When a satellite in a circular orbit around the earth enters the atmospheric region, it encounters small air resistance to its motion. Then

Options

A. its kinetic energy increases.
B. its kinetic energy decreases
C. its angular momentum about the earth decreases.
D. its period of revolution around the earth increases

Explanation

To solve the problem regarding the behavior of a satellite in a circular orbit around the Earth when it enters the atmospheric region, we can break it down into the following steps:

### Step-by-Step Solution:

1. **Understanding the Initial Conditions**:
- A satellite is in a stable circular orbit around the Earth, which means it has a certain velocity and is experiencing gravitational force acting towards the center of the Earth.

**Hint**: Remember that a satellite's motion is governed by gravitational forces and its velocity is balanced by the centripetal force.

2. **Entering the Atmospheric Region**:
- As the satellite enters the atmosphere, it begins to encounter air resistance (drag). This drag force opposes the motion of the satellite.

**Hint**: Consider how air resistance affects the motion of objects moving through the atmosphere.

3. **Effect of Air Resistance on Kinetic Energy**:
- The satellite experiences a gravitational pull towards the Earth, which causes it to accelerate. Although air resistance does work against the satellite, the gravitational force is stronger, leading to an increase in the satellite's speed (kinetic energy).

**Hint**: Think about the relationship between gravitational force and kinetic energy when an object moves closer to the Earth.

4. **Change in Potential Energy**:
- As the satellite descends into the atmosphere, its height decreases, which means its gravitational potential energy decreases.

**Hint**: Remember that potential energy is given by the formula \( U = -\frac{GMm}{r} \), where \( r \) is the distance from the center of the Earth.

5. **Total Mechanical Energy**:
- The total mechanical energy (sum of kinetic and potential energy) is not conserved due to the work done by air resistance. However, the decrease in potential energy is greater than the work done against air resistance, leading to an overall increase in kinetic energy.

**Hint**: Consider how energy conservation principles apply when non-conservative forces (like air resistance) are present.

6. **Angular Momentum Considerations**:
- The satellite's angular momentum about the Earth is affected by air drag. The drag force reduces the angular momentum as it acts in the opposite direction to the satellite's motion.

**Hint**: Recall that angular momentum \( L \) is given by \( L = mvr \) and how external torques can affect it.

7. **Conclusion**:
- Therefore, as the satellite enters the atmosphere, its kinetic energy increases, its potential energy decreases, and its angular momentum decreases due to air drag.

### Final Answer:
- Kinetic Energy: Increases
- Potential Energy: Decreases
- Angular Momentum: Decreases