A book is lying on the horizontal table top. If we tilt the table a little, then the book starts down slowly. This happens because :

Options

A. sliding friction is greater than static friction
B. sliding friction is less than force of gravity
C. static friction is greater than sliding friction
D. force of gravity is less than sliding friction

Answer

To solve the question, "A book is lying on the horizontal table top. If we tilt the table a little, then the book starts down slowly. This happens because:", we can break down the explanation into the following steps:

### Step-by-Step Solution:

1. **Understanding the Forces at Play**:
- When the table is horizontal, the book remains stationary due to the balance of forces acting on it. The primary forces are the gravitational force pulling the book downwards and the frictional force acting against the motion of the book.

**Hint**: Identify the forces acting on the book when it is on a flat surface.

2. **Tilting the Table**:
- When the table is tilted, the gravitational force can be resolved into two components: one acting perpendicular to the surface (which is balanced by the normal force) and the other acting parallel to the surface (which tries to slide the book down).

**Hint**: Think about how the direction of the gravitational force changes when the table is tilted.

3. **Role of Friction**:
- The frictional force acting on the book is static friction initially, which prevents the book from sliding. However, as the angle of tilt increases, static friction reaches its maximum limit and cannot hold the book anymore.

**Hint**: Consider the difference between static friction and sliding (kinetic) friction.

4. **Sliding Down**:
- Once the static friction is overcome, the book starts to slide down slowly. This indicates that the force of gravity acting parallel to the surface is greater than the maximum static friction force.

**Hint**: Relate the sliding motion of the book to the forces acting on it.

5. **Conclusion**:
- The book slides down slowly because the force of gravity acting down the slope is greater than the opposing force of friction. Therefore, the correct explanation is that "sliding friction is less than the force of gravity."

**Hint**: Summarize the relationship between the forces to arrive at the correct answer.

### Final Answer:
The correct explanation for why the book starts to slide down slowly when the table is tilted is that **sliding friction is less than the force of gravity**.