Lecture-05-08-posted[1]

# Lecture-05-08-posted[1] - Relative motion If reference...

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9/30/2008 1 Relative motion If reference frames A and B move relative to each other with a constant velocity BA v G Then BA PB PA v v v G G G + = BA PB PA r r r G G G + = Acceleration measured in both reference frames will be the same PB PA a a G G = A boat travels at a speed v bw relative to the water. The water in a river moves at v wg relative to the ground. How long does it take for the boat to travel a distance L up the river and back to the starting point? How long for the boat to travel a distance L directly across the river and back?

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9/30/2008 2 River Race • Race boats up and down a river v wg v bw up bg L t v = bw wg L vv = down bw wg L t = + 2 Lv wg v β ≡ / 22 bw up down bw wg t = bw v β / 2 21 1 up down bw L t v ⎛⎞ = ⎜⎟ −β ⎝⎠ River Race • Race boats across river and back 2 across bg L t v = 2 bw wg L = wg v v β≡ 2 across L t v = / 2 1 up down bw L t v = v wg v bw bw 1 bw v bg
9/30/2008 3 The Michelson-Morley experiment mirror mirror beam-splitter Dark-light pattern Light source telescope Chapters 4 & 5 Force and Motion

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9/30/2008 4 Newtonian mechanics Sir Isaac Newton (1643 – 1727) Describes motion and interaction of objects Applicable for speeds much slower than the speed of light Applicable on scales much greater than the atomic scale Applicable for inertial reference frames – frames that don’t accelerate themselves Force & Motion
9/30/2008 5 Interacting Systems The Earth and Moon exert long-range gravitational forces on each other. The Earth’s ocean tides are raised A hammer and a nail exert contact forces on each other. 9 by the Moon’s force. A pair of blocks touch and exert contact forces on each other. Such pairs of forces we call “action/reaction pairs” . Dynamics Defining Force Questions: • What is a force? • What is the connection between force and motion? What is the connection between force and motion? Definitions: • A force is a push or pull on an object. • Forces in nature come in pair (Action/Reaction). • A force is a vector . It has both magnitude and direction ( superposition apply). • A force requires an agent . Something must do the pushing and pulling. • A force is either a contact force or a long-range force . Gravity & EM is the only long-range force we will consider in First year Physics.

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9/30/2008 6 Examples of Force Vectors Pul Push Gravity Pull (contact force) (contact force) (long-range force)
9/30/2008 7 Force in the Quantum World: Two players exchanging a ball Every action has an equal and opposite reaction force carriers : bosons, spin=1 particle interactions mass intensity 1 e+ γ γ em 0 ~ 10 -1 (charged part.) W, Z weak 80 GeV ~ 10 -5 (q, W, Z) 91 GeV g strong 0 1 (q, g) e- e- W - & e q q g

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9/30/2008 8 Equilibrium An object on which the net force is zero is said to be in equilibrium . The object might be at rest in static equilibrium , or
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Lecture-05-08-posted[1] - Relative motion If reference...

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