ASTR 101 Lecture 6 Outline of Last Lecture I. Galileo Overcame Major ObjectionsII. Order of Astronomers and Their ContributionsIII. Idealized Scientific MethodOutline of Current Lecture I. Describing MotionA. Define speed, velocity, and accelerationII. Acceleration of GravityIII. Momentum and ForceIV. Mass and WeightV. Newton’s Laws of MotionA. First, Second, and ThirdVI. Conservation Laws in AstronomyCurrent LectureChapter 4How do we describe motion?- Precise definitions: -Speed: Rate at which object moves -Velocity: Speed and direction -Acceleration: Any change in velocity, unit of speed/timeAcceleration of gravity- All falling objects accelerate at the same rate (not counting friction of air resistance)- On earth g= about 10m/s2: speed increases 10 m/s with each second falling- Galileo showed that g is the same for all falling objects, regardless of their mass. (Apollo 15 demonstration: hammer and feather drop)Momentum and force- Momentum = mass x velocity- A net force changes momentum, which generally means an acceleration (change in velocity)- The rotational momentum of a spinning or orbiting object is known as angularmomentumQ: Is there a net force for each of the following?- A car coming to a stop? –yes- A bus speeding up? –yes- An elevator moving up at constant speed? –no- A bicycle going around a curve? –no- A moon orbiting Jupitier? -noHow is mass different from weight?- Mass: the amount of matter in an object- Weight: the force that acts upon mass- (You are weightless in free-fall!!)Q: On the Moon___________.A: Your weight is less; your mass is the same.Why are astronauts weightless in space?- There IS gravity in space.- Weightlessness is due to a constant state of free-fall4.2 NEWTON'S LAWS of MOTIONHow did Newton change out view of the universe?- He realized the same physiccal laws that operate on Earth also in the heavens: one universe- He discovered laws of motion and gravity- & Much more: experiments with light, first reflecting telescope, calculus, etc.What are Newton's three laws of motion?FIRST: an object moves at constant velocity unless a net force acts to change its speed ordirectionSECOND: Force = mass X accelerationTHIRD: For every force, there is always an equal and opposite reaction forceQ: Is the force that Earth exerts on you larger, smaller, or the same as the force you exert on it?A: Earth and you exert equal and opposite forces on each other4.3 CONSERVATION LAWS IN ASTRONOMYConservation of Momentum- the total momentum of interacting objects cannot change unless an external force is acting on them- interacting objects exchange momentum through equal and opposite forcesConservation of angular momentum- angular momentum = mass x velocity x radius- The angular momentum of an object cannot change unless an external twisting force (torque) is acting on it.- Earth experiences no twisting force as it orbits the sun, so its rotation and orbit will continue indefinitely- Angular momentum conservation also explains why objects rotate faster as they shrink in radius.Basic Types of Energy-Kinetic (motion)-Radiative (light)-stored or potentialEnergy can change type but cannot be
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