EXAM II, PHYSICS 1403
July 21, 2004
Dr. Charles W. Myles
INSTRUCTIONS: Please read ALL of these before doing anything else!!!
1. PLEASE put your name on every sheet of paper you use and write on one side of the paper
only!! PLEASE DO NOT write on the exam sheets, there will not be room! Yes, this wastes
paper, but it makes my grading easier!
2. PLEASE show all work, writing the essential steps in the solutions. Write appropriate
formulas first, then put in numbers. Partial credit will be LIBERAL, provided that essential
work is shown. Organized, logical, easy to follow work will receive more credit than
disorganized work.
3. The setup (PHYSICS) of a problem will count more heavily than the math of working it out.
4. PLEASE write neatly. Before handing in your solutions, PLEASE: a) number the pages and
put the pages in numerical order, b) put the problem solutions in numerical order, and c)
clearly mark your final answers. If I can’t read or find your answer, you can't expect me to
give it the credit it deserves.
NOTE: I HAVE 55 EXAMS TO GRADE!!! PLEASE HELP ME
GRADE THEM EFFICIENTLY BY FOLLOWING THE ABOVE
SIMPLE INSTRUCTIONS!!! FAILURE TO FOLLOW THEM
MAY RESULT IN A LOWER GRADE!! THANK YOU!!
An 8.5’’ x 11’’ piece of paper with anything written on it and a calculator are allowed.
NOTE: Problem 1 consists of Conceptual Questions and IS REQUIRED! You may
work any three (3) of the remaining four problems for four (4) problems total for this
exam. Each problem is equally weighted and worth 25 points, for a total of 100 points on
this exam.
1. THIS PROBLEM IS MANDATORY!!! CONCEPTUAL QUESTIONS: Answer
these briefly, in a few complete and grammatically correct English sentences. You
may supplement these sentences with equations, but keep these to a minimum!!
(Newton’s Laws are about forces! For full credit in parts a. & b., you must mention FORCES!)
a. State Newton’s 1st Law.
b. State Newton’s 3rd Law.
c. See figure. A hockey puck slides to the right at constant
velocity across a flat, horizontal, frictionless ice surface.
Which of the sketches in the figure is the correct free body
diagram for this puck? WHY? Explain your answer using
Newton’s Laws! (Hint: Is there a force in the direction of the
puck’s motion?)
d. See figure. A Ferris wheel rider moves in a vertical circle of radius r at
constant speed v. He thus experiences a centripetal acceleration a. The
free body diagrams for the rider at the top and at the bottom are shown in
the figure. Is the normal force FN that the seat exerts on the rider at the
top of the circle less than, more than, or the same as the normal force at
the bottom of the ride? (Is FN equal to the weight?) Explain your answer
using Newton’s 2nd Law with centripetal acceleration.
NOTE: WORK ANY THREE (3) OF PROBLEMS 2., 3., 4., or 5.!!!!!
2. See figure. Two masses (m1 = 15 kg and m2 = 25 kg) are
m1, a 
connected by a massless cord over a massless, frictionless pulley
as shown. m1 sits on a flat, horizontal frictionless table. The
system is released and m1 accelerates to the right while m2
accelerates downward.
a. Draw the free body diagrams for the two masses, properly
labeling all forces. For m1, be sure to include both horizontal
and vertical forces.
b. Compute (find a numerical value for) the normal force FN between m1 and the
table.
c. The two unknowns in this problem are the acceleration, a, of the masses (assumed
to be the same magnitude for both) and the tension, FT, in the cord. By applying
Newton’s 2nd Law to m1 and m2, find the two equations needed to solve for a and
FT. Writing them in symbols, without substituting in numbers, will receive more
credit than writing them with numbers substituted in!
d. Using the equations from part b, compute (find numerical values for) the
acceleration a and the tension FT (in any order).
3. See figure. A box, mass m = 25 kg, is placed on a flat, horizontal
surface. There is friction. The coefficient of kinetic friction between the
box and the surface is μk = 0.2. The box is pulled a by a force FP = 70 N
using a cord that makes a 37º angle with the horizontal.
a. Draw the free body diagram for the box, properly labeling all forces.
b. Compute the horizontal and vertical components of the force FP.
c. Compute the weight of the box and the normal force FN between it and the
surface. Is this normal force equal (& oppositely directed) to the weight? If so,
why? If not why not? Justify your answer using Newton’s 2nd Law in the
vertical direction.
d. Compute the frictional force Ffr that the box experiences as it moves to the right.
e. Use Newton’s 2nd Law to find the acceleration experienced by the box. What
forces cause this acceleration?
m2
a

NOTE: WORK ANY THREE (3) OF PROBLEMS 2., 3., 4., or 5.!!!!!
4. See figure. A planet, mass m = 3.0 x 1024 kg, is in a circular orbit at a constant
speed v around a star, which is assumed to be a uniform sphere of constant
density. The radius of the orbit (measured from the star’s center) is
r = 3.0 x 1011 m. The mass of the star is M = 2.0 x 1030 kg. The gravitational
constant is G = 6.67 x 10-11 N m2/kg2.
a. The planet’s orbit is circular, so it experiences a centripetal acceleration.
Using words (not equations, for which I will give zero credit!) tell me
what the cause of this acceleration is. (Hint: See part b!)
Circular orbit!
b. Compute the gravitational force of attraction FG between the planet and the
star. What is the “centripetal force” on the planet? (Hint: Answers to a & b
should be consistent! You do not need to know the planet’s speed to answer
this!)
c. Compute the centripetal acceleration experienced by the planet. What is the
direction of this acceleration? (You do not need to know the planet’s speed to
answer this!).
d. Compute the speed v of the planet in orbit. (Hint: Einstein taught us that the
largest speed possible for anything is the speed of light c = 3 × 108 m/s. If you get
a speed larger than this, you’ve done something wrong!)
e. Compute the period T of the planet’s orbit.
5. See figure. A ball, mass m = 2.5 kg is twirled at constant speed v = 4 m/s at the end
of a massless string in a vertical circle of constant radius r. The period for
this uniform circular motion is T = 1.6 s. Free body diagrams for the ball at
the top and at the bottom of the circle are shown.
a. Compute the radius r of the circle in which the ball is moving.
b. Compute the ball’s centripetal acceleration.
c. Compute the “centripetal force” on the ball.
d. When the ball is at the top (point A in the figure) compute the
tension FTA in the string.
e. When the ball is at the bottom (point B in the figure) compute the
tension FTB in the string.