Which of the following is not correct?

Options

A. If we drop a 10 lb iron ball and 1 lb aluminium ball from the top of a high platform inside a mine, then both will reach the floor at the same time.
B. The gravitational force of attraction between the two bodies increases when their masses are increased and distance is reduced.
C. We do not observe small objects moving towards each other because frictional force balances the gravitational pull.
D. When a body is thrown up, the acceleration due to gravity at the topmost point is zero because velocity of the body is zero.

Explanation

To solve the question of which statement is not correct, we will analyze each statement provided in the options.

### Step-by-Step Solution:

1. **Analyze Statement 1**:
- Statement: If we drop a 10LB iron ball and a 1LB aluminum ball from the same height, they will reach the floor at the same time.
- Explanation: According to the laws of physics, specifically Galileo's principle of falling bodies, in the absence of air resistance, all objects fall at the same rate regardless of their mass. Since both balls are dropped from the same height, they will indeed reach the ground at the same time.
- Conclusion: This statement is **correct**.

2. **Analyze Statement 2**:
- Statement: The gravitational force of attraction between two bodies increases when their masses are increased and distance is reduced.
- Explanation: This statement is based on Newton's law of universal gravitation, which states that the gravitational force (F) between two masses (m1 and m2) is directly proportional to the product of their masses and inversely proportional to the square of the distance (r) between them: \( F = \frac{G \cdot m1 \cdot m2}{r^2} \). Thus, increasing the masses or decreasing the distance will indeed increase the gravitational force.
- Conclusion: This statement is **correct**.

3. **Analyze Statement 3**:
- Statement: We do not observe small objects moving towards each other because the frictional force balances the gravitational pull.
- Explanation: While it is true that small objects experience gravitational attraction, the force is often too weak to overcome frictional forces present in everyday situations. Thus, they do not visibly move towards each other.
- Conclusion: This statement is **correct**.

4. **Analyze Statement 4**:
- Statement: When a body is thrown up, the acceleration due to gravity at the topmost point is zero because the velocity of the body is zero.
- Explanation: At the highest point of the trajectory, the velocity of the object is indeed zero, but the acceleration due to gravity (g) is not zero. The acceleration due to gravity is always directed downwards and has a constant value near the Earth's surface, regardless of the object's velocity. Therefore, at the highest point, while the velocity is zero, the acceleration due to gravity is still present and acts on the object.
- Conclusion: This statement is **incorrect**.

### Final Answer:
The statement that is not correct is **Statement 4**.