[填空题]
1.Define gravitationfzz+zue9l1o 8f l sq)2al fif ff7-evvno;7eld strength.
2.The Earth's average radius is given as $6.37\times10^3\,km$. Calculate the height above the Earth's surface at which gravitational field strength becomes half of its surface value.
3.Calculate the gravitational force acting on a satellite of mass $2.63\times10^6\,kg$, placed at the height in part (b). State your answer to the appropriate number of significant figures.
4.Using the available information, calculate the mass of Earth. $M_{earth}$ = $10^{24}\,kg$
