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From Aristotle to Newton

From Aristotle to Newton. The history of the Solar System (and the universe to some extent) from ancient Greek times through to the beginnings of modern physics. Clicker Question:. Why didn’t my hand get crushed by the hammer? A: My bones are actually stronger than steel.

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From Aristotle to Newton

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  1. From Aristotle to Newton The history of the Solar System (and the universe to some extent) from ancient Greek times through to the beginnings of modern physics.

  2. Clicker Question: Why didn’t my hand get crushed by the hammer? A: My bones are actually stronger than steel. B: The plate has a lot of inertia C: The plate is very strong D: The force of gravity kept the plate from moving

  3. Fnet m a = Newton's Laws of Motion Newton’s Zeroeth Law of Motion Objects are dumb. They do not know the past and they are not good predictors of the future. They only know what forces act on them right now. Newton’s First Law of Motion Every object continues in a state of rest or a state of motion with a constant speed in a straight line unless acted on by an unbalanced force. Newton’s 2nd Law of Motion When a force, F, acts on an object with a mass, m, it produces an acceleration, a, equal to the force divided by the mass. Newton’s Third Law of Motion To every action there is an equal and opposite reaction. Or, when one object exerts a force on a second object, the second exerts an equal and opposite force on first.

  4. Gravitational Force on a Planet For an object of massm at or near the surface of a planet the force of their gravitational attraction is given by: F = mg F is the gravitational force. g is the planetary "gravitational constant". Your "weight" is just the gravitational force between the Earth and you.

  5. Newton's Law of Gravity For two objects of massm1 and m2, separated by a distance R, the force of their gravitational attraction is given by: G m1 m2 R2 F = F is the gravitational force. G is the universal "gravitational constant". An example of an "inverse-square law". Your "weight" is just the gravitational force between the Earth and you.

  6. Throwing a ball into Orbit

  7. Clicker Question: Suppose Matt weighs 120 lbs on his bathroom scale on Earth, how much will his scale read if he standing on a platform 6400 km high (1 Earth radius above sea-level)? A: 12 lbs B: 30 lbs C: 60 lbs D: 120 lbs E: 240 lbs

  8. Newton's Correction to Kepler's First Law The orbit of a planet around the Sun has the common center of mass (instead of the Sun) at one focus.

  9. Escape Velocity Velocity needed to completely escape the gravity of a planet. The stronger the gravity, the higher the escape velocity. Examples: Earth 11 km/s Jupiter 60 km/s Deimos (moon of Mars) 7 m/s = 15 miles/hour

  10. Timelines of the Big Names Galileo 1564-1642 Copernicus Newton Brahe 1473-1543 1473-1543 1546-1601 1642-1727 Kepler 1571-1630

  11. Electromagnetic Radiation (How we get most of our information about the cosmos) Examples of electromagnetic radiation: Light Infrared Ultraviolet Microwaves AM radio FM radio TV signals Cell phone signals X-rays

  12. Radiation travels as waves. Waves carry information and energy. Properties of a wave wavelength (l) crest amplitude (A) trough velocity (v) l is a distance, so its units are m, cm, or mm, etc. Period (T): time between crest (or trough) passages Frequency (n): rate of passage of crests (or troughs), n = Also, v = l n h 1 T (units: Hertz or cycles/sec)

  13. Waves Demo: making waves - wave table Demo: slinky waves

  14. Radiation travels as Electromagnetic waves. That is, waves of electric and magnetic fields traveling together. Examples of objects with magnetic fields: a magnet the Earth Clusters of galaxies Examples of objects with electric fields: Power lines, electric motors, … } Protons (+) Electrons (-) "charged" particles that make up atoms.

  15. Scottish physicist James Clerk Maxwell showed in 1865 that waves of electric and magnetic fields travel together => traveling “electromagnetic” waves.

  16. The speed of all electromagnetic waves is the speed of light. c = 3 x 10 8 m / s or c = 3 x 10 10 cm / s or c = 3 x 10 5 km / s light takes 8 minutes Earth Sun or, bigger l means smaller n c = l n

  17. The Electromagnetic Spectrum 1 nm = 10 -9 m , 1 Angstrom = 10 -10 m l n c =

  18. A Spectrum Demo: white light and a prism

  19. Refraction of light All waves bend when they pass through materials of different densities. When you bend light, bending angle depends on wavelength, or color.

  20. Clicker Question: Compared to ultraviolet radiation, infrared radiation has greater: A: energy B: amplitude C: frequency D: wavelength

  21. Clicker Question: The energy of a photon is proportional to its: A: period B: amplitude C: frequency D: wavelength

  22. Clicker Question: A star much colder than the sun would appear: A: red B: yellow C: blue D: smaller E: larger

  23. Rainbows rred orangeyellowgreenblueviolet

  24. What's happening in the cloud? raindrop Sun's ray 42o 40o

  25. Double Rainbows

  26. We form a "spectrum" by spreading out radiation according to its wavelength (e.g. using a prism for light). What does the spectrum of an astronomical object's radiation look like? Many objects (e.g. stars) have roughly a "Black-body" spectrum: • Asymmetric shape • Broad range of wavelengths • or frequencies • Has a peak Brightness Frequency also known as the Planck spectrum or Planck curve.

  27. Approximate black-body spectra of astronomical objects demonstrate Wien's Law and Stefan's Law cold dust hotter star (Sun) “cool" star very hot stars frequency increases, wavelength decreases

  28. Laws Associated with the Black-body Spectrum Wien's Law: 1 T lmax energya (wavelength at which most energy is radiated is longer for cooler objects) Stefan's Law: Energy radiated per cm2 of area on surface every second a T 4 (T = temperature at surface) 1 cm2

  29. Betelgeuse Rigel

  30. Betelgeuse

  31. The total energy radiated from entire surface every second is called the luminosity. Thus Luminosity = (energy radiated per cm2 per sec) x (area of surface in cm2) For a sphere, area of surface is 4pR2, where R is the sphere's radius.

  32. The "Inverse-Square" Law Applies to Radiation Each square gets 1/4 of the light Each square gets 1/9 of the light 1 D2 D is the distance between source and observer. apparent brightness a

  33. at rest velocity v1 you encounter more wavecrests per second => higher frequency! velocity v2 velocity v1 fewer wavecrests per second => lower frequency! velocity v3 velocity v1 The Doppler Effect Applies to all kinds of waves, not just radiation.

  34. Doppler Effect Demo: buzzer on a moving arm Demo: The Doppler Ball

  35. The frequency or wavelength of a wave depends on the relative motion of the source and the observer.

  36. Things that waves do 1. Refraction Waves bend when they pass through material of different densities. air water swimming pool prism glass air air

  37. 2. Diffraction Waves bend when they go through a narrow gap or around a corner.

  38. 3. Interference Waves can interfere with each other

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