# An exoplanet, 0.01% the mass of the Sun, orbits a star 2 times the mass of the Sun at a distance of 1AU = 1.5x10^8 km. Using Newton's Law of Universal Gravitation, determine the force between the exoplanet and the star. Mass of Sun = 2x10^30kg.

This question is not particularly tricky as long as it is read carefully.
First, calculate the masses of the Star and the Exoplanet. Double the mass to find the star mass (M). To find the planet mass, multiply by 0.01/100 = 1x10-4. Mass of Star: m1 = 2 x 2x1030 = 4x1030kg Mass of Exoplanet: m2 = 1x10-4 x 2x1030 = 2x1026kg
Notice that the distance was given in km. You must convert to metres as the equation will only give the correct answer if Standard Units (SI units) are used. Therefore:
r = 1.5x1011m
Now all that's left to do it substitute into the equation:
F = Gm1m2/r2
Where:G = Gravitational Constant (from data sheet) = 6.67x10-11 m3 kg-1 s-2and m1, m2 and r are as they are above.
F = ((6.67x10-11)(4x1030)(2x1026))/((1.5x1011)2)
F = 2.372x1024NF = 2.4x1024N