Let us assume that our galaxy consists of 2.5 × ${10}^{11}$ stars each of one solar mass. How long will a star at a distance of 50,000 ly from the galactic centre take to complete one revolution? (The diameter of the milky way is ${10}^5$ ly.)
1. 3.55 × 10⁸ years
2. 4.55 × 10⁸ years
3. 3.55 × 10⁷ years
4. 4.55 × 10⁷ years

A satellite orbits the earth at a height of 400 km above the surface. How much energy must be expended to rocket the satellite out of the earth’s gravitational influence? (Mass of the satellite = 200 kg; mass of the earth = 6.0×${10}^{24}$ kg, radius of the earth = 6.4 × ${10}^6$ m, G = 6.67 × ${10}^{-11}$ $N m^2 k{g}^{-2}$)

1. 7.5 × 10⁹ J
2. 4.7 × 10⁹ J
3. 4.4 × 10⁹ J
4. 5.9 × 10⁹ J