Law of Universal Gravitation Or Is gravity responsible
Law of Universal Gravitation Or Is gravity responsible for people falling in love?
The Law of Universal Gravitation • Any two massive bodies in the universe are attracted to each other. – – Force is always attractive 3 rd Law still holds Fgravity =G msource mtest / d 2 This equation is always true, W=mg only true near surface of planet – Action at a distance force (influence extends beyond object)
Check Question 1 The Earth and Moon exert gravitational forces on one another. How do the force on and accelerations of the Moon and Earth compare? a) The force and acceleration are greater for the Moon b) The force is the same, the acceleration is greater for the Moon c) The force is greater on the Moon, the acceleration is the same d) The force and acceleration are greater for the Earth
Newton’s Discovery • Gravity is a universal force – Showed Moon and apple fall proportional distances indicating same force is acting (1/60 x 5 m =. 0014 m = 1. 4 mm each second) – Needs to be a force on Moon because otherwise it would fly off in straight line – Same force acts between planets, stars, and galaxies – Accounts for spherical shape of planets and stars
The Universal Gravitational Constant (G) • 6. 67 x 10 -11 N m 2 /kg 2 • Stays same no matter – What planet you are on – Your altitude above the planet – If you in outer space • Difficult to measure – Cavendish’s torsion balance • Small size indicates gravity is a relatively weak force torsion balance in garage
The Inverse Square Law • Force varies as inverse square because force’s effect spreads out in both the vertical and horizontal directions. – Butter Gun Video and Analogy – Force never goes to zero – Other phenomenon that follow this include light brightness, and electric force Butter Gun Video
Check Question 2 • Two bodies have a gravitational force between them of 10 N. What would be the new gravitational force between the bodies if both bodies were doubled in mass and the distance between them cut to 1/3 the original amount? a) 40 N b) 90 N c) 120 N d) 360 N e) none of these
Uses of the Formula: Perturbations • Perturbations are wobbles in an astronomical bodies motion due to the gravity from another body. • Resulted in discovery of – Neptune and Pluto – Charon, Hydra and Nix (New moon 2011) – extra-solar planets (1235 candidates by Kepler as of 2011)
Uses of the Formula: Birth and End of the Universe • Universe started with Big Bang – galaxies are moving away from us – Further galaxies are moving faster • Fate of Universe – Universes expansion is increasing with time (2011 Nobel Prize) – Universe will eventually suffer heat death
Uses of the Formula: What is the Universe Made Of? • 4% Normal Matter (protons, neutrons, electrons and atoms) • 23% Dark Matter – Does not emit light – Produces gravity – Unknown material • 73% Dark Energy – Unknown material – Responsible for accelerating expansion
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