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How do you use Newton's Law of Universal Gravitation to find the value for the acceleration of gravity?

The law of force of gravity. Physical equations, formulas and schemes on whiteboard.

Using Newton's Universal Law of Gravitation to find the acceleration of gravity is a matter of comparing that law with Newton's Second Law when finding weight.

Newton's Universal Law of Gravitation for g

\(\large\mathsf{ g = \frac{GM}{r^2} }\)

...where G is the universal gravitational constant of 6.673 x 10-11 Nm2/kg2, M is the mass of the planetary object (i.e. Moon, planet, Sun, etc.) and r is the distance between the centers of the objects.

Much like the law in its original form, using this equation is a matter of substituting what you know into the equation and solving using your algebra skills. Consider the problem: Find the acceleration due to gravity on an astronaut who is 105 km above the surface of a moon near Jupiter. The moon's mass is 4.56 x 1020 kg and its radius is 1.34 x 106 m.

\(\mathsf{ g = \frac{GM}{r^2} }\)

\(\mathsf{ g = \frac{(6.673 \times 10^{-11} \text{ Nm}^2 \text{/kg}^2)(4.56 \times 10^{20} \text{ kg})}{(1.34 \times 10^{6} \text{ m} + 1.05 \times 10^{5} \text{ m})^2} }\)

\(\mathsf{ g = \frac{3.04 \times 10^{10} \text{ Nm}^2 \text{/kg} }{(1.445 \times 10^{6} \text{ m})^2} }\)

\(\mathsf{ g = 0.0146 \text{ m/s}^2}\)

Question

Using this equation, show that the acceleration of gravity on surface of the Earth is 9.81 m/s2 if the mass of the Earth is 5.972 x 1024 kg and the radius is 6.371 x 106 m.

\(\small\mathsf{ g = \frac{GM}{r^2} }\)

\(\small\mathsf{ g = \frac{GM}{r^2} }\)

\(\small\mathsf{ g = \frac{(6.673 \times 10^{-11} \text{ Nm}^2 \text{/kg}^2)(5.972 \times 10^{24} \text{ kg})}{(6.371 \times 10^{6} \text{ m})^2} }\)

\(\small\mathsf{ g = 9.818 \text{ m/s}^2}\)