ASTR 101 Lecture 6 Outline of Last Lecture I Galileo Overcame Major Objections II Order of Astronomers and Their Contributions III Idealized Scientific Method Outline of Current Lecture I Describing Motion A Define speed velocity and acceleration II Acceleration of Gravity III Momentum and Force IV Mass and Weight V Newton s Laws of Motion A First Second and Third VI Conservation Laws in Astronomy Current Lecture Chapter 4 How do we describe motion Precise definitions Speed Rate at which object moves Velocity Speed and direction Acceleration Any change in velocity unit of speed time Acceleration 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 angular momentum Q 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 no How 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 fall 4 2 NEWTON S LAWS of MOTION How 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 or direction SECOND Force mass X acceleration THIRD For every force there is always an equal and opposite reaction force Q 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 other 4 3 CONSERVATION LAWS IN ASTRONOMY Conservation 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 forces Conservation 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 potential Energy can change type but cannot be destroyed
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