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Lecture slides available at physics.ucdavis/physics7

Physics 7B - AB Lecture 4 April 24 Chapter 6 Galilean Space-Time Model, lots of Vectors, Intro. to Force, Momentum. Lecture slides available at http://physics.ucdavis.edu/physics7. Course Website http://physics.ucdavis.edu/physics7 Click on Physics 7B-A/B. Today Quiz 2!.

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Lecture slides available at physics.ucdavis/physics7

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  1. Physics 7B - ABLecture 4April 24Chapter 6Galilean Space-Time Model, lots ofVectors, Intro. to Force, Momentum Lecture slides available athttp://physics.ucdavis.edu/physics7

  2. Course Website http://physics.ucdavis.edu/physics7 Click on Physics 7B-A/B Today Quiz 2! May is a busy month. There will be four Quizzes.

  3. What is Galilean Space-Time model about? The Galilean Space-Time Model In our ordinary experience, three spatial dimensions and one time dimensions are all independent of each other. z Ex. You walk on a moving bus, what is your Vw.r.t.the ground? y x What if the bus was moving really fast? Like close to the speed of light? (i.e., C = 3 x 108 m/s)

  4. If the speed of the bus was close to the speed of light… The Special Relativity Model of Space-Time The three spatial dimensions are NOTindependent of time. i.e. Someone on the moving bus and someone on the ground will measure different velocity. What is Galilean Space-Time model about? The Galilean Space-Time Model In our ordinary experience, three spatial dimensions and one time dimensions are all independent of each other. z Ex. You walk on a moving bus, what is your Vw.r.t.the ground? y x

  5. z y x Why does she start spinning muchfaster when she pulls her arms and legs in? What are the forces exerted on the airplane for it to accelerate? Models in 7B are based on Galilean Space-Time model. Good news is, The predictions of special relativity agree well with Galilean Space-Time model in their common realm of applicability, specifically in experiments in which all velocities are small compared to the speed of light.

  6. New physical quantities! • Linear momentum vector p = mv • Angular momentum vector L = rptangential • To describe the motion of objects, we use several vector quantities such as… • Position vector R e.g. Rinitial, Rfinal • Displacement vector ∆R = Rfinal – Rinitial • Velocity vector v = dr/dt • Acceleration vector a = dv/dt • Force vector F Ok… What were vectors again??

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  19. Example #1 I take four steps right and three steps up, what is my displacement?

  20. A completely different path Example #2 If I take a different path from point A to point B, would my displacement be different as well? B ∆RAB A

  21. vave = ∆R/ ∆t, v= dR/ dt Therefore, velocity (vector) points in the same direction as the displacement (vector) The magnitude of velocity is a positive number called speed 21

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  41. Introduction to Conservation of MomentumMomentum is another (vector) quantity Nature chooses to conserve (for a closed system).

  42. Momentum • For a particle: Defined by p = mv • Is a vector, points in the same direction as v(see above equation) • For a system: Defined by adding together the momentum vectors of everything that makes up the system, I.e. ptotal = ∑pi = p1+ p2+ p3+… • Is conserved for a system if nothing external pushes or pulls on it • Has units of kg m/s

  43. Conservation of MomentumExample Rifle recoil Before shooting (at rest)

  44. Conservation of MomentumExample Rifle recoil Before shooting (at rest) After shooting vbullet pbullet

  45. Conservation of MomentumExample Rifle recoil Before shooting (at rest) After shooting vbullet vRifle pbullet pRifle

  46. Conservation of MomentumRailroad cars collide A 10,000kg railroad car A, traveling at a speed of 24m/s strikes an identical car B, at rest. If the car lock together as a result of the collision, what is their common speed afterward? vAi vBi =0 At rest Before collision A B pAi vA+Bf After collision A+B

  47. Conservation of MomentumRailroad cars collide A 10,000kg railroad car A, traveling at a speed of 24m/s strikes an identical car B, at rest. If the car lock together as a result of the collision, what is their common speed afterward? vAi vBi =0 At rest Before collision A B pAi vA+Bf After collision A+B pA+Bf

  48. When does momentum of something change?? … when a force F acts on the something during a time interval e.g. A bat hits a baseball • change in momentum is called: Impulse • Impulse Is related to the net external force in the following way: Net Impulseext = ∆ p = ∫ ∑ Fext(t)dt Approximate a varying force as an average force acting during a time interval ∆t Net Impulseext = ∆ p = ∑ Fave.ext x ∆ t

  49. DLM8&9 : Use of vectors, Force Model, Some new ideas: Force diagram, Momentum chart Next weekMay1 Quiz3(20min) will cover:Today’s lecture (exclude momentum, force, Impulse)Activities and FNTs from DLM7 and Activities from DLM8Bring Calculator!Closed-book, formulas will be provided.

  50. Be sure to write your name, ID number & DL section!!!!!

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