Astronomy 100 Tuesday, Thursday 2:30 - 3:45 pm Tom Burbine [email protected] www.xanga.com/astronomy100Help DeskAnother Homework Assignment (Due Thursday March 3rd)Homework assignments due today (or it is late)2nd HW assignment due today (or it is late)Next assignmentPRSMonkey Drop ExperimentShow videoSo what is happeningAccelerationDifference between mass and weightWho came up with the first understanding of how gravity and forces really work?Isaac Newton (1642-1727)1st Law2nd LawShow video3rd LawConservation of MomentumConservation of Angular MomentumUniversal Law of GravitationSo what should you know about this formulaWhat is the attraction of two people in this room?PRS questionPRS questionCan you use this formula to calculate acceleration of gravity?Escape velocityShow animationRememberForce due to Moon on side closest to EarthForce due to Moon on side farthest from EarthIs the force due to the Sun stronger or weaker on the Earth?PRS QuestionPRS QuestionWhat is the difference in forces on the two sides of the Earth?When Sun and Moon are pulling in same direction, strongest tidesWhen Sun and Moon are pulling in perpendicular directions, weakest tidesQuestions?Astronomy 100Tuesday, Thursday 2:30 - 3:45 pmTom [email protected]/astronomy100Help Desk• There is an Astronomy Help Desk in HAS 205. It will be open from Monday through Thursday from 7-9 pm.Another Homework Assignment(Due Thursday March 3rd)• Make up a test question• Multiple Choice• A-E possible answers• 1 point for handing it in• 1 point for me using it on testHomework assignments due today (or it is late)• Two due today• One is from this sheet that I handed out• Two calculations• Worth 1 HW point• I will discuss them next class2ndHW assignment due today (or it is late)• I want you to pick an element• Write its symbol• Is it a solid, metal, or gas?• Atomic Number• Atomic Mass• Its isotopes• What this element is used for?• http://www.webelements.com/• Worth 1 HW pointNext assignment• Handed out today• More calculations• Due Feb. 24 (Thursday) by 3:45 pm• You need to show work to get creditPRS• You need to make sure that you have a clicker registered• And your name appears on screen• If your name does not appear onscreen, I don’t know you are when you use the clickerMonkey Drop Experiment• A hunter shoots at a monkey in the tree• The monkey drops the second that the hunter pulls the trigger• What happens?Show video• http://www.ap.stmarys.ca/demos/content/mechanics/monkey_and_hunter/monkey_and_hunter.htmlSo what is happening• The monkey and the bullet are accelerating at the same speed downwards• g = 9.8 m/s2 = acceleration due to gravity• Objects fall faster by 9.8 m/s with every second• Even though the bullet has a velocity in the horizontal direction, it is also moving downwards in the vertical directionAcceleration• Acceleration is when your velocity is changing• Velocity not changing, no accelerationDifference between mass and weight• Mass is the amount of matter in your body• Weight is the amount of force acting on your body• So on the Moon, you would have the same mass as on Earth but weigh less on the Moon since the Moon is less massive than EarthWho came up with the first understanding of how gravity and forces really work?Isaac Newton (1642-1727)• Supposedly saw an apple fall to the ground• He then understood that gravity was universal, meaning it affected both the planets and us on Earth• Came up with 3 Laws of MotionIsaac Newton (1642-1727)1stLaw• In the absence of a net (overall) force acting upon it, an object moves with a constant velocity• An object at rest remains at rest• An object in motion tends to remain in motion unless a force is acting upon it2ndLaw• Force = mass x acceleration• Units of Force kg⋅m/s2= newton• So much do you weigh• Say your mass is 100 kg• F = 100 kg x 9.8 m/s2• F = 980 NewtonsShow video3rdLaw• For any force, there is an equal and opposite reaction force• Gravity is holding you on the ground• The ground is also pushing back up on you with the same amount of forceConservation of MomentumConservation of Angular Momentum• Angular momentun = m x v x r• M = mass• V = velocity• R = radius• In the absence of a net torque (twisting force), the angular momentum of a system remains constantUniversal Law of Gravitation• Every mass attracts every other mass through the force called gravity• Newton came up with this formulaF = G M1M2d2M1, M2 are the masses of the two objectsd is the distance between the objectsG = constant = 6.67 x 10-11 m3/(kg⋅s2)So what should you know about this formulaF = G M1M2d2• The force of attraction between any two objects is directly proportional to the product of their masses• The force of attraction between two objects decreases with the square of the distance between their centers• G is a very small numberWhat is the attraction of two people in this room?F = G M1M2d2• Say their masses are both 100 kg• Their distances are 10 meters apart• F = 6.67 x 10-11 m3/(kg⋅s2) * 100*100 kg2/(10*10 m2)• F = 6.67 x 10-9N = 0.0000000067 N• Remember the person weighs 980 NPRS questionF = G M1M2d2• How would the force between the two people change if they were only 5 meters apart instead of 10 meters?• A) Stay the same• B) Double (Increase by a Factor of 2)• C) Quadrupul (Increase by a Factor of 4)• D) halve (decrease by a factor of 2)PRS questionF = G M1M2 =G M1M2 =4 G M1M2 (d/2)2 d2/4 d2• How would the force between the two people change if they were only 5 meters apart instead of 10 meters?• A) Stay the same• B) Double (Increase by a Factor of 2)• C) Quadrupul (Increase by a Factor of 4)• D) halve (decrease by a factor of 2)Can you use this formula to calculate acceleration of gravity?•F = M2a = G M1M2you are M2d2 M1 is the Earth’s massa = G M1d is the Earth’s radiusd2a = 6.67 x 10-11 m3/(kg⋅s2) * (6.0 x 1024kg)(6.4 x 106m) * (6.4 x 106m) a = 9.8 m/s2Escape velocity• Velocity above this will allow an object to escape Earth’s gravityv = square root[(2 x G x M)/R]v = square root[(2 x 6.67 x 10-11 m3/(kg⋅s2) x (6.0 x 1024kg)](6.4 x 106m) v = square root [1.25 x 108m2/s2]v = 11.2 x 103 m/s = 11.2 km/sShow animation• Why do we have tides?Remember• Rock is stretching too• However, water is easier to