Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW1Questions of the Day• What are the four forces of nature?• Why is gravity the most important force for astronomy?• How does the strength of the gravitational force change with increasing mass? Increasing distance?• What is an “Inverse Square Law”?• What changes during an acceleration? Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UWAdministrative Stuff• First reading quiz due tonight at midnight!• A given week’s quiz is on the same week’s readings (i.e. the “Week 6 Reading Quiz” will cover all the readings assigned for Week 6).• Don’t have colored index card set? Talk to your TA.• Links to PDFs of lecture are in place for the whole quarter, but they only work for past lectures2Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW3Why does everything in the universe move?FORCES!Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW4A Force is a pull (attraction) or a push (repulsion).•Forces cause the direction and/or speed of motion to change.BUT•Forces can cancel each other outAstronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW5FORCES!Speed changesordirection changesThere must be a force acting!Both speed anddirection are constantThere cannot be a force acting OR the forces are in balance!We say: “no net force”Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW6A Thought ExperimentWhat are the forces acting on:•A falling ball?•Me?•A parachutist at terminal velocity? •A ball on a string, swung in a circle?Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW7There are “central forces” acting on objects moving in circles (or other curved paths)Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW8Central Forces are common in astronomy:•Moons around planets•Planets around stars•Stars around the centers of galaxiesPS. Most of the paths are actually ellipses, and while that was a big realization in the history of astronomy, we’re going to pretty much ignore it and treat most paths (i.e. orbits) as being circular.Circular OrbitsCentral ForceGravity!Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW9Why does gravity lead to roughly circular orbits?The Moon tries to travel in a straight path, but the gravitational pull of the Earth keeps tugging it back onto a circular path.It’s the same for the Earth around the Sun, or the Sun around the center of the Galaxy.Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW10Gravity also keeps astronomical objects bound togetherAstronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW11Gravity is one of the four fundamental forces of nature.1.Gravity2.Electromagnetic 3.Strong Nuclear4.Weak NuclearAstronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW121. Gravity• Only attractive.• Only depends upon mass and separation of the center of mass.• Weakest force but operates over any distance.Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW13Electromagnetic Force: A Brief Review of the Atom• Protons Positive (+) charge• Neutrons No charge (neutral)• Electrons Negative (-) chargeNot drawn to scale!Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UWXElectromagnetic Force: A Brief Review of the Atom• Protons Massive Positive (+) charge• Neutrons Massive No charge (neutral)• Electrons Negligible Mass (me = mp / 1830) Negative (-) chargeNot drawn to scale!Note: “massive”!1.67x10-27kg!Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW142. Electromagnetic• Attractive (opposite charges) or repulsive (same charges)• Only works between particles with electrical charge, in proportion to the amount of charge.• Falls off with distance like gravity & operates over any distance-+-+Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UWXGravity responds to MASS &Electromagnetic force responds to CHARGE“Charge” and “Mass” are flags we use to indicate BEHAVIOR!Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW153. Strong Nuclear Force• Strongest of all forces, needed to overcome repulsive electromagnetic force of protons.• Effective over only very tiny distances (the scale of the atomic nucleus).Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW164. Weak Nuclear• Responsible for radioactivity and particle decay• Even smaller range than the strong force (10-18m)Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW17Which forces will be significant in the following situations?A. (yellow) GravityB. (pink) Electromagnetic ForceC. (blue) Strong Force1. Holding together the particles in your body.2.Two protons 1 km apart?3.Two protons in a nucleus?Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW18Gravity is (almost) the only one that matters for astronomy!•Strong and Weak Nuclear forces have very short ranges (<10-12 cm).•Electromagnetic force has a long range, but astronomical objects are not charged.BUT IN CONTRAST•Gravity operates over any range, and affects anything with mass.Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW19The gravitational force Fg between two objects depends upon the masses of the two objects, and their separationAstronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW20MASSSEPARATIONGravitational Force gets stronger when…..Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW21Gravitational Force gets stronger when…..MASSSEPARATIONAstronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW22Bonding with the gravitational force equation…Don’t think “Ewww, scary math”!Instead, try to read the equation.Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW23Depends on the inverse of distance squared (d X d)! “Inverse Square Law” Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW24Amount of force Depends on the mass of both objects. Masses on the top implies the force goes up when M goes up.Depends on distance squared (d X d)! Much stronger dependence than on mass of either object. On bottom, so force is weaker when distance is bigger.“Newton’s Constant”: sets how strong the force is. If it were different, gravity would be stronger or weaker.Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW25Which of the following is the weakest force? The
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