Because the formulas for Coulomb's law and Newton's law of gravity have the same inverse-square law dependence on distance, a formula analogous to the formula for Gauss's law can be found for gravity. The gravitational ficld ĝ at a location is the force per unit mass on a test mass mo placed at that location. (Then, for a point mass m at the origin, the gravitational ficld g at some position 7' is j = – (Gym/r²) î.) (a) Compute the flux of the gravitational ficld through a sphcrical surface of radius r, and verify that the gravitational analog of Gauss's law is dÃ= -4TGNMencl· (b) What do you think is the differential form of Gauss's law for gravitation? (c) Determine the gravitational ficld, g, a distance r from the center of the Earth (where r < RE, where RE is the radius of the Earth), assuming that the Earth's density is uniform. Express your answer in terms of GN, the mass of the Earth, МЕ, and Rg.

Classical Dynamics of Particles and Systems
5th Edition
ISBN:9780534408961
Author:Stephen T. Thornton, Jerry B. Marion
Publisher:Stephen T. Thornton, Jerry B. Marion
Chapter10: Motion In A Noninertial Reference Frame
Section: Chapter Questions
Problem 10.13P
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Because the formulas for Coulomb's law and Newton's law of gravity have the same
inverse-square law dependence on distance, a formula analogous to the formula for
Gauss's law can be found for gravity. The gravitational ficld g at a location is the force
per unit mass on a test mass mo placed at that location. (Then, for a point mass m
at the origin, the gravitational field g at some position is g = - (GNm/r²)î.)
(a) Compute the flux of the gravitational ficld through a spherical surface of radius
r, and verify that the gravitational analog of Gauss's law is
*, = f i
dà = -4TGNMenel·
(b) What do you think is the differential form of Gauss's law for gravitation?
(c) Determine the gravitational field, g, a distance r from the center of the Earth
(where r < RE, where RE is the radius of the Earth), assuming that the Earth's
density is uniform. Express your answer in terms of GN, the mass of the Earth,
МЕ, and Rp-
Using the results of the problem above, suppose that you were to drill a tunnel all
the way through the Earth, and then drop a ball down the tunnel. Neglecting any
friction/air resistance/cffects due to the rotation of the Earth, cxplain what would
happen to the ball and how long you would have to wait (in minutes!) to get it back!
Hint: consider the force on the ball when it is inside the Earth. Does this form of the
force look familiar? This same method also works for a small charge falling through an
oppositely-charged big ball of charge, of course.
Transcribed Image Text:Because the formulas for Coulomb's law and Newton's law of gravity have the same inverse-square law dependence on distance, a formula analogous to the formula for Gauss's law can be found for gravity. The gravitational ficld g at a location is the force per unit mass on a test mass mo placed at that location. (Then, for a point mass m at the origin, the gravitational field g at some position is g = - (GNm/r²)î.) (a) Compute the flux of the gravitational ficld through a spherical surface of radius r, and verify that the gravitational analog of Gauss's law is *, = f i dà = -4TGNMenel· (b) What do you think is the differential form of Gauss's law for gravitation? (c) Determine the gravitational field, g, a distance r from the center of the Earth (where r < RE, where RE is the radius of the Earth), assuming that the Earth's density is uniform. Express your answer in terms of GN, the mass of the Earth, МЕ, and Rp- Using the results of the problem above, suppose that you were to drill a tunnel all the way through the Earth, and then drop a ball down the tunnel. Neglecting any friction/air resistance/cffects due to the rotation of the Earth, cxplain what would happen to the ball and how long you would have to wait (in minutes!) to get it back! Hint: consider the force on the ball when it is inside the Earth. Does this form of the force look familiar? This same method also works for a small charge falling through an oppositely-charged big ball of charge, of course.
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