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Download Basic Physics Topics For Today`s Class Newton`s Laws of Motion (1
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Transcript
PHYS 1411 Introduction to Astronomy Basic Physics Chapter 5 Topics For Today’s Class • • • • • • • Keep your voting card ready Recap of Newton’s Laws Mass and Weight Work, Energy and Conservation of Energy Rotation, Angular velocity and acceleration Centripetal Force Angular Momentum Universal Law of Gravity Newton’s Laws of Motion (1) 1. A body continues at rest or in uniform motion in a straight line unless acted upon by some net force. An astronaut floating in space will continue to float forever in a straight line unless some external force is accelerating him/her. Newton’s Laws of Motion (2) 2. The acceleration a of a body is inversely proportional to its mass m, directly proportional to the net force F, and in the same direction as the net force. a = F/m F = m a 1 Newton’s Laws of Motion (3) How to find the Weight on Earth? Weight is a Force = mass x acceleration due to gravity 3. To every action, there is an equal and opposite reaction. Weight in Newtons = mass in kg x 9.8 m/s2 for Earth Mass is a property of matter, it is not equal to weight M = 70 kg V=? The same force that is accelerating the boy forward, is accelerating the skateboard backward. Apple (m) Earth (M) m = 1 kg v = 7 m/s Weight of apple = mg = m x 9.8 m/s2 = Fg https://www.youtube.com/watch?v=PrJnWTcW55s Work and Energy Work • Work = Force x distance SI units: Newton's x meter = Joules – Example: A 1 kg object at a height of 1 m from ground experience a force of 9.8 N, when it falls to the ground it does work that is W = 9.8 N x 1 m = 9.8 N.m = 9.8 J Work and Energy Gravitation Potential Energy • Work due to Gravity is called Gravitational Potential Energy (GPE) measured in Joules GPE = mass x g x h (h = height with respect to a reference point) – If a 1 kg object falls from a height of 1 m, its GPE is GPE = 1kg x 9.8m/s2 x 1m = 9.8 J Work and Energy Energy • Ability of a body to do work • Work due to motion is called Kinetic Energy (KE) KE = ½ x mass x velocity2 measured in Joules – Example: If a 1 kg object fall from a height of 1 m with a velocity of 4.43 m/s KE = ½ x 1kg x (1 m/s)2 = 9.8 J Fundamental Law of Nature Conservation of Energy • In all the three examples above we find that Work = KE = GPE • Energy can neither be created or destroyed: It only transforms from one form to the other. Example: Dropping a ball converts GPE to KE and at impact KE to heat, sound and work 2 Work and Energy? Work and Energy This is called conservation of energy. Energy is transformed from gravitational to kinetic and to heat, sound and mechanical As you walk towards the front of this class room from your seat, which type(s) of energy do you exhibit? a) potential energy only b) kinetic energy only c) both potential and kinetic energy d) neither, only thermal energy http://hea-www.harvard.edu/~pgreen/ Power Power • Rate at which energy is expended Power (P) = Work / time = Joules /seconds = watts (w) Conservation of Energy Demo https://www.youtube.com/watch?v=Ehx1P4adv6I Mindcraftwounderhowto.com Conservation of Energy and Kepler's Second Law of Planetary Motion Things that Move in Circles: Units Radian: An angle at the center of a circle whose arc is equal in length to the radius Units of Measure: Radian, Degrees and Revolutions 1 radian = 57.3 degrees Source: wikipedia 3 Things that Move in Circles Angular velocity (ω) = change in angle / change in time rad/s or rev/s, or deg/s Angular Acceleration (α) = change in ω / change in time rad/s2 or rev/s2, or deg/s2 Newton’s Second Law for Rotating Bodies • Any object that moves in a circle or an arc has centripetal force • Centripetal force (Fc) r measured in Newton’s Fc = ma = m v2 / r • Centripetal (radial) Circumference = 2 π r acceleration ac = v 2 / r v=2πr/t v Source: share.ehs.uen.org Source: faculty.wcas.northwestern.edu Visual Demo of Centripetal Force Consequences of Centripetal Force Why planets are not perfect spheres? https://www.youtube.com/watch?v=Tctr8CIMOZA Image source: NASA, STSI Angular Momentum Rotating and or orbiting object poses angular momentum Solar System Source: Wikipedia Galaxy Angular Momentum is Conserved I is called Moment of Inertia ~ MR2 M is mass and R is radius of disk Source: wikipedia https://www.youtube.com/watch?v=V3UsrfHa4MQ https://www.youtube.com/watch?v=0RVyhd3E9hY 4 Angular Momentum? Attendance Which of the following equation gives angular momentum> a) L = mv b) L = Iω c) L = mα d) L = mr2 http://hea-www.harvard.edu/~pgreen/ Newton’s Law of Gravitation Any two bodies are attracting each other through gravitation, with a force (Fg) proportional to the product of their masses (M,m) and inversely proportional to the square of their distance (r). G is called gravitational constant. Fg = GMm r2 Campus.kellerisd.net Gravity and Distance: The InverseSquare Law Inverse-square law -• relates the intensity of an effect to the inversesquare of the distance from the cause • in equation form: intensity = 1/distance2 • for increases in distance, there is decreases in force • even at great distances, force approaches but never reaches zero YouTube Inverse-Square Law Force of Gravity and Inverse-Square Law 5 Gravitational Constant How to find the value of G? G=6.67x10-11 N.m2/kg2 Fg = Gm1m2 d2 Gravity? Gravity is a) sometimes a repulsive force and sometimes an attractive force. b) always a repulsive force. c) always an attractive force. d) none of the above. Gravitational force is significant only for very large masses and small separation distance http://hea-www.harvard.edu/~pgreen/ Gravity? Compared to your mass here on Earth, your mass out in the space between the stars would be __________. a) zero b) negligibly small c) much much greater d) the same e) the question cannot be answered from the information given http://hea-www.harvard.edu/~pgreen/ ClassAction: Astronomy Education at the University of Nebraska-Lincoln Web Site (http://astro.unl.edu) ClassAction: Astronomy Education at the University of Nebraska-Lincoln Web Site (http://astro.unl.edu) ClassAction: Astronomy Education at the University of Nebraska-Lincoln Web Site (http://astro.unl.edu) 6 How to find g of a Planet? How to find the mass of Earth? Weight = Gravitational Force Weight = Gravitational Force = mg = GMm Apple (m) Apple (m) r2 = M= Earth (M) g r2 Earth (M) G If we know G, M and r then we can find g Since g, r and G are known we can calculate the mass of Earth = 6 x 1024 kg Exam 1 Statistics • • • • • Hi – 90 Low – 46 Average – 70 Includes 10 points curve Good News: I drop the lowest exam grade so there is still a chance to make an A. Suggestions • • • • • • Read Chapter ahead of class Study power points after class Practice all questions in power points Use MindTap study resource Come to help session with questions Try other questions from ClassAction at UNL site (see links in the PowerPoint). Acknowledgment • The slides in this lecture is for Tarleton: PHYS1411/PHYS1403 class use only • Images and text material have been borrowed from various sources with appropriate citations in the slides, including PowerPoint slides from Seeds/Backman text that has been adopted for class. 7