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Chapter 3 Newton’s First Law of Motion

Chapter 3 Newton’s First Law of Motion. Aristotle on Motion. Natural Motion Not caused by a force Objects seek their natural resting place. Typically either straight up or straight down Examples: Water flows over a waterfall Smoke rises. Aristotle on Motion. Violent Motion

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Chapter 3 Newton’s First Law of Motion

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  1. Chapter 3 Newton’s First Law of Motion Conceptual Physics Chapter 3

  2. Aristotle on Motion Natural Motion • Not caused by a force • Objects seek their natural resting place. • Typically either straight up or straight down • Examples: • Water flows over a waterfall • Smoke rises Conceptual Physics Chapter 3

  3. Aristotle on Motion Violent Motion • Motion imposed on an object • Caused by a push or pull on an object • Examples: • A ship is pushed across the water by the wind • A cart is pulled by a horse Conceptual Physics Chapter 3

  4. Copernicus’ Contribution • It had been thought for centuries that the earth was stationary – the center of the universe. • Copernicus suggested that the earth and other planets orbit around the sun. • This concept supported all astronomical observations, but was extremely controversial. Conceptual Physics Chapter 3

  5. Galileo on Motion • Galileo suggested that only when friction is present is a force required to keep an object moving. • Experimented with balls rolling on inclined planes. Conceptual Physics Chapter 3

  6. Galileo on Motion • A ball rolling down a ramp speeds up as a result of gravity pulling downward, with the motion. • A ball rolling up a ramp slows down as a result of gravity pulling downward, against the motion. • Reasoned that when a ball rolls across a very smooth and level surface, where gravity acts perpendicular to the motion, the ball neither speeds up nor slows down. Conceptual Physics Chapter 3

  7. Galileo on Motion • Only friction keeps the ball from rolling forever. • It is not the nature of the ball to come to rest as Aristotle had suggested. Conceptual Physics Chapter 3

  8. Aristotle versus Galileo A ball rolls across a level floor and gradually slows to a stop. How does Aristotle explain this event? How does Galileo explain this event? Conceptual Physics Chapter 3

  9. Inertia • Galileo stated that every object resists any change to its state of motion; we call this resistance inertia. • Inertia is not a measurable quantity in the way that temperature or time are. • There are no units that are used for quantifying inertia. Conceptual Physics Chapter 3

  10. Inertia • Inertia is directly related to mass. • The more massive an object is, the more the object will resist changes to its motion; the more inertia it has. • Only relative statements can be made regarding inertia. • A 2 kg mass has twice as much inertia as a 1 kg mass, but the 2 kg mass does not have “2 kg of inertia”. Conceptual Physics Chapter 3

  11. Newton’s 1st Law Conceptual Physics Chapter 3

  12. Newton’s 1st Law Every object continues in a state of rest, or of motion in a straight line at a constant speed, unless it is compelled to change that state by forces exerted upon it. Simply put, things tend to keep on doing whatever they have been doing. Conceptual Physics Chapter 3

  13. Mass • Mass is a measure of how much matter makes up an object. • It does not depend on the gravitational attraction of an object to the earth; it is not dependent on location. • Mass is a scalar quantity. • Mass is measured in kilograms (kg). • Example: m = 27 kg Conceptual Physics Chapter 3

  14. Mass and Weight • Mass and weight are NOT the same, but are related to one another by where g is the acceleration due to gravity. On earth, g = -9.8 m/s2 or approximately -10 m/s2. W = mg Conceptual Physics Chapter 3

  15. Mass and Weight What is the weight of a 7 kg rock resting on the surface of the earth? W = mg = 7 kg · 10 m/s2 - = 70 N It is understood that the weight acts downward, toward the center of the earth. We drop the negative sign, as it only indicates direction. Conceptual Physics Chapter 3

  16. Weight versus Mass If the rock were taken to the moon, would the mass change? Would the weight change? Conceptual Physics Chapter 3

  17. Volume • Volume is a measure of the amount of space occupied by an object. • Volume and mass are not the same; objects with larger volumes do not necessarily have more mass. • Volume is a scalar quantity. • Volume is measured in cubic meters (m3) or liters (ℓ). • Example: V = 1.5 m3 Conceptual Physics Chapter 3

  18. Question Does a 2 kg loaf of bread have twice as much mass as a 1 kg banana? Twice as much inertia? Twice as much weight, when weighed in the same location? Twice as much volume? Conceptual Physics Chapter 3

  19. The Moving Earth What is wrong with the following scheme? • To travel from Washington, D.C. to San Francisco on minimal fuel, simply ascend in a helicopter high above D.C. and wait for San Francisco to pass beneath you. Conceptual Physics Chapter 3

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