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Problem Set – Inverse Square Law/Gravity & Light Nov. 2011 : The force due to gravity decreases as the square of the distance from the source. The force due to gravity is proportional to the product of the two masses Force = g GMm r2 acceleration = a GM r2 The Inverse Square Law for Light A "standard candle" is a star whose luminosity is known. The brightness of a star depends on its distance. Its brightness decreases as 1/d2. A star of the same luminosity that is twice as far away will be ¼ as bright. Gravity Questions 1. What is the acceleration due to gravity on the surface of a white dwarf? Let the mass of a white dwarf by approximately 1 solar mass, or 2.0 x 1030 kg, and its radius be approximately that of Earth or 6.4 x 106 m. (J63) 2. What is the acceleration due to gravity on the surface of a neutron star? Let the mass of a neutron star be two solar masses or 4 x 1030 kg and its radius be 10 km. (J64) Inverse Square Law If two stars have the same intrinsic luminosity, but star A is ten times further away than star B, how much fainter than star B will star A appear to be? Answer Key – Gravity Problems Gravity Questions 1. 3.2 x 106 m/s2 (J63) 2. 2.7 x 1012 m/s2 (J64)