Physics 106 Homework #1
Submitting Answers
Remember to be careful about units.
Implicit multiplication is not understood. Type 3*x, not 3x
Defined Constants
The following constants can be used by name throughout this assignment:
.
Problem 1.1
Purpose: This problem is a simple exercise in
using Coulomb's Law.
Problem
with data for try #1
:
A charge
q1
= +5.58×10
–9
C is located 3.20 m
from a charge of –2.80×10
–9
C. Find the
magnitude of the electrostatic force exerted by
one charge on the other.
Additional input values q1 (C): 3.26E9, 5.57E
9
Defined symbols:
q1
first charge in C
q2
second charge in C
r
distance between charges in meters
The answer is
F
magnitude of the force in N
Hints:
Remember that the magnitude of a vector is always positive.
Range of answers: 7.00×10
–9
N to 1.50×10
–8
N
.
Problem 1.2
Purpose: This problem uses the same concepts
as Problem 1.1, but requires you to think of
Coulomb's Law in conjunction with Hooke's
Law for springs. It's a good problem for MCAT
review.
Problem
with data for try #1
:
A molecule of DNA (deoxyribonucleic acid) is
2.17 μm long. The ends of the molecule become
singly ionized negative on one end, positive on
the other. The helical molecule acts like a spring
and compresses upon becoming charged. If the
molecule compresses 1.10 percent, determine
the effective spring constant of the molecule. Be
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View Full Documentsure to put everything into SI units, so the
answer is in SI units!
Additional input values pct: 1.1, 1
Defined symbols:
pct
percentage compression of the molecule
d
length of the DNA molecule
The answer is
k
the effective spring constant
of the molecule
Hints:
The Coulomb force between the charges at the ends of the molecule causes the molecule to
compress. The Spring force is
F
=–
kx
where
k
is the spring constant and
x
is the amount of
compression. Note that when the molecule compresses, the distance between the charges is no
longer 2.17 μm.
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 Fall '09
 Charge, Work, Electric charge, µC, Additional input values

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