Choose the correct statement(s).

Options

A. Motion necessarily takes place along the direction of force
B. If no force acts, the body stays at rest
C. A body in motion need not be acted upon by a force
D. A change in speed is always there, whenever a force is applied on a moving body

Explanation

To solve the question of choosing the correct statements regarding motion and force, let's analyze each statement one by one.

### Step-by-Step Solution:

1. **Statement 1: Motion necessarily takes place along the direction of force.**
- **Analysis:** This statement is incorrect. A force can change the velocity of a body, but it does not dictate the direction of motion. For example, if a car is moving at a constant velocity and a force is applied in the opposite direction, the car will slow down but will not stop immediately. Therefore, motion does not necessarily take place along the direction of the force.
- **Conclusion:** This statement is **false**.

2. **Statement 2: If no force acts, the body stays at rest.**
- **Analysis:** This statement is also incorrect. According to Newton's first law of motion, a body will remain in its state of rest or uniform motion unless acted upon by a net external force. If a body is already in motion (like a car moving at a constant velocity), it will continue to move even if no additional force is applied. Thus, a body can be in motion without a force acting on it.
- **Conclusion:** This statement is **false**.

3. **Statement 3: A body in motion need not be acted upon by a force.**
- **Analysis:** This statement is correct. As established in the analysis of the previous statements, a body can maintain its motion (constant velocity) without any force acting on it. This aligns with Newton's first law of motion.
- **Conclusion:** This statement is **true**.

4. **Statement 4: A change in speed is always there whenever a force is applied on a moving body.**
- **Analysis:** This statement is true. When a force is applied to a moving body, it causes an acceleration (or deceleration) depending on the direction of the force relative to the motion. If the force is in the same direction as the motion, the speed increases; if it is in the opposite direction, the speed decreases. Therefore, there is always a change in speed when a force is applied.
- **Conclusion:** This statement is **true**.

### Final Answers:
- **Correct Statements:**
- Statement 3: A body in motion need not be acted upon by a force.
- Statement 4: A change in speed is always there whenever a force is applied on a moving body.