Final answer:
In Robert Heinlein's The Moon is a Harsh Mistress, the colonial inhabitants of the Moon threaten to launch rocks down onto Earth if they are not given independence (or at least representation). Assuming a gun could launch a rock of mass m at twice the lunar escape speed, the speed of the rock as it enters Earth's atmosphere would be 4800 m/s.
Step-by-step explanation:
In Robert Heinlein's The Moon is a Harsh Mistress, the colonial inhabitants of the Moon threaten to launch rocks down onto Earth if they are not given independence (or at least representation). Assuming a gun could launch a rock of mass m at twice the lunar escape speed, we can calculate the speed of the rock as it enters Earth's atmosphere.
Based on the information given, the escape velocity of the Moon is 2.4 km/s (or 2400 m/s). If the rock is launched at twice the escape speed, its velocity would be 2 × 2400 = 4800 m/s. This would be the speed of the rock as it enters Earth's atmosphere.