Which of the following is not an example of upthrust?

Options

A. Upward force in water
B. Force acting on a stone
C. Upward force in milk
D. Upward force in oil

Answer

To determine which of the following is not an example of upthrust, we first need to understand what upthrust is.

### Step-by-Step Solution:

1. **Definition of Upthrust**: Upthrust, also known as buoyant force, is the upward force exerted by a fluid (liquid or gas) on an object that is submerged or floating in it. This force opposes the weight of the object and is responsible for the object's ability to float.

2. **Understanding the Options**:
- **Option 1: Upward Force on Water**: This is an example of upthrust because water can exert an upward force on objects submerged in it.
- **Option 2: Force Acting on a Stone**: This option is problematic. While a stone can experience forces when placed in a fluid, the force acting on the stone is not an example of upthrust. Upthrust specifically refers to the buoyant force acting on an object due to the fluid, and a stone does not produce this force by itself.
- **Option 3: Upward Force in Milk**: Similar to water, milk is a fluid and can exert an upward force (upthrust) on objects submerged in it.
- **Option 4: Upward Force in Oil**: Oil is also a fluid, and it can exert an upward force on submerged objects, making it an example of upthrust.

3. **Conclusion**: After analyzing all the options, we find that the only option that does not represent an example of upthrust is **Option 2: Force Acting on a Stone**. This is because the force acting on the stone does not originate from the fluid; rather, it is the stone itself that is being acted upon by gravity and not producing upthrust.

### Final Answer:
**Option 2: Force Acting on a Stone** is not an example of upthrust.

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