Fly-wheel is used with an engine because it

Options

A. Increases the efficiency of engine
B. Converts linear motion into rotatory motion
C. Is computsory by law
D. Makes the motion smooth and cuts down the vibrations

Answer

### Step-by-Step Solution:

1. **Understanding the Function of a Flywheel**:
A flywheel is a mechanical device that stores rotational energy. It is typically used in engines to maintain a consistent speed and smooth out the delivery of power.

2. **Analyzing the Options**:
- **Option A**: "It increases the efficiency of the engine."
- While a flywheel can help in maintaining efficiency indirectly by providing consistent power, its primary function is not to increase efficiency directly.
- **Option B**: "Converts linear motion into rotatory motion."
- This is not a primary function of a flywheel. Flywheels are primarily used to store rotational energy, not to convert motion types.
- **Option C**: "It is compulsory by law."
- This is incorrect; there is no legal requirement for a flywheel in engines.
- **Option D**: "Makes the motion smooth and cuts down the vibrations."
- This option accurately describes the main function of a flywheel. It helps smooth out the power delivery from the engine and reduces fluctuations in speed, which in turn minimizes vibrations.

3. **Conclusion**:
Based on the analysis, the correct answer is **Option D**: "Makes the motion smooth and cuts down the vibrations."