Calculate the gravitational force of attraction between a satellite of mass $1.4 \mathrm{x}{10}^4 \mathrm{kg}$ orbiting the Earth at a distance of $6800 \mathrm{km}$ from the Earth's centre. (The mass of the Earth is$6.0\times {10}^{24}\mathrm{kg}$).

Calculate the Earth's gravitational field strength at the position of the Moon. 

During the manned Moon landings in the $1960\mathrm{s}$, the command module orbited the Moon in an elliptic orbit with a maximum height of $310 \mathrm{km}$ above the surface of the Moon, whilst the lunar module descended and landed on the Moon's surface. Explain why the potential energy of the command module varied during its orbit. 

During the manned Moon landings in the $1960\mathrm{s}$, the command module orbited the Moon in an elliptic orbit with a maximum height of $310 \mathrm{km}$ above the surface of the Moon, whilst the lunar module descended and landed on the Moon's surface. Calculate the maximum gravitational potential difference between the lunar surface and the position of the command module.