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Chapter 4: Newton and Universal Motion

Chapter 4: Newton and Universal Motion. Mechanics. Mechanics = laws of motion Aristotle Rest = Natural State of Motion Heavy objects fall faster Galileo Object continues in motion unless something pushes on it Heavy and light objects fall at same rate. Study of Motion (Mechanics).

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Chapter 4: Newton and Universal Motion

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  1. Chapter 4: Newton and Universal Motion

  2. Mechanics • Mechanics = laws of motion • Aristotle • Rest = Natural State of Motion • Heavy objects fall faster • Galileo • Object continues in motion unless something pushes on it • Heavy and light objects fall at same rate

  3. Study of Motion (Mechanics) • Velocity • Change in location Speed (mph) and direction (NE) • Acceleration • Change in velocity (speed and/or direction) • Force • Push or pull (pounds) • Mass • How much stuff (grams, kilograms)

  4. Mass vs Weight mass on Moon = mass on Earth • Mass Produces Gravity • Mass intrinsic to object (never changes) • Gravity proportional to mass • Weight = Force of Gravity • Stand on scale scale pushes back with equal force • Weight proportional to mass • Free-Fall (falling elevator, astronauts) • Acceleration of gravity = weight / mass All objects fall at same rate • Objects appear “weightless” weight on Moon = 1/6 weight on Earth in space, force ofgravity isnot zero

  5. Newton • Laws of Motion • Moving object keeps moving • Same speed • Same direction Objects want to move in straight line • Change in motion (speed or direction) • caused by force acceleration = force / mass • Equal, but opposite, forces between pairs of objects Push on object; it pushes back (just as hard)

  6. Newton • Invents mathematics (calculus) • Used to solve force equations • Circular motion • Direction of motion changes • Requires force • Force changes direction; speed unaltered • Force points toward center of circle

  7. Newton • Gravity • Pulls apple toward earth • makes apple fall Weight = force of gravity • Orbits similar to circles • Newton’s Hypothesis • All objects produce gravity • Sun’s gravity • planets orbit sun • Planet’s gravity • moon orbits planet Gravity Sun

  8. Launching Rockets • Fire Cannon Sideways; keep increasing velocity • Rocket moves sideways; offsets falling • Circular Orbit Speed =17,000 mph • Escape Speed = 25,000 mph

  9. Newton M1 = mass 1st object (sun) M2 = mass 2nd object (planet) R = distance between them G = Newton’s constant (a number) • Law of Gravity Force = G M1M2 / R2 • Double either mass: force increases by 2 • Double distance: force decreases by 4 • Larger (smaller) mass causes larger (smaller) gravitational force. • Larger (smaller) distance causes smaller (larger) gravitational force.

  10. Newton and Planets • Law of Gravity Force = G MsunMplanet / R2 Acceleration = Force / Mplanet = G Msun / R2 • Planet motion: • independent of planet massdepends on: mass of sundistance

  11. Newton and Planets Laws of motion + Gravity • Predicts Kepler’s Laws: • 1st Law (orbits are ellipses) • 2nd Law (equal area in equal time) • conservation of angular momentum • Skater pulls arms in; spins faster • Planet gets closer to sun; goes faster • Extended 3rd Law a3 = M P2 • use to measure mass M (of central body) M in solar masses

  12. Consider a planet orbiting the Sun. If the mass of the planet doubled but the planet stayed at the same orbital distance, then the planet would take • a) more than twice as long to orbit the Sun. • b) exactly twice as long to orbit the Sun. • c) the same amount of time to orbit the Sun. • d) exactly half as long to orbit the Sun. • e) less than half as long to orbit the Sun.

  13. Imagine a new planet in our solar system located 3 AU from the Sun. Which of the following best approximates the orbital period of this planet? • a) 1 year • b) 3 years • c) 5 years • d) 9 years P2=a3, so if a=3, then a3=3x3x3=27; then P2=27, so P~5 (since 5x5=25)

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