AP Physics 1 Question 313: Answer and Explanation

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Question: 313

1. Two identical spheres of mass 1 kg are placed 1 m apart from each other. Each sphere pulls on the other with a gravitational force, Fg. If each sphere also holds 1 C of positive charge, then the magnitude of the resulting repulsive electric force is

  • A. 1.82 × 1040Fg
  • B. 1.35 × 1020Fg
  • C. 7.42 × 10-21Fg
  • D. 5.50 × 10-41Fg

Correct Answer: B

Explanation:

B

The force of gravity between two objects is given by Fg = GmM/r2. This comes out to (6.67 × 10-11)(1)(1)/(1)2 = 6.67 × 10-11. Electric force is given by Fe = kqQ/r2. This comes out to (8.99 × 109)(1)(1)/(1)2 = 8.99 × 109. Therefore, in terms of Fg, Fe = [(8.99 × 109)/(6.67 × 10-11)]Fg = (1.35 × 1020)Fg.