EXAM III, PHYSICS 1403, July 30, 2014, Dr. Charles W. Myles
INSTRUCTIONS: Please read ALL of these before doing anything else!!!
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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! This wastes paper, but it
makes my grading easier!
PLEASE show all work, writing the essential steps in the solutions. Write formulas first, then put
in numbers. Partial credit will be LIBERAL, provided that essential work is show n. Organized,
logical, easy to follow work will receive more credit than disorganized work.
The setup (PHYSICS) of a problem will count more than the math of working it out.
PLEASE write neatly. Before handing in your solutions, PLEASE: a) number the pages & put the
pages in numerical order, b) put the problem solutions in numerical order, & 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!! The words “EXPLAIN”, “DISCUSS” & “DEFINE” below mean to answer mostly in
ENGLISH, NOT math symbols!
I HAVE 41 EXAMS TO GRADE!! PLEASE HELP ME GRADE
THEM EFFICIENTLY BY FOLLOWING THESE SIMPLE
INSTRUCTIONS!!! FAILURE TO FOLLOW THEM MAY
RESULT IN A LOWER GRADE!! THANKS!
An 8.5’’ x 11’’ piece of paper with anything written on it & a calculator are allowed. NOTE: Problem 1,
Conceptual Questions & Problem 2, Work & Energy ARE REQUIRED! You may work any two (2)
of the remaining 3 problems for four (4) problems total. (This means 4 complete problems, with all of
their parts!). Each problem is equally weighted & worth 25 points, for 100 points on this exam. Also be
sure to look at the BONUS Questions in #6!!
1. MANDATORY CONCEPTUAL QUESTIONS!!! Answer briefly all parts in a few complete,
grammatically correct English sentences. Give answers using mainly ENGLISH WORDS, NOT
symbols or equations! If you insist on using symbols, DEFINE all symbols! NO
Fig. 1
credit will be given for answers with ONLY symbols! If a part contains more than
one question, please answer each one!
a. See Fig 1. A ball of mass m is twirled at the end of a string in a circle of
constant radius r and constant speed v. Free body diagrams for the ball at the top
& at the bottom of the circle are shown. Is the tension FTA that the string exerts
on the ball at the top of the circle (point A) less than, more than, or the same as
the tension FTB at the bottom of the circle (point B)? WHY? Explain (in
English!) your answer using Newton’s 2nd Law with centripetal acceleration.
b. State Newton’s Universal Law of Gravitation.
c. State the Work-Energy Theorem. (Hint: This ISN’T the same as the definition of the work done
by a constant force! Answers giving the definition of the work done by a constant force will get
ZERO credit!).
d. In class, I stated that the Work-Energy Theorem is one of Newton’s Laws of Motion, but
expressed in Work-Energy language rather than Force language. Fill in the blank: The Work
Energy Theorem is Newton’s ___________ Law of Motion in Work-Energy Language.
e. This was discussed yesterday: State The Principle of Conservation of
both start
Mechanical Energy. (Hint: This ISN’T the same as The Law of’
here
Fig. 2
Conservation of Total Energy! Answers discussing that Law will get ZERO credit!).

f. See Fig 2. This idea was discussed at the end of class yesterday: Two water
slides are shaped differently, but start at the same height, h. Two riders, Paul &
Kathleen, start from rest at the same time & at the same height h but on different
slides. (The figure shows them at different heights because it shows them AFTER
they’ve started down!) The slides are frictionless. Who is traveling faster at the
bottom? What Physical Principle did you use to answer this?
g. 2 BONUS POINTS: In part f, who gets to the bottom first? Why? (Answer in
words!!) Also be sure to look at the BONUS questions in #6!!!!!
2.
3.
4.
NOTE: YOU MUST WORK PROBLEM 2! WORK ANY TWO OF PROBLEMS 3., 4., or 5!!
Also be sure to look at the BONUS questions in #6!!!!!
MANDATORY WORK-ENERGY PROBLEM!! See figure,
which is a free body diagram of a box, mass m = 55 kg sitting
on a flat, horizontal floor. A cord is wrapped around it &
someone pulls on it with a constant force FP = 110 N at angle θ
= 37 with the horizontal. This causes the box to move to the
right across the floor. The friction force between the box & the
surface is known to be Ffr = 45 N. That force acts to the left, as
shown in the figure. Starting from rest at left in the figure, the
box moves to the right a displacement of x = 40 m.
a. Calculate the work done by the normal force FN, as the box moves through the displacement x =
40 m. Calculate the work done by the weight mg during the same process.
b. Calculate the work done by the friction force Ffr during the process described in part a. Calculate
the work done by the pulling force FP during the same process.
c. Use the results of parts a, & b to calculate the net work done Wnet due to all forces on the box
during the process described in part a.
d. If the box starts from rest on the left of the figure, calculate it’s kinetic energy after it has moved
through the x = 40 m displacement. What Physical Principle did you use to do this calculation?
e. Calculate the box’s speed after it has moved through the x = 40 m displacement.
f. Calculate the normal force FN acting on the box in the vertical direction. Is it equal & oppositely
directed to the box’s weight in this case? What Physical Principle did you use to do this
calculation?
NOTE: YOU MUST WORK PROBLEM 2! WORK ANY TWO OF PROBLEMS 3., 4., or 5!!
Also be sure to look at the BONUS questions in #6!!!!!
See figure. A car, mass m = 2,400 kg, rounds a curve on a flat road at speed v = 20 m/s.
The figure shows a top view. The curve’s radius of curvature is r = 60 m. There is static
friction between the road & the car tires, or the car would not stay on the curve.
a. Sketch the free body diagram for the car, properly labeling all forces. (Hint: This
will show a front view of the car, as opposed to the top view of the figure! Note: If
you don’t make a sketch, you will lose points!!)
b. Calculate the normal force FN between the road and the car tires. Is FN equal (&
v = 20 m/s
oppositely directed) to the weight? If so, why? If not why not? Which Newton’s

Law of Motion did you use to justify your answer?
c. Calculate the centripetal acceleration experienced by the car. What is the direction of this
acceleration?
d. Calculate the “centripetal force” experienced by the car. What physical phenomenon is the cause
of this “centripetal force”?
e. Calculate the static friction force between the tires and the road.
f. If the given speed v = 20 m/s is known to be the maximum speed for which the car will not skid
on this curve, calculate the coefficient of static friction μs between tires and the road.
NOTE: YOU MUST WORK PROBLEM 2! WORK ANY TWO OF PROBLEMS 3., 4., or 5!!
Also be sure to look at the BONUS questions in #6!!!!!
See figure. A child, mass m = 38 kg, is on a Ferris wheel which moves her at
v=?
constant speed v in a vertical circle of radius r = 12 m. The period for this
uniform circular motion is T = 18 s. The free body diagrams for the rider at the
top & at the bottom are shown. FN is the normal force experienced by the rider
(due to the seat pressing upward on her body). FN is not necessarily the same at
the top & at the bottom.
a. Calculate the speed v of the rider as she moves around the Ferris wheel in
the circular path just described.
b. Calculate the centripetal acceleration aC experienced by the rider. What
forces cause this acceleration? (Answer in words!)
c. Calculate the “centripetal force” on the rider.
PROBLEM 4 CONTINUES ON THE NEXT PAGE!!
h
aC
r = 60 m
d.
e.
f.
PROBLEM 4 CONTINUED!
What is the DIRECTION of the centripetal acceleration & centripetal force found in parts b
and c? Are they in the same direction when the rider is at the top & when she is at the bottom?
Write the equation resulting from applying Newton’s 2nd Law in the vertical direction to the
rider when she is at the top of the Ferris wheel. Calculate the normal force FN on the rider at
that point. Is FN equal (& oppositely directed) to the child’s weight? WHY or WHY NOT?
Write the equation resulting from applying Newton’s 2d Law in the vertical direction to the
rider when she is at the bottom of the Ferris wheel. Calculate the normal force FN on the rider
at that point. Is FN equal (& oppositely directed) to the child’s weight? WHY or WHY NOT?
NOTE: YOU MUST WORK PROBLEM 2! WORK ANY TWO OF PROBLEMS 3., 4., or 5!!
Also be sure to look at the BONUS questions in #6!!!!!
5.
See figure. Note: YOU MUST use scientific (power of 10) notation to solve this problem.
PLEASE be careful in accounting for powers of 10! A satellite, mass m = 3,600 kg, is in a
circular orbit at constant speed v around planet X, assumed to be a uniform sphere of constant
density. (Resemblance of planet X to Earth is coincidental!) The radius of the orbit (measured
from the planet’s center) is r = 5.5  107 m. The planet mass is M = 7.0  1024 kg. The
gravitational constant is G = 6.67  10-11 (N m2)/(kg2).
a. The satellite’s orbit is circular, so it experiences a centripetal acceleration. Using words (not equations,
r 
Circular
which will get zero credit!) tell me what the cause of this acceleration is.
Orbit!
Calculate the gravitational force between the satellite and the planet. What is the “centripetal force” on
the satellite? (Hints: Answers to a & b should be consistent! You don’t need to know the satellite’s speed to
answer this!).
c. Calculate the centripetal acceleration experienced by the satellite. What is it’s direction? (Hint: This will be
very small! You don’t need to know the satellite’s speed to answer this!).
d. Calculate the speed of the satellite in orbit. (Hints: Einstein told us that the largest speed possible for any
mass is the speed of light c = 3 × 108 m/s! If your speed is larger than c, or even a significant fraction of it,
you’ve done something wrong! But, v should be large enough for the satellite to go the HUGE distance
around the orbit in a reasonable time. FYI: Earth satellites can make 2 or more full orbits per day. If you find
a v as slow as that of ordinary objects moving on the Earth’s surface, such as, for example, 100 m/s, that’s
much too slow & you’ve done something wrong!).
e. Calculate the period of the satellite’s orbit. (Hint: This should be a reasonable period for a satellite around a
planet similar in size to Earth. For a typical period, see the hint for part d.)
BONUS QUESTIONS, 8 BONUS POINTS TOTAL!! For the extra points indicated, briefly answer the following
questions using a few complete, grammatically correct English sentences. The answers to most of these are
NOT in the book! If you were in class when I discussed them, you likely will be able to answer them. If you
skipped, as some often do, you likely won’t be able to answer them.
a. 2 BONUS POINTS! When starting the Gravitation discussion, I said that Newton was the first to do
Gravitation Theory. I also said that he used his Gravitation Law (+ his 2 nd Law + Calculus!) to make
predictions which were in Excellent Agreement with some observations. Tell me, briefly, exactly what the
observations were that Newton explained with his theory. This result also eventually came to be called his
“Greatest Achievement”. It’s a major reason why he’s called “the Greatest Scientist who Ever Lived”!.
h
Correct answers should give some details of the problem he successfully applied his Gravitation Law to.
NOTE: Answers ONLY saying that he developed the Gravitation Law aren’t sufficient (this is obvious!) &
will get ZERO credit. The problem I’m referring to here is NOT the same as his idea for putting objects into
orbit around Earth. Answers here that discuss that idea will get ZERO credit. I ask about that idea in part b!
b. 2 BONUS POINTS!!! Near the end of the Gravitation discussion, we talked about small objects orbiting
larger ones, like artificial satellites around the Earth. As a historical comment, I mentioned that Newton
himself was the first to suggest putting objects in Earth orbit. Briefly describe his idea for HOW to launch
such objects into orbit. Be specific! He didn’t know about rockets! Zero credit will be given for answers
which talk about rockets. (He had the physics correct for this, but the technology to actually do this didn’t
exist in the 1600’s).
c. 2 BONUS POINTS!!! Near the end of the Gravitation discussion, we discussed the “effective
weightlessness” concept & the fact that reporters are VERY wrong when they say things like “the space
shuttle has escaped the Earth’s gravity & is now in orbit.” In a few complete, grammatically correct
sentences, explain the reason that this statement is wrong. What would happen if the shuttle didn’t
experience the Gravitational force of the Earth?
d. 2 BONUS POINTS!!! During of the Gravitation discussion, I briefly discussed some of Newton’s scientific
achievements. Obviously, his Laws of Motion and his Law of Gravitation are among his major scientific
achievements. He also had many others besides these. Briefly tell me what ONE of his OTHER scientific
achievements was. (His alchemy & his biblical research are not scientific achievements & will be given zero
credit on this part!)
b.
6.
v