IGCSE›Edexcel IGCSE Physics›Mind mapsMotion in the UniverseEdexcel IGCSE Physics: Mind mapStudy pack PDFAlso for this subtopic:Revision notesFlashcardsSubtopic testCover factsThe universeUniverse holds billions of galaxiesGalaxy holds billions of stars, held by gravitySolar System is in the Milky WayField strengthGravitational field strength g: force per unit massDepends on mass of the bodyDecreases with distance from its centreMoon 1.6 N/kg, Earth 9.8 N/kgOrbitsGravity provides the centripetal forceMoons orbit planetsPlanets and comets orbit the SunSatellites orbit the EarthNo gravity → straight line pathMotion in spacegravity and orbitsgrTOrbit shapesPlanets and moons: nearly circularComets: highly ellipticalComet speed varies hugely around its orbitOrbital speedv=2πrTv = \frac{2\pi r}{T}v=T2πrDistance in one orbit is the circumferencer=7×106 mr = 7\times10^6\,\text{m}r=7×106m, T=6000 sT = 6000\,\text{s}T=6000s → 7330 m/sExam tipsMass is constant, weight changes with gWeight: W=mgW = mgW=mgUse radius from the centre, in metres