Physics 235, Midterm Exam # 1

October 16, 2017: 10.25 am - 11.40 am



Do not turn the pages of the exam until you are instructed to do so.


Exam rules: You may use only a writing instrument while taking this test.  You may not consult any calculators, computers, books, nor each other.


Problems 1 and 2 must be answered in exam booklet 1.  Problems 3 and 4 must be answered in exam booklet 2.  The answers need to be well motivated and expressed in terms of the variables used in the problem.  You will receive partial credit where appropriate, but only when we can read your solution.  Answers that are not motivated will not receive any credit, even if correct.


At the end of the exam, you need to hand in your exam, the blue exam booklets, and the equation sheet.  All items must be clearly labeled with your name, your student ID number, and the day/time of your recitation.  If any of these items are missing, we will not grade your exam, and you will receive a score of 0 points.


You are required to complete the following Honor Pledge for Exams.  Copy and sign the pledge before starting your exam.


“I affirm that I will not give or receive any unauthorized help on this exam, and that all work will be my own.”








Name:  ______________________________________________________________________


Signature:  ____________________________________________________________________


Problem 1 (25 points)

Consider a thin uniform disk of mass M and radius a.  The center of the disk is located at the origin of our coordinate system, and the disk is located in the xy plane.


a)     Find the potential F on the z axis, a distance z from the center of the disk.


b)    Find the graviational field on the z axis, a distance z from the center of the disk.  Indicate the direction and magnitude of the gravitational field.


c)     Find the gravitational force on a mass m located at this position.  Indicate the direction and magnitude of the gravitational force.


Problem 2 (25 points)

a)     Which of the following forces are conservative?



In these equations a, b, and c are constants.  Note: your answer must be wel motivated.


b)    For the conservative forces identified in step a), find the potential energy U.


Problem 3 (25 points)

Two masses m1 and m2 slide freely on a horizontal frictionless track and are connected by a spring whose force constant is k.  Assume the rest length of the spring is L. 



a)     Write down the differential equation that describes the acceleration of mass 1 in terms of the position of mass 1 and 2.


b)    Write down the differential equation that describes the acceleration of mass 2 in terms of the position of mass 1 and 2.


c)     Use the two differential equations obtained in parts a) and b) to obtain a differential equation that desrcribes the acceleration of mass 1 in terms of just the position of mass 1.


d)    Find the frequency of the oscillatory motion for this system.


Problem 4 (25 points)

a.     Consider the Lyapunov exponent as a function of a for the logistic equation map, shown in the Figure below.  How many solutions are possible when a = 3.5?  What is the smallest value of a for which chaotic motion will occur?



b.     Which of the curves in the following figure shows the time dependence of the amplitude of an over-damped oscillator?






c.     Consider a damped and driven pendulum.  The Poincare sections for seven different driving force amplitudes are shown on the right: F = 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and 1.0 N (from top to bottom).


For each Poincare section, indicate if the motion is

1.     Simple Harmonic.

2.     Periodic.

3.     Chaotic.


d.     Consider a system that can be described by a second order differential equation.  Consider the following three second order differential equations:



The constants in these differential equations are positive constants.  The mass m is at rest at time t = 0 s and located at x = 1 m.

Consider the following phase diagrams.  Match each phase diagram to one of these three differential equations.



Phase diagram # 1.



Phase diagram # 2.



Phase diagram # 3.


e.     What is the nationality of Didi Gregorius?



___   USA

___   The Netherlands

___   South Africa

___   China

___   Brazil



How many times did the Yankees reach the post season since 1921?


___   Between 0 and 9

___   Between 10 and 19

___   Between 20 and 29

___   Between 30 and 39

___   Between 40 and 49

___   Between 50 and 59

___   Between 60 and 69

___   Between 70 and 79

___   Between 80 and 89

___   Between 90 and 99


Exam # 1.

First midterm exam in Phy 235, covering the material of Chapters 1 - 5.