Physics 121C Midterm 3 Information

Where:

• All students in PAB A102
• Seat Assignments are available on Tycho  -- the same as for the first two midterms.
  

When:

• Monday, November 24  10:30 - 11:20 am
  

Coverage:

You are still responsible for material  covered on the first and second midterms, but the focus of the third midterm is on material covered in the past 3-4 weeks.   Note that rotational kinematics and dynamics requires and expands most of the material on the 1st two midterms, so that you may well be asked a question that you could have answered earlier in the quarter.

• Lecture/Text material: Chapter 8:  Conservation of Linear Momentum; Chapter 9:  Rotation; Chapter 12:1-3 Static Equilibrium (only 1st half of chapter); Chapter 10:1-3  Angular Momentum (only when torque = 0 or is parallel to angular momentum -- 3D will be covered on the final, but not this midterm)
  
• Tutorials 5, 6:  conservation of energy, conservation of momentum ((NOTE:  because of the Veteran's Day Holiday, Tutorial 7, Rigid body rotation, will be covered on the final but not on MT 3).
  
• Labs 6, 7  Work and Energy, momentum & collisions (NOTE:  because of the Veteran's Day Holiday, Lab 8, angular kinematics, will be covered on the final but not on MT 3). 
• Homework:  Tycho week 5, 6, 7, 8 (though not 3D torque/angular momentum); Supplemental through Nov 17; tutorials 5, 6.
  

Important Lecture Concepts:

• Chapter 8:  Linear Momentum
• Concept of Momentum.
• generalization of Newton's 2nd Law
• Momentum Conservation
• Impulse = change in momentum; can give you average force during a collision.
• Change in Total Momentum requires Net Force on System
• In general, 2-item collision in 1D has four unknowns:  momentum of each item before and after.  Momentum conservation gives you one equation.
• In more than one-dimension, get 4 more unknowns and one more equation for each dimension from momentum conservation
• Elastic Collisions
• Energy is conserved in an elastic collision
• one-dimensional elastic collisions
• You should be able to solve for final velocities in terms of initial velocities with combined momentum and energy conservation and vice versa.
  
• Inelastic collisions
• momentum is still conserved
• kinetic energy of center of mass motion cannot be lost
• maximum energy loss for items sticking together after collision
• Two dimensional collisions
• momentum conserved individually in the different dimensions
• Problem Solving
• keep your variables clear, with consistent definitions of positive and negative velocity
• keep your algebra in terms of the unknowns as long as possible -- many will cancel to simplify the math later.
• conserve momentum, and where appropriate, conserve energy
• Center of Mass
• all motion can be separated into CM translation plus other motion relative to the CM
• the total momentum relative to the CM is zero
• you should be able to find the CM position, velocity, acceleration of an object or system of objects (though you will not be required to do an integral on the exam)
• The acceleration of the CM is always given by the net force/total mass.
• You are not responsible for rocket propulsion (section 8-5).
• Chapter 9:  Rotational Motion about a Fixed Axis
• Angular Kinematics
• definitions of angle, angular velocity and angular acceleration
• kinematic equations relating θ, ω, and α
• Relationship between angular velocity and acceleration to the actual velocity and acceleration of a point (including centripetal acceleration).
  
• Rolling motion, with and without slipping
• v_cm = ωR for rolling WITHOUT slipping
• static frictional force is only as large as it needs to be to maintain rolling without slipping.
  
• no definite relationship between v_cm and ω if there is slipping.  Friction is kinetic.
  
• Torque: τ = Iα = R_perpF=RF_perp= RFsinφ
• Moment of Inertia:  You should know I for a point object and for a hoop, solid disk, and rod around their centers.  Any others will be given on the exam.  You should also know how to calculate I using superposition.  You should also understand qualitatively how to rank moments of inertia (where mass is distributed).
• When a pulley has mass, accelerating it requires torque, and the tension in a string can be different on the two sides of a pulley.
  
• Rotational Kinetic Energy
• KE_rot = Iω²/2
• Work = τθ
• Power = τα.
• Energy conservation must be expanded to include both rotational and translational KE
• Total KE = KE of translation of CM + KE of rotation around CM
• if a point (not CM) is stationary, you may calculate only the KE of rotation around that point.  Remember to use moment of inertia calculated around that point.
• Problem Solving
• You should know when to use energy vs. dynamical equation approach to find angular variables as a function of space or time.
• Be careful about definitions of velocity, relative velocity, and how they relate to angular velocity.
• Chapter 12:  Static Equilibrium
  
• vector nature of torque:  τ = r x F
  
• static equilibrium means torque about any axis must be zero
• pick the one that enables you to solve for the variables you are asked for
• Static equilibrium also means net force is zero
• Rigid body acts for torque as though all the mass were at the center of gravity (same location as c center of mass)
• You are not responsible for sections on accelerated reference frames, deformable bodies, stress and strain.
• Problem solving
• draw an extended free body diagram first
• set up net force and net torque equations
• solve only those components you need to.
  
• Chapter 10:  Angular Momentum
  
• vector nature of angular momentum:  L = r x p
• angular momentum depends on your choice of origin
• angular momentum for symmetric object L = Iω, with vector along the axis of rotation
• counterclockwise motion viewed down the angular momentum vector (tip to tail)
• vector cross product
  
• angular momentum is conserved if there is no net torque on a system
• torque impulse gives a change in angular momentum
• Problem solving
• find initial and final angular  momentum
• relate them via the net torque (which may be zero, in which case they are equal)
• remember to calculate both torque and angular momentum about the same point
• linear motion can result in angular momentum
• gyroscopes and precession are covered on the final exam, but not on midterm 3.
• combined motion of the center of mass with rotation is covered on the final exam, but not on midterm 3 except for contexts covered in chapter 9 (e.g., rolling)
• quantization of angular momentum is not covered.
• Chapter R:  Relativity is NOT COVERED.
  

Format:

• Total of 100 points, distributed as:
• 56 points multiple choice (2 problems with multiple sections)
• 24  point short answer lecture question
• 20  point short answer tutorial question

You Must Bring:

Scan-Tron Form for Multiple-choice part of exam

Dark Pencil or Pen that marks scan-tron form so that it can easily be read by an optical scanner.

You May Bring:

• Calculator (you may need trig functions)
  
• Ruler or straight edge
• one 8.5x11 page of notes (both sides)
You will be expected to know (or have on your notes) basic formulas and the appropriate trigonometry or geometry. You should also know the important constants.

You May NOT Bring (or at least they must be turned off and not taken out of your backpack):

• Wireless devices, including laptops, blackberries, cell phones, etc.
  

Review on your own.  I reccomend making a list of questions about the points you don't understand, discussing it with your classmates, and asking me (or any of the other instructors at the Study Center) during my office hours about any questions that you can't answer.  Make sure you understand how to do all the Tycho, supplemental, and tutorial problems, as well as understanding the material covered in the text, lecture, tutorial, and lab.  Work through additional problems from the book.  Finally, your goal in this class should be to understand the physics, not to get a good grade on the exam.  I know it's hard to believe, but you will actually do better on the exam if you approach it with the goal of understanding physics.

If you had problems on the first two midterms, it may need you need to change your study approach.  Practice doing old homework problems with the book closed.  It shouldn't take more than a few minutes/problem if you really understand it.  Practice setting up problems by looking through the end-of-book questions and problems.   Think in advance about how you might explain your reasoning in one or two sentences.  Get a good-night's sleep on Sunday, and eat a healthy breakfast before the exam on Monday.

Back to 121C Home Page