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Physics 218 Chapter 15 Prof. Rupak Mahapatra Physics 218, Lecture XXII 1 Checklist for Today • Midterm 3 average 61 – Collect your exams from your TAs. • Last lecture next Monday – Will cover up to Chpater 18 • Ch 14 and 15 Home work due Wed this week • Ch 18 Home work due Mon next week Physics 218, Lecture XXII 2 Angular Quantities • Position Angle q • Velocity Angular Velocity w • Acceleration Angular Acceleration a • Force Torque t • Mass Moment of Inertia I Today we’ll finish: – Momentum Angular Momentum L – Energy Physics 218, Lecture XXII 3 Rotational Kinetic Energy 2 ½mv KEtrans = 2 KErotate = ½Iw Conservation of Energy must take rotational kinetic energy into account Physics 218, Lecture XXII 4 Rotation and Translation • Objects can both Rotate and Translate • Need to add the two KEtotal = ½ mv2 + ½Iw2 • Rolling without slipping is a special case where you can relate the two V = wr Physics 218, Lecture XXII 5 Rolling Down an Incline You take a solid ball of mass m and radius R and hold it at rest on a plane with height Z. You then let go and the ball rolls without slipping. What will be the speed of the ball at the bottom? What would be the speed if the ball didn’t roll and there were no friction? Note: Isphere = 2/5MR2 Z Physics 218, Lecture XXII 6 A bullet strikes a cylinder A bullet of speed V and mass m strikes a solid cylinder of mass M and inertia I=½MR2, at radius R and sticks. The cylinder is anchored at point 0 and is initially at rest. What is w of the system after the collision? Is energy Conserved? Physics 218, Lecture XXII 7 Rotating Rod A rod of mass uniform density, mass m and length l pivots at a hinge. It has moment of inertia I=ml/3 and starts at rest at a right angle. You let it go: What is w when it reaches the bottom? What is the velocity of the tip at the bottom? Physics 218, Lecture XXII 8 Person on a Disk A person with mass m stands on the edge of a disk with radius R and moment ½MR2. Neither is moving. The person then starts moving on the disk with speed V. Find the angular velocity of the disk Physics 218, Lecture XXII 9 Same Problem: Forces Same problem but with Forces Physics 218, Lecture XXII 10 Chapter 18: Periodic Motion • This time: – Oscillations and vibrations – Why do we care? – Equations of motion – Simplest example: Springs – Simple Harmonic Motion • Next time: – Energy } Physics 218, Lecture XXIV Concepts } The math 11 Physics 218, Lecture XXIV 12 What is an Oscillation? • The good news is that this is just a fancy term for stuff you already know. It’s an extension of rotational motion Stuff that just goes back and forth over and over again “Stuff that goes around and around” • Anything which is Periodic • Same as vibration • No new physics… Physics 218, Lecture XXIV 13 Examples Lots of stuff Vibrates or Oscillates: – Radio Waves – Guitar Strings – Atoms – Clocks, etc… In some sense, the Moon oscillates around the Earth Physics 218, Lecture XXIV 14 Why do we care? Lots of engineering problems are oscillation problems – Buildings vibrating in the wind – Motors vibrating when running – Solids vibrating when struck – Earthquakes Physics 218, Lecture XXIV 15 What’s Next 1.First we’ll “model” oscillations with a mass on a spring •You’ll see why we do this later 2.Then we’ll talk about what happens as a function of time 3.Then we’ll calculate the equation of motion using the math Physics 218, Lecture XXIV 16 Simplest Example: Springs What happens if we attach a mass to a spring sitting on a table at it’s equilibrium point (I.e., x = 0) and let go? What happens if we attach a mass, then stretch the spring, and then let go? k Physics 218, Lecture XXIV 17 Questions • What are the forces? Hooke’s Law: F= -kx • Does this equation describe our motion? x = x0 + v0t + ½at2 Physics 218, Lecture XXIV 18 The forces No force Force in –x direction Force in +x direction Physics 218, Lecture XXIV 19 More Detail Time Physics 218, Lecture XXIV 20 Some Terms Amplitude: Max distance Period: Time it takes to get back to here Physics 218, Lecture XXIV 21 Overview of the Motion • It will move back and forth on the table as the spring stretches and contracts • At the end points its velocity is zero • At the center its speed is a maximum Physics 218, Lecture XXIV 22 Simple Harmonic Motion Call this type of motion Simple Harmonic Motion (Kinda looks like a sine wave) Next: The equations of motion: Use SF = ma = -kx (Here comes the math. It’s important that you know how to reproduce what I’m going to do next) Physics 218, Lecture XXIV 23 Equation of Motion A block of mass m is attached to a spring of constant k on a flat, frictionless surface What is the equation of motion? k Physics 218, Lecture XXIV 24 Summary: Equation of Motion Mass m on a spring with spring constant k: x = A sin(wt + f) Where w2 = k/m A is the Amplitude f is the “phase” (phase just allows us to set t=0 when we want) Physics 218, Lecture XXIV 25 Simple Harmonic Motion At some level sinusoidal motion is the definition of Simple Harmonic Motion A system that undergoes simple harmonic motion is called a simple harmonic oscillator Physics 218, Lecture XXIV 26 Understanding Phase: Initial Conditions A block with mass m is attached to the end of a spring, with spring constant k. The spring is stretched a distance D and let go at t=0 – What is the position of the mass at all times? – Where does the maximum speed occur? – What is the maximum speed? Physics 218, Lecture XXIV 27 Check: This looks like a cosine. Makes sense… Spring and Mass Paper which tells us what happens as a function of time Physics 218, Lecture XXIV 28 Example: Spring with a Push We have a spring system – Spring constant: K – Mass: M – Initial position: X0 – Initial Velocity: V0 Find the position at all times Physics 218, Lecture XXIV 29 What is MOST IMPORTANT? Simple Harmonic Motion X= A sin(wt + f) • What is • What is • What is • What is points? • What is the the the the amplitude? phase? angular frequency? velocity at the end the velocity at the middle? Physics 218, Lecture XXIV 30