IMPORTANT: Fill in the circle “A” after “TEST FORM”
under your signature on the answer sheet.
PHYS 172 Fall 2008
Wednesday, September 17
EXAM 1 - TEST FORM A
There are two parts to Exam 1: the machine-graded part of this test, and the last page that
you turn in to be graded by hand.
Machine Answer Sheet:
• Using a pencil, fill in Last Name, First Name, & Middle Initial, plus your 10-digit
Purdue University ID number. Enter Instructor (Peterson, for the 11:30 lecture, or
Ritchie, for the 12:30 & 1:30 lectures), Course (PHYS 172), Date (9/17/08), and
Test (1). Leave “Section” blank. You must include your Signature.
•
The seven machine-graded problems are worth 10 points each (70 points total). If
your answer is slightly different from any of the choices due to round-off errors,
etc., then select the closest choice.
Hand-Graded Sheet:
• Enter your Name, Signature, PUID, and circle your Recitation Section.
•
The hand-graded portion is worth a total of 30 points.
When finished with both parts, bring them to the front of the classroom, show your
Purdue ID card to the Instructor, and turn in the machine-graded answer sheet and the
hand-graded answer sheet at the same time. These two parts together are worth 100
points total. Your grades will appear in the CHIP gradebook soon. An answer key will be
posted soon on the course home page. You may keep this copy of the exam.
************************************************************************
Some useful information:
!
r r
"p = Fnet "t
v
Mm
Fgrav = G v 2 ; approx. mg near surface of Earth
r
G = 6.7 ! 10 "11 N·m2/kg2
g = 9.8 N/kg!
r
r
p = m"v
"=
c = 3 ! 10 8 m/s
r
r
r
p " mv , if v is small compared to c
1
2
1#
!
r
"r = v avg "t
v
Fspring = ks s , opposite the stretch
v
c2
!
!
!
1
Problem 1: In your lab #3 on motion, you studied the motion of a ball falling under the
influence of gravity near the Earth’s surface. Below is an incomplete program used to
model the motion of that ball.
from visual import *
ball = sphere(pos=vector(0.0,1.0,0.0), radius=0.05, color=color.green)
ball.m = 0.10
# mass of the ball
ball.p =vector(0.0,0.0,0.0)
# zero initial momentum
Fgrav = vector(??,??,??) # force of gravity near earth’s surface
deltat = 0.01
t = 0.0
while t<4.0:
ballpold=ball.p
ball.p= ??
ball.pos =
t=t+deltat
ball.pos
+
(ball.p+ballpold)/2./ball.m*deltat
Which of the following correctly completes the bolded code for defining the force due to
the gravity (Fgrav) and the update of the momentum (ball.p) of the disk?
1) Fgrav = -9.8*ball.m
ball.p = ball.p + Fgrav*deltat
2) Fgrav = vector(0, -9.8*ball.m, 0)
ball.p = ball.p + Fgrav*deltat
3) Fgrav = vector(0, -9.8*ball.m ,0)
ball.p = Fgrav*deltat
4) Fgrav = -9.8*ball.m
ball.p = Fgrav*deltat
5) Fgrav = vector(0,-9.8*ball.m,0)
ball.p = ball.p + Fgrav*t
2
Problem 2: The baseball player Kosuke Fukudome (Chicago Cubs) steps up to the plate.
A fastball is thrown towards him with velocity,
. He swings, and pops the
ball straight up vertically in the air with a velocity,
, as in the figure below.
What is the direction of the force applied by the bat on the ball? (You have nine choices
shown in the diagram on the right.)
9) no change in momentum
Problem 3: Bob (mass = 90 kg) is standing still on a frozen pond, minding his own
business when Alice throws a snowball (mass = 100 g) at him. The snowball hits Bob in
the chest and sticks to him. The velocity of the snowball just prior to hitting Bob is
<15,0,25> m/s. What is Bob’s velocity after being hit? (The x-z plane is parallel to the
ice.)
1)
2)
3)
4)
5)
<1.7,0,2.8> cm/s
<0.0,0,0.0> cm/s
<2.2,0,1.7> cm/s
<1.1,0,2.8> cm/s
<2.2,0,2.8> cm/s
Problem 4: A stationary particle in space decays into two fragments. One of the
fragments is observed to have a momentum of <9.1,-5.4,0.5> kg m/s just after the
decay. The other particle is invisible to the detector and so its momentum cannot be
directly measured. What is the momentum of the invisible particle?
1)
2)
3)
4)
5)
<0.5,5.4,9.1> kg m/s
<9.1,-5.4,0.5> kg m/s
<5.4,9.1,-0.5> kg m/s
<-5.4,9.1,0.5> kg m/s
<-9.1,5.4,-0.5> kg m/s
3
Problem 5: A 5 kg block is attached to the end of spring with relaxed length 0.5 m and
spring constant 250 N/m. The free end of the spring is held stationary and the block is
made to go in a circle of radius 0.75 m, thus stretching the spring. What is the speed of
the block (neglecting gravity and air resistance)?
1)
2)
3)
4)
5)
3.4 m/s
3.1 m/s
1.0 m/s
2.7 m/s
2.4 m/s
Problem 6: Protons with mass of 1.7 x 10-27 kg can be accelerated to speeds close to the
speed of light by applying forces from magnets over a period of time in particle
accelerators like the Large Hadron Collider. What force is required to accelerate a
proton to a speed of 2 x 108 m/s after 60 seconds?
1)
2)
3)
4)
5)
3.0 x 10-20 N
1.1 x 10-20 N
7.6 x 10-21 N
4.6 x 10-20 N
5.1 x 10-21 N
Problem 7: If you drop a 0.5 kg object at rest on Earth, what is its velocity after 1.2
seconds?
1. <0,11.8,0> m/s
2. <0,9.8,0> m/s
3. <0,24.5,0> m/s
4. <0,20.6,0> m/s
5. <0,17.7,0> m/s
4
PHYS 172 - Fall 2008
Name(Print):_____________________________
Signature:________________________________
PUID:___________________________________
1. [10 pts] An expert pool (billiards) player hits the cue
ball on one side of the table with her cue stick. The cue
ball rolls across the table and collides the eight ball at an
angle. The cue ball and the eight ball then separate and
roll in different directions. First, draw a sketch of this
event and label your coordinates so that the line
connecting the cue ball to the eight ball is the x-axis and
the z-axis is perpendicular. Then, if the pool player
applies a force of 5 Newtons to the ball with the cue
stick for 0.1 seconds and the mass of the cue ball is 0.17
kg, then what is the initial velocity of the cue ball?
(Write the velocity in vector notation).
Circle your
Recitation:
W
W
Th
Th
Th
Th
Th
Th
Th
Th
Th
Fri
Fri
Fri
Fri
Fri
Fri
Fri
Fri
8:30
9:30
8:30
9:30
10:30
11:30
12:30
1:30
2:30
3:30
4:30
8:30
9:30
10:30
11:30
12:30
1:30
2:30
3:30
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2. [10 pts] The cue ball slows down to 90% of its initial speed as it rolls across the table
and before it hits the eight ball 0.75 seconds later. Calculate the cue ball’s average
velocity and then determine the distance the cue ball was from the eight ball originally.
3. [10 pts] If the velocity of the cue ball after the collision is v= < 0.3,0,0.5 > and the
velocity before the collision is 90% of the initial speed that you calculated in problem 1,
then what is the magnitude of the velocity of the eight ball (assuming it has the same
mass as the cue ball)?
6