Answer:
0.017 N
Explanation:
The relevant relation is ...
F = GMm/r²
where G is the universal gravitational constant, 6.67408 × 10^-11 m^3·kg^-1·s^-2, M and m are the masses of the objects, and r is the distance between them.
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Filling in the given numbers, we find the force to be ...
F = (6.67408 × 10^-11 m^3·kg^-1·s^-2)(8.7 × 10^20 kg)(77 kg)/(1.6 × 10^7 m)^2
where m in this expression is the unit "meters".
F = 6.67408 · 8.7 · 77/2.56 × 10^(-11 +20 -2·7) N ≈ 0.017 N
The asteroid exerts a force of about 0.017 N on Sally.
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Additional comment
That's about 0.000023 times the force of Earth's gravity.
