AP Physics 1 Question 164: Answer and Explanation

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Question: 164

4.

Question below refer to the following information:

Two metal balls of equal mass (100 g) are separated by a distance (d). In state 1, shown in the figure, the left ball has a charge of +20 µC, while the right ball has a charge of –20 µC. In state 2, both balls have an identical charge of –40 µC.

Which of the following statements about the magnitudes of the electrostatic and gravitational forces between the two balls is correct?

  • A. The electrostatic force is greater in state 2 than in state 1. The electrostatic force is greater than the gravitational force in state 1, but the gravitational force is greater than the electrostatic force in state 2.
  • B. The electrostatic force is greater in state 1 than in state 2. The electrostatic force is greater than the gravitational force in state 1, but the gravitational force is greater than the electrostatic force in state 2.
  • C. The electrostatic force is greater in state 2 than in state 1. The electrostatic force is greater than the gravitational force in both states.
  • D. The electrostatic force is greater in state 1 than in state 2. The electrostatic force is greater than the gravitational force in both states.

Correct Answer: C

Explanation:

C-The magnitude of the electrostatic force depends on the product of the charges, regardless of sign. In state 2, the charges are both bigger than in state 1, giving a bigger electrostatic force in state 2. Without even doing the calculation, you can recognize that since Newton's gravitational constant G is orders of magnitude less than the coulomb's law constant k, the electrostatic force will be much greater than the gravitational force between two objects in virtually any laboratory situation. The only time these forces become comparable is when objects the size of planets exert gravitational forces.