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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