Physics 21 Fall, 2005 Solution to HW-1 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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