Select the correct alternative(s).

Options

A. Work done by static friction is always zero
B. Work done by kinetic friction can be positive also
C. Kinetic energy of a system can not be increased without applying any external force on the sytem
D. Work energy theorem is valid in non-inertial frames also

Answer

To solve the question regarding the correct alternatives related to work, power, and energy, let's analyze each statement step by step.

### Step-by-Step Solution:

1. **Statement about Work Done by Static Friction**:
- The statement claims that the work done by static friction is always zero.
- **Analysis**: This is true only in the case of pure rolling motion. In pure rolling, there is no relative motion between the surfaces in contact, hence static friction does not do any work. However, in other scenarios, static friction can do work.
- **Conclusion**: This statement is incorrect in general terms.

2. **Statement about Work Done by Kinetic Friction**:
- The statement suggests that work done by kinetic friction can be positive.
- **Analysis**: Kinetic friction always acts opposite to the direction of displacement. Therefore, the work done by kinetic friction is negative when it opposes the motion of the object.
- **Conclusion**: This statement is incorrect.

3. **Statement about Kinetic Energy Increase without External Force**:
- The statement claims that kinetic energy of a system cannot be increased without applying any external force.
- **Analysis**: This is false. For example, when an object falls under the influence of gravity, its potential energy converts to kinetic energy without any external force being applied.
- **Conclusion**: This statement is incorrect.

4. **Statement about Work-Energy Theorem in Non-Inertial Frames**:
- The statement asserts that the work-energy theorem is valid in non-inertial frames.
- **Analysis**: The work-energy theorem can be applied in non-inertial frames if we consider pseudo forces acting on the objects. This allows us to account for the effects of acceleration in a non-inertial frame.
- **Conclusion**: This statement is correct.

### Final Answer:
- The correct alternative is **D**: Work-energy theorem is valid in non-inertial frames when considering pseudo forces.