlander safely down to earth.
BBC KS3 Bitesize Revision:
Mass and gravity
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
Planetary orbits
Notes questions from pages 226 & 227
1.
2.
3.
4.
5.
6.
7.
Copy the table on page 227.
(a) What force is responsible for planetary motion? (b) Why
is this force an example of centripetal force?
Explain how orbital speed affects the shape of a planet’s
orbit.
State how (a) the speed and (b) the time taken to complete
one orbit depends on a planet’s distance from the Sun.
Copy and answer questions (a) and (b) on pages 226 and
227.
Copy the ‘Key points’ table on page 227.
Answer the summary questions on page 227.
Planetary orbits
ANSWERS
In text questions:
(a) There would probably
be a bigger variation of
temperature each year.
The tides would be
more variable.
(b) Its orbit is about 5 times
bigger and it takes
about 12 times longer,
so it must travel slower
than the Earth.
Summary questions:
1. (a) Satellite, Earth.
(b) Earth
(c) Satellite, Earth.
(d) Planet, Sun.
2. (a) (i) Jupiter (ii) Venus
(b) 49 km/s
Planetary Motion Simulations
My Solar System - PhET- Build your own system of heavenly bodies and watch the gravitational
ballet. With this orbit simulator, you can set initial positions, velocities, and masses of 2, 3, or 4
bodies, and then see them orbit each other.
Multiple planets - 7stones
Planet orbit info - Fendt
Orrery of Inner Solar System - CUUG
The Solar System - Powerpoint presentation by KT
Solar system quizes - How well do you know the solar system? This resource contains whiteboard
activities to order and name the planets corrrectly as well as a palnet database - by eChalk
Hidden Pairs Game on Planet Facts - by KT - Microsoft WORD
Fifty-Fifty Game on Planets with Atmospheres - by KT - Microsoft WORD
Fifty-Fifty Game on Planets that are smaller than the Earth - by KT - Microsoft WORD
Sequential Puzzle on Planet Order - by KT - Microsoft WORD
Sequential Puzzle on Planet Size - by KT - Microsoft WORD
Projectile & Satellite Orbits - NTNU
Kepler Motion - NTNU
Kepler's 2nd Law - Fendt
Two & Three Body Orbits - 7stones
Orbits - Gravitation program
BBC KS3 Bitesize Revision:
Gravitational Forces - includes planet naming applet
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
Satellites
Notes questions from pages 228 & 229
1.
2.
3.
4.
5.
6.
7.
8.
With the aid of a diagram explain how a satellite can remain in
orbit about the Earth.
How does (a) the speed and (b) the period of a satellite vary with
its height above the Earth?
Copy and answer questions (a) and (b) on page 228.
(a) What is meant by a ‘geostationary orbit’? (b) Why must
satellite TV use geostationary satellites?
What are ‘monitoring satellites’? What type of orbit is used for
this type of satellite?
Copy and answer question (c) on page 229.
Copy the ‘Key points’ table on page 229.
Answer the summary questions on page 229.
Satellites
ANSWERS
In text questions:
(a) It can give the location
and the height above
sea level.
(b) 12
(c) They would be slowed
by drag from the
atmosphere and would
fall back to Earth.
Summary questions:
1. (a) High, equator.
(b) Low, poles.
2. (a) (i) Below (ii) Above
(b) Less energy is
needed because the
orbit is nearer the
ground than a
geostationary orbit is.
Satellite Simulations
Electromagnetic Spectrum & Communications - BT
Inside a communication satellite - BT
Projectile & Satellite Orbits - NTNU
Newton's Cannon Demo - to show how orbits occur - by Michael
Fowler
Kepler Motion - NTNU
Kepler's 2nd Law - Fendt
Space craft control - NTNU
How a satellite orbits - BT
Satellite orbits - BT
Inside a communication satellite - BT
BBC KS3 Bitesize Revision:
Satellites & Space Probes
Turning issues
Notes questions from pages 230 & 231
1. No questions.