Question
The two spheres pictured above have equal densities and are subject only to their mutual gravitational attraction. Which of the following quantities must have the same magnitude for both spheres?
(A) Acceleration (B) Kinetic energy (C) Displacement from the center of mass (D) Gravitational force
▶️Answer/Explanation
Ans:D
Solution: Newton’s third law
Question
A table supports a wooden block placed on the tabletop. Which fundamental force of nature is responsible for this interaction, and why?
(A) The electric force, because the protons in the nuclei of the top atomic layer of the table repel the nuclei in the bottom atomic layer of the wood.
(B) The gravitational force, because by \(F=GMm/r^{2}\), the force of the table on the wood at that close range is sufficient to balance the force of the Earth on the wood.
(C) The electric force, because the outer electrons in the top atomic layer of the table repel the outer electrons in the bottom atomic layer of the wood.
(D) The strong nuclear force, because the protons in the nuclei of the top atomic layer of the table repel the nuclei in the bottom atomic layer of the wood.
▶️Answer/Explanation
Ans:C
Choice B is ridiculous because not only is gravity the weakest of the fundamental forces, it makes no sense that the gravitational force of the maybe 100-kg table is similar to the gravitational force of the \(10^{24}\) -kg Earth in the equation \(F=G\frac{M_{1}M_{2}}{d^{2}}\). Choices A and D are ridiculous because the protons are about \(10^{5}\) (100,000) times smaller than the diameter of an atom—any interactions between atoms have to involve the electrons at the outside of the atoms, not protons far away in the center.