Which one of the following is not used to reduce friction

The correct answer is Sand.

To understand why, let's first look at what friction is and what it means to "reduce" it.

What is Friction?
Friction is a force that opposes the relative motion between two surfaces in contact. In many machines and mechanical systems, friction causes wear, heat, and energy loss — so we often want to reduce it.

Now let's examine each option:

1. Oil (Lubricant)
Oil is a classic lubricant. When applied between two surfaces, it forms a thin film that separates them, preventing direct contact and significantly reducing friction. This is why engines and machines use oil.

2. Ball Bearings
Ball bearings replace sliding friction with rolling friction. Since rolling friction is much smaller than sliding friction, ball bearings are widely used in wheels, motors, and rotating machinery to reduce friction effectively.

3. Sand ✗ (Does NOT reduce friction)
Sand actually increases friction. Sand particles are rough and act as an abrasive material between surfaces. This is precisely why sand is sprinkled on icy or slippery roads — to give vehicles better grip by increasing friction and preventing skidding. It works in the opposite direction — it is a friction-increasing agent, not a friction-reducing one.

4. Graphite
Graphite is a dry lubricant. Its layered atomic structure allows layers to slide over each other easily, making it an excellent material for reducing friction. It is used in locks, bearings, and other mechanical parts where oil-based lubricants are not suitable.

Summary Table:

Therefore, Sand is the odd one out — it is not used to reduce friction. Instead, it is used in situations where more friction is needed, such as on slippery roads and rail tracks.