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AST 112 Lecture 2

AST 112 Lecture 2. Newton’s Laws, Gravity, Structure of the Atom. Newton’s 1st Law of Motion. An object at rest remains at rest and an object in motion moves at the same speed and in the same direction unless acted upon by a force. A Question.

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AST 112 Lecture 2

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  1. AST 112 Lecture 2 Newton’s Laws, Gravity,Structure of the Atom

  2. Newton’s 1st Law of Motion An object at rest remains at rest and an object in motion moves at the same speed and in the same direction unless acted upon by a force.

  3. A Question If a ball is moving along in outer space far away from anything, does it slow down on its own?

  4. Another Question If I slide a book across the table, why does it slow down and stop?

  5. Gravity • We all know that Earth pulls us toward its surface via gravity. • What is it about Earth that causes that to happen? • What is the prerequisite for something to have gravity?

  6. Gravity • More questions to ponder… • Does gravity only exist at Earth’s surface? What about higher up? • How far does Earth’s gravitational influence extend?

  7. Gravity • Consider the Moon orbiting Earth. • Why doesn’t it continue in a straight line? • What does this say about the range of Earth’s gravity?

  8. Gravity

  9. Gravity

  10. Gravity

  11. Gravity • When the apple hit Newton, he realized that the force that caused the apple to fall is the same force that keeps the Moon in orbit around Earth. • Newton immediately concluded that the Sun has gravity that keeps Earth in it’s (the Sun’s) orbit

  12. Experiments in Gravity • What if all bodies, large or small, exert gravity? • What if smaller bodies exert a force that’s so small, it can’t be noticed?

  13. Cavendish Experiment • Henry Cavendish measured the force of gravity. • Two small (1.6 lb) lead spheres on the end of a torsion balance • Two MASSIVE (348 lb) lead spheres placed 9” away from the smaller spheres

  14. Newton’s Law of Gravitation • The force of gravity between two objects: • Increases when either mass increases • Mass is the amount of “stuff” in an object • Gets weaker (but never zero!) as the objects’ separation is increased • Always attractive

  15. Energy • An abstract but powerful concept in physics • Roughly: the ability of something to either move or cause something else to move • Applies on macroscopic and microscopic scales • Converts from one form to another and is “conserved”

  16. Matter • Just like the “hierarchy” of objects in the universe, so is there one in the structure of matter

  17. Structure of Matter • Matter is NOT infinitely divisible • It is made up of discrete particles called atoms. • Evidence: • Brownian motion • Gases combine in whole-number ratios

  18. Atoms • We can interact with individual atoms. • STM image of gold is shown

  19. Structure of Matter • Interest in matter surged in 1700’s • Alchemists wanted to turn lead into gold • Experiments done by Benjamin Franklin and Robert Millikan showed: • Matter contains two types of charge: + and – • Like charges repel, opposites attract

  20. Structure of Matter • J. J. Thomson: • Matter is like plum pudding • Negative charge is embedded in positive charge

  21. Structure of Matter • Ernest Rutherford (1900’s): • Shoot a beam of + particles at the “plum pudding atom”, should go right through since + particles all spread out

  22. Structure of Matter Most of the beam particles went through thetarget foil. A few of them bounced right backtoward where they came from!

  23. Structure of Matter Bohr Model • Dense nucleus holds the + charge • Protons • Number of protons defines the element • Smaller negative charges exist in clouds around the dense nucleus • Electrons Quantum Mechanics

  24. Ionization • A neutral atom has equal numbers of + and – charge • Ions have an imbalance of charge • Removing electrons from an atom ionizes it

  25. Structure of Matter • Smaller: • Quarks combine to make up nuclear particles • Forces carried by messenger particles • This is the domain of particle physics • Larger: • Molecules • Electron clouds of atoms combine and the atoms form a molecule • Can rearrange in chemical reactions • This is the domain of chemistry

  26. Temperature • Another rough definition: • The amount of microscopic motion in a substance

  27. Phases of Matter • Plasma • Free electrons move among + ions • Gas • Atoms / molecules do not interact much • Liquid • Atoms / molecules share weak bond, move freely • Solid • Atoms / molecules held in place, often in crystal structure

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