Turning ForcesCircular, Satellite

& Planetary Motion

AQA GCSE SpecificationCIRCULAR MOTION13.2 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 MOTION13.3 What provides the centripetal force forplanets 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 MotionAn 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 ForceCENTRIPETAL 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

Electron orbiting a nucleus ELECTROSTATIC attraction due to opposite charges

Factors affecting centripetal forceCentripetal 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:An object will only move along a __________ path if it is constantly acted on by a centripetal _________. The force is always directed __________ the centre of the circular path.

Centripetal force ___________ if the object moves in a smaller radius path or at a __________ speed.

An example of a _________ force is the Moon orbiting the Earth due to the Earth’s _____________ pull on the Moon.

centripetalcirculargravitational

increasestowardsgreater

WORD SELECTION:force

centripetal

circular

gravitational

increases

towards

greater

force

Circular Motion SimulationsLadybug 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 attractionGravity 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 strengthGravitational 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/kgMars’ surface = 3.7 N/kg

Weight is the force of gravity on an object.

CompleteAnswersSurface Field Strength

(N/kg)Object mass

(kg)Object weight

(N)

Earth 10 80

Moon 1.6 80

Mars 3.7 740

Jupiter 60 1500

Pluto 0.07 80

800

128

200

25

5.6

Choose appropriate words to fill in the gaps below:Gravity is a force exerted by all ________ on each other because of their ________.

Gravitational force __________ if the distance between the objects is increased but __________ if their masses are increased.

_________ is the force of gravity on an object. On the Earth’s surface an object of mass 1kg has a weight of 10 __________.

The Moon’s gravity is about one sixth the strength of the Earth’s because its _________ is much lower.

decreasesnewtons objectsincreasesmass weight

WORD SELECTION:

masses

decreases

newtons

objects

increases

mass

weight

masses

Gravity SimulationsFree-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 orbitsThe 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 daysVenus 225 days

Earth 1 year

Mars 2 years

Jupiter 12 years

Saturn 29 years

Neptune 165 years

Uranus 84 years

Planetary Motion SimulationsMy 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

SatellitesA 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 orbitsTo stay in orbit at a satellite must move at a particular speed.

too slow

correct speedtoo fast

Communication satellitesThese 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 satellitesThey 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.

QuestionWhat 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 / SatNavThe 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:A satellite is a ________ mass object orbiting around a ________ mass body.

The larger the orbit of a satellite the more ________ it moves and the ________ it takes to complete one orbit.

Geostationary satellites are used for _____________ and have an orbital period of _____ hours.

_____________ satellites normally use polar orbits.

24lowerlonger slowly

communicationsWORD SELECTION:

highermonitoring

24

lower

longerslowly

communications

higher

monitoring

Satellite SimulationsElectromagnetic 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