IMPORTANT: Fill in the circle “A” after “TEST FORM”
under your signature on the answer sheet.
PHYS 172 Fall 2009
Wednesday, October 14th
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 (Ritchie or Csathy), Course
(PHYS 172), Date (10/14/09), and Test (2). 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 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:
 
Δp = Fnet Δt

Mm
Fgrav = G  2 ; approx. mg near surface of Earth
r
G = 6.7 × 10 −11 N·m2/kg2
1
= 9x10 9 Nm 2 /C 2
4 πε0


p = mγv
g = 9.8 N/kg
γ=
1
1−
v2
c2
 
Δr = v avg Δt

Fspring = ks s , opposite the stretch
c = 3 × 10 8 m/s



p ≈ mv , if v is small compared to c
1
U electric =
1 q1q2
4 πε0 r
U gravity =
−GMm
r
ΔE system = W surroundings + Q
E = mγc 2 for a single particle
1
E = mc 2 + mv 2 for a single particle if v << c
2
1
Fair = Cρ air Av 2
2
Fbuoyancy =
ρ air
mg
ρ object
ΔE thermal = mCv ΔT
________________________________________________________________________
2
Problem 1: Two protons are held fixed at a distance of 3 angstroms (3 x 10-10 m) from
one another. The protons have a charge of +1.6 x 10-19 C. After they are released they
repel each other and fly apart and each acquires some kinetic energy. What is the final
total kinetic energy of the two particles when they are at a large distance from each other?
1)
2)
3)
4)
5)
7.7 x 10-19 J
3.3 x 10-19 J
1.6 x 10-23 J
4.6 x 10-19 J
1.2 x 10-18 J
Problem 2: A car of mass 1200 kg is constructed by supporting its frame on four
springs, each with a spring constant of 20000 N/m. The driver and passenger in the car
have a combined mass of 150 kg. What is the frequency of vibration of the car when the
car drives over a bump in the road? (Note: Give the frequency, f = ω/2π)
1)
2)
3)
4)
5)
0.21 Hz
1.50 Hz
1.93 Hz
1.23 Hz
1.73 Hz
Problem 3: A child of mass 35 kg slides down a slide at the park. The slide starts at a
height of 4 m and ends at the ground. If the child starts at rest and ends at a speed of 7
m/s, how much heat has been generated as the child goes down the slide?
1)
2)
3)
4)
5)
1215 J
20 J
515 J
1092 J
934 J
Problem 4: A ball of mass m falls from a height h to the surface of the Earth. Which one
of the following is correct? (assume that h is much smaller than the radius of the Earth)
1)
2)
3)
4)
If we choose our system to be ball+Earth then the external work done on the
system is mgh
If we choose our system to be just the ball, then the external work done on the
system is –mgh.
If we choose our system to be ball+Earth, then the external work done on the
system is zero.
If we choose our system to be just the ball, then the change in the potential energy
is mgh.
3
Problem 5: A boat carrying a large boulder is floating in a lake. The boulder is thrown
overboard and sinks. Relative to the shore line, the water in the lake will:
1)
2)
3)
4)
rise
fall
remain the same
not enough information is given
Problem 6: The inter-atomic bonds in metals can be modeled as springs. For a given
metal, the effective spring constant of the inter-atomic bond is k (unit N/m), the mass of
each atom in the metal is m (unit kg), and the distance between atoms in the metal is d
(unit m). In this model, the speed of sound (unit m/s) in the metal is given by:
1)
2)
3)
d k /m
4)
m d k
5)
kd / m
d m/k
k d m
4