In a household cream separator, fat (cream) is separated from milk mainly due to the action of which of the following forces?

Introduction / Context:
This question relates to mechanics and everyday applications of circular motion. Cream separators are commonly used in dairies and rural homes to separate lighter fat from heavier skim milk. The device spins milk rapidly, causing the components to move differently according to their densities. Understanding which force is effectively responsible for this separation helps you connect rotational dynamics with a familiar household process.

Given Data / Assumptions:

• Milk is a mixture of fat globules and watery skim milk.
• The cream separator spins milk at high speed in a circular motion.
• The options mention cohesive, gravitational, centripetal and centrifugal forces.
• We adopt the common rotating frame description where centrifugal force acts outward on rotating masses.

Concept / Approach:
When a liquid mixture like milk is rotated rapidly in a separator, the components experience an outward apparent force in the rotating frame known as centrifugal force. Heavier components (higher density) tend to move outward more strongly and concentrate near the outer wall, while lighter components remain closer to the axis. In the case of milk, the denser skim milk moves outward, while the less dense fat globules concentrate nearer the centre, allowing separation. In an inertial frame, this effect is described by centripetal acceleration and the different radial positions at equilibrium, but in practical and exam language, the separation is usually attributed to centrifugal force. Cohesive forces and gravity play roles but are not the main cause of rapid separation in a rotating cream separator.

Step-by-Step Solution:
Step 1: Recognise that the cream separator bowl or drum rotates at high speed, causing the milk to move in circular paths. Step 2: In the rotating frame, each small element of the liquid experiences an outward centrifugal force proportional to its mass and the square of the rotational speed, acting away from the axis. Step 3: Because skim milk is slightly denser than fat, the heavier portion tends to move further outward under this apparent force. Step 4: The lighter fat globules remain closer to the axis, forming a separate layer (cream) that can be collected. Step 5: Therefore, the separation of fat from milk in a cream separator is mainly due to centrifugal force produced by rapid rotation.

Verification / Alternative check:
If the milk were simply left standing under gravity without rotation, cream would eventually rise to the top due to density differences, but this process would be slow. The cream separator speeds up separation dramatically by imposing strong radial acceleration, which is much larger than gravitational acceleration. This confirms that gravitational force alone is not the main driver in the machine. Cohesive forces keep the liquid together but do not separate components. In technical descriptions, the mechanism is consistently described as centrifugal separation.

Why Other Options Are Wrong:
Cohesive force refers to attraction between molecules of the same substance and does not selectively separate fat from milk. Gravitational force alone can cause slow separation when milk stands still, but the cream separator specifically uses rapid rotation to enhance the process. Centripetal force is the inward force required to maintain circular motion; in the rotating frame, the separation is described more directly by the outward centrifugal effect on components of different densities.

Common Pitfalls:
Some students confuse centripetal and centrifugal forces, forgetting that centripetal is an inward real force while centrifugal is an outward apparent force in rotating frames. Exam questions about cream separators and centrifuges typically use the word centrifugal because it directly describes the outward tendency that leads to separation. Remembering this association between centrifuges, cream separators and centrifugal force will help you answer such questions quickly and accurately.

Final Answer:
Fat is separated from milk in a cream separator mainly due to centrifugal force in rapid rotation.