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.

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 is the general name given to a centrally directed force that causes circular motion.
Tension provides the
CENTRIPETAL FORCE required by the hammer thrower.

Situation Centripetal force
Earth orbiting the Sun GRAVITY of the Sun
FRICTION on the car’s tyres Car going around a bend.
Airplane banking (turning) PUSH of air on the airplane’s wings
Electron orbiting a nucleus ELECTROSTATIC attraction due to opposite charges

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:
WORD SELECTION: gravitational towards force greater centripetal circular increases

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 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.

Surface
Earth
Moon
Mars
Jupiter
Pluto
Field Strength
(N/kg)
10
Object mass
(kg)
80
Object weight
(N)
800
1.6
80
3.7
128
740 200
60 1500
25
0.07
80 5.6

Choose appropriate words to fill in the gaps below: increased.
The Moon’s gravity is about one sixth the strength of the increases
WORD SELECTION: newtons masses objects mass weight decreases

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

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
Mars
1 year
2 years
Jupiter 12 years
Saturn 29 years
Uranus 84 years
Neptune 165 years

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

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.

To stay in orbit at a satellite must move at a particular speed.
too slow too fast correct speed

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.

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.

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

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:
WORD SELECTION: monitoring higher communications longer lower 24 slowly

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