Question

A spherical rock weighs $1000N$ on a certain planet. The planet is a billion times as massive as the rock and has a radius that is a billion times as great as that of the rock's radius.
How much force does the rock exert on the planet?

• A. A thousand-billion Newtons
• B. A thousand-billion-billion Newtons
• C. A thousand billionths of a Newton
• D. A thousand billion-billionths of a Newton
• E. A thousand Newtons

Answer 1

Answer: (E)

A thousand Newtons

According to Newton's 3rd law of motion, every action has equal and opposite reaction.

Thus the force exerted by rock on the planet is equal to the force exerted by the planet on the rock i.e $1000$ N