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Chapter 4: Newton and Universal Motion - PowerPoint PPT Presentation


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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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Presentation Transcript
mechanics
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
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)
mass vs weight
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

newton
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)

newton1
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
newton2
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

launching rockets
Launching Rockets
  • Fire Cannon Sideways; keep increasing velocity
    • Rocket moves sideways; offsets falling
  • Circular Orbit Speed =17,000 mph
  • Escape Speed = 25,000 mph
newton3
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.
newton and planets
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
newton and planets1
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

slide12

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.
slide13

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)