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Physics 21
Fall, 2005
Solution to HW1
21
Q19
The form of Coulomb’s law is very similar to that
for Newton’s law of universal gravitation. What are the dif
ferences between these two laws? Compare also gravitational
mass and electric charge.
Both laws describe a force between two point objects 1
and 2 that depends inversely on the square of the distance
r
between the objects, and is directed from one object toward
the other. For gravity, the magnitude of the force depends
on the “mass” property of the object and is given by
F
=
G
M
1
M
2
r
2
,
where
G
is the gravitational constant. For Coulombs law,
the magnitude of the force depends on the “charge” property
and is given by
F
=
1
4
π±
0
Q
1
Q
2
r
2
,
where 1
/
4
π±
0
is a di±erent proportionality constant.
Masses are always positive, and the gravitational force is
always attractive. Charges may be positive or negative, and
the electrical force may be attractive or repulsive.
21
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 Fall '07
 Hickman
 Physics, Charge, Mass, Work

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