Question
A block is submerged in a container of water and remains at rest with the only forces exerted on it being gravity and forces exerted by the water. The force exerted by the water on the bottom of the block is greater than the force exerted by the water on the top of the block. Which of the following is an explanation of the microscopic cause of the difference in the forces?\
A The block exerts a downward force on the water equal to the block’s weight.
B The net force exerted by the water on the block is equal in magnitude and opposite in direction to the force the block exerts on the water, as required by Newton’s third law.
C The molecules of water hitting the bottom of the block have a greater average speed than those hitting the top of the block, and thus they exert a greater force.
D The contact force between the water and the bottom of the block is fundamentally due to interatomic electric forces.
▶️Answer/Explanation
Ans:C
This is a description that correctly indicates the microscopic difference in the interactions at the two surfaces.
Question
Four cubes with equal volumes are at rest in a container of water, as shown in the figure. A student claims that cube D is experiencing a greater number of water molecule collisions per second with its bottom surface than the other cubes. Which of the following indicates whether or not the student is correct and provides evidence?
A The student is correct, because the bottom surface of cube D is deepest, which means the pressure is greatest.
B The student is correct, because cube D is the most massive cube and therefore exerts the largest gravitational forces on the water molecules.
C The student is incorrect, because cube C is the furthest out of the water, so the pressure on the bottom of its surface is greatest.
D The student is incorrect, because the cubes all have the same volume, so they experience the same number of collisions per second.
▶️Answer/Explanation
Ans:A
The macroscopic pressure is caused by the net impulse exerted on the surface by collisions with water molecules. Since pressure increases with depth, so does the rate of collisions.
Question
Two metal cylinders of equal density and volume are submerged in water and remain at rest with their top surfaces at the same height, as shown. Cylinder A has a smaller height and a larger cross-sectional area than cylinder B. Which of the following explains why the force exerted on the top of cylinder A is greater than the force exerted on the top of cylinder B?
A The water molecules striking the bottom surface of cylinder A have a greater average speed than the water molecules striking the bottom surface of cylinder B, so the force on the top of cylinder A must be greater to compensate.
B The water molecules striking the top surface of cylinder A have the same average speed as the water molecules striking the top surface of cylinder B, but more water molecules strike the top surface of cylinder A than the top surface of cylinder B .
C The difference in the average speed of the water molecules striking the top surface and bottom surface of cylinder A is less than that of cylinder B .
D The difference in the number of the molecules striking the top surface and bottom surface of cylinder A per unit time is less than that of cylinder B .
▶️Answer/Explanation
Ans:B
Since cylinder A has a greater area, more molecules strike its top surface than cylinder B ’s top surface. This results in a larger force even though the average molecular speed is the same.
Question
The small dots in the figure represent the molecules of a solid object, and the dashed lines represent the molecular bonds between them. The large dots represent the molecules of a liquid in which the object floats. Which of the following best describes the force that explains the interaction of the object and the liquid?
A Gravitational attraction between all the molecules present
B An electrical force that repels the electrons of the object’s molecules from the electrons of the liquid’s molecules
C Molecular bonds between the molecules of the object and the molecules of liquid
D A strong nuclear force that causes repulsion between the molecules of the solid and the molecules of the liquid
▶️Answer/Explanation
Ans:B
The individual molecules of the liquid do not pass through the solid, and the individual molecules of the solid do not pass through the liquid, because the outer electrons repel each other.
Question
A plastic bowl is floating in a sink full of water. Which of the following describes the microscopic cause of the buoyant force exerted on the bowl by the water?
A Gravitational attraction between the molecules of the bowl and the molecules of the water
B Electrostatic attraction between the nuclei in the molecules in the bowl and the electrons in the molecules of the water
C Electrostatic repulsion between the nuclei in the molecules in the bowl and the nuclei in the molecules of the water
D Electrostatic repulsion between the electrons in the molecules in the bowl and the electrons in the molecules of the water
▶️Answer/Explanation
Ans:D
Contact forces are due to interatomic electric forces. The electrons of the molecules on the surface of the materials in contact repel each other.