Given a scientific passage and an answer, generate a question for the given answer.
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Question: Passage: Figure 6.11 The frictional force supplies the centripetal force and is numerically equal to it. Centripetal force is perpendicular to velocity and causes uniform circular motion. The larger the F c , the smaller the radius of curvature r and the sharper the curve. The second curve has the same v , but a larger. Answer: centripetal force

Answer: What force is perpendicular to velocity and causes uniform circular motion?

Question: Passage: A metallic bond is the force of attraction between a positive metal ion and the valence electrons that surround itboth its own valence electrons and those of other ions of the same metal. The ions and electrons form a lattice-like structure. Only metals, such as the copper pictured in the Figure below , form metallic bonds. Answer: metallic bond

Answer: What is the force of attraction between a positive metal ion and valence electrons?

Question: Passage: The iris is the colored part of the eye. It controls the size of the pupil. Answer: iris

Answer:
What part of the eye controls the size of the pupil?