A satellite in vacuum

Options

A. is kept in orbit by solar energy
B. previous energy from gravitational field
C. is kept in an orbit by remote control
D. does not require any energy for revolving

Answer

To solve the question regarding a satellite in vacuum, we will analyze the forces acting on the satellite and the nature of its motion.

### Step-by-Step Solution:

1. **Understanding the Forces Acting on the Satellite:**
- A satellite in orbit around a planet experiences two main forces: the gravitational force pulling it towards the planet and the centrifugal force due to its circular motion.

2. **Gravitational Force:**
- The gravitational force \( F_g \) acting on the satellite can be described by Newton's law of gravitation:
\[ F_g = \frac{G \cdot M \cdot m}{r^2} \]
where \( G \) is the gravitational constant, \( M \) is the mass of the planet, \( m \) is the mass of the satellite, and \( r \) is the distance from the center of the planet to the satellite.

3. **Centrifugal Force:**
- The centrifugal force \( F_c \) experienced by the satellite due to its circular motion is given by:
\[ F_c = \frac{m \cdot v^2}{r} \]
where \( v \) is the orbital speed of the satellite.

4. **Equilibrium of Forces:**
- For the satellite to maintain a stable orbit, the gravitational force must equal the centrifugal force:
\[ F_g = F_c \]
This means:
\[ \frac{G \cdot M \cdot m}{r^2} = \frac{m \cdot v^2}{r} \]
- Simplifying this equation leads to:
\[ v^2 = \frac{G \cdot M}{r} \]
- This shows that the satellite's speed is determined by the gravitational pull of the planet and the distance from the planet.

5. **Energy Considerations:**
- Once the satellite is in orbit, it does not require any additional energy to maintain its motion. The forces are balanced, and it continues to revolve due to its initial velocity and the gravitational pull.

6. **Conclusion:**
- Therefore, the correct option is that a satellite in vacuum does not require energy for revolving. This corresponds to option four.

### Final Answer:
The correct option is: **A satellite in vacuum does not require energy for revolving.**