50N.
The magnitude of the reaction to any force is equal to the magnitude of the first force. (See Newton #3.)
The magnitude of the gravitational force exerted by ball A on ball B is equal to 36.31 × 10⁻¹¹ N.
<h3>What is gravitational force?</h3>
Newton’s law of gravitation can be defined as every particle of matter in the universe attracting every other particle with mass with a certain force. The magnitude of gravitational force has a direct relationship with their masses.
Therefore, the gravitational force between two bodies can be written as:

Where G denotes the universal gravitational constant.
Given, the mass of the Ball A = Ball B = 7 Kg
The separation between the two balls, r = 3 m
The magnitude of the gravitational force applied by ball A on ball B can be determined as:

F = 36.31 × 10⁻¹¹ N
Therefore, the gravitational force is equal to 36.31 × 10⁻¹¹ N.
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We have that for the Question "A 2kg book is held against a vertical wall. The <em>coefficient </em>of friction is 0.45. What is the minimum force that must be applied on the <em>book</em>, perpendicular to the wall, to prevent the book from slipping down the wal" it can be said that the minimum force that must be applied on the <em>book is</em>
From the question we are told
A 2kg book is held against a vertical wall. The <em>coefficient </em>of friction is 0.45. What is the minimum force that must be applied on the <em>book</em>, perpendicular to the wall, to prevent the book from slipping down the wal
Generally the equation for the Force is mathematically given as

F=44N
Therefore
the minimum force that must be applied on the <em>book is</em>
F=44N
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<span>standard unit of resistance is OHM</span>