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Transcript

Making Sense of the Universe: Understanding Motion, Energy, and Gravity How do we describe motion? Precise definitions to describe motion: • Speed: Rate at which object moves speed = distance time units of m s example: speed of 10 m/s • Velocity: Speed and direction example: 10 m/s, due east • Acceleration: Any change in velocity units of speed/time (m/s2) The Acceleration of Gravity •Galileo dropped cannon balls and balsa wood balls • All falling objects accelerate at the same rate (not counting friction of air resistance). • On Earth, g ≈ 10 m/s2: speed increases 10 m/s with each second of falling. The Acceleration of Gravity (g) • Galileo showed that g is the same for all falling objects, regardless of their mass. • Demonstrated by Apollo 15 Hammer and Feather expt. http://www.youtube.com/watch?v=5C5_dOEyAfk Momentum and Force • 'Momentum' -- your total amount of motion • Momentum = mass velocity • Momentum is 'conserved' • A net force changes momentum, which generally means an acceleration (change in velocity) • Rotational momentum of a spinning or orbiting object is known as angular momentum Conservation of Angular Momentum angular momentum = mass x velocity x radius • A net external twisting force (torque) must be acting on an object in to change the angular momentum of the object. • Earth experiences no twisting force as it orbits the Sun, so its rotation and orbit will continue indefinitely Angular momentum conservation explains why objects rotate faster as they shrink in radius: What is mass? How is mass different from weight? • Mass – the amount of matter in an object • Weight – the force that acts upon an object You are weightless in free-fall! Thought Question On the Moon: A. B. C. D. My weight is the same, my mass is less. My weight is less, my mass is the same. My weight is more, my mass is the same. My weight is more, my mass is less. Why are astronauts “weightless” in space? • There is gravity in space • Weightlessness is due to a constant state of free-fall • How is mass different from weight? – Mass = quantity of matter – Weight = force acting on mass – Objects are 'weightless' in free-fall Newton’s Laws of Motion Our goals for learning: • How did Newton change our view of the universe? • What are Newton’s three laws of motion? How did Newton change our view of the universe? • Realized the same physical laws that operate on Earth also operate in the heavens one universe • Discovered laws of motion and gravity (Philosophiae Naturalis Prinipia Mathematica – 1687) Sir Isaac Newton (1642-1727) • Much more: Experiments with light; first reflecting telescope (1672), calculus… What are Newton’s three laws of motion? Newton’s first law of motion: An object moves at constant velocity unless a net force acts to change its speed or direction. Examples of First Law Which of these has a force acting on it and when? • Ball on a string • Inside planes and cars • Drying • Orbiting Objects: Satellites, planets Thought Question: Is there a net force? Y/N 1. 2. 3. 4. 5. A car coming to a stop. A bus speeding up. An elevator moving up at constant speed. A bicycle going around a curve. A moon orbiting Jupiter. Newton’s second law of motion Force = mass acceleration Shotput vs. baseball • If I throw each one, I put the same amount of force into it. F=mshot a F=mbsball a If the shotput is twice as massive how far does it go? Thought Question: Is the object accelerating? Y/N 1. 2. 3. 4. 5. A car coming to a stop. A bus speeding up. An elevator moving up at constant speed. A bicycle going around a curve. A moon orbiting Jupiter. Newton’s third law of motion: For every force, there is always an equal and opposite reaction force. Understanding Our Universe, 1st Edition Copyright © 2012 W. W. Norton & Company Thought Question: Is the force the Earth exerts on you larger, smaller, or the same as the force you exert on it? A. Earth exerts a larger force on you. B. I exert a larger force on Earth. C. Earth and I exert equal and opposite forces on each other. What have we learned? • How did Newton change our view of the universe? – He discovered laws of motion & gravitation – He realized these same laws of physics were identical in the universe and on Earth • What are Newton’s Three Laws of Motion? – 1. Object moves at constant velocity if no net force is acting. – 2. Force = mass acceleration – 3. For every force there is an equal and opposite reaction force