A metal sphere is released from rest in oil and begins sinking with increasing speed. Which statement about forces is correct?
- The buoyant force is greater than the weight because the sphere is moving downward.
- The net force is downward, so at that moment. (correct answer)
- Because it started from rest, the net force must be zero initially.
- The sphere sinks because buoyant force depends on mass and is too small for heavy objects.
Explanation: This question tests Newton's laws for objects accelerating in fluids. When the metal sphere sinks with increasing speed, it has downward acceleration, requiring a net downward force by Newton's second law. Since only buoyant force (upward) and weight (downward) act on the sphere, the weight must exceed the buoyant force: mg > FB. The buoyant force depends on the volume of fluid displaced and the fluid's density, not the object's mass or density. Choice A incorrectly reverses the force relationship, claiming FB > mg would cause downward motion. To analyze motion in fluids, first determine the direction of acceleration, then use Newton's second law to find which force must be larger.