A `20 N` metal block is suspended by a spring balance. A beaker containing some water is placed on a weighing machine which reads `40 N`. The spring balance is now lowered so that the block gets immersed in the water. The spring balance now reads `16 N`. The reading of the weighing machine will be.

Options

A. 36 N
B. 60 N
C. 44 N
D. 56 N

Explanation

To solve the problem step by step, let's analyze the situation carefully.

### Step 1: Understand the Initial Conditions
Initially, we have:
- A metal block with a weight of \( W_{block} = 20 \, N \) suspended by a spring balance.
- A beaker filled with water placed on a weighing machine, which reads \( W_{beaker} = 40 \, N \).

### Step 2: Analyze the Forces Before Immersion
Before the block is immersed in water, the total weight supported by the weighing machine is the weight of the beaker and the block:
- Total weight = \( W_{beaker} + W_{block} = 40 \, N + 20 \, N = 60 \, N \).

### Step 3: Immersion of the Block
When the block is immersed in the water, the reading on the spring balance changes:
- After immersion, the spring balance reads \( T = 16 \, N \).

### Step 4: Determine the Buoyant Force
The difference in the readings of the spring balance before and after immersion gives us the buoyant force acting on the block:
- Buoyant force \( F_b = W_{block} - T = 20 \, N - 16 \, N = 4 \, N \).

### Step 5: Analyze the Forces After Immersion
After the block is immersed, the forces acting on the system change:
- The spring balance now reads \( 16 \, N \), indicating that the tension in the spring is less due to the buoyant force acting upwards.
- The total weight supported by the weighing machine now includes the weight of the beaker and the effective weight of the block (which is now reduced by the buoyant force).

### Step 6: Calculate the New Reading of the Weighing Machine
The new effective weight of the block when immersed in water is:
- Effective weight of the block \( W_{effective} = W_{block} - F_b = 20 \, N - 4 \, N = 16 \, N \).

Now, the total weight supported by the weighing machine is:
- Total weight = \( W_{beaker} + W_{effective} = 40 \, N + 16 \, N = 56 \, N \).

### Conclusion
Thus, the reading of the weighing machine after the block is immersed in water will be \( 56 \, N \).

### Final Answer
The reading of the weighing machine will be **56 N**.

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