Melinda was surprised at the Macnair girl's almost flawless explanation, but years of working with social deviants had taught her not to let anything change show on her face.
"An adequate summation, Miss Macnair," she agreed. "Ten points to Slytherin, since I'm sure you didn't learn that in any Hogwarts classroom."
Having made her way back to the front of the classroom, Melinda conjured an image all the students ought to have been familiar with. The solar system. The planets wheeled around the room until the Earth sat spinning slowly on her desk.
"As you should know from your Astronomy classes," Melinda began, "gravity is the force that keeps everything in the universe from flying off wherever it likes. The force of gravity on Earth is 9.8 metres per second, squared. Of course, that is an approximation, but it is the closest we need for most practical uses."
The words Acceleration due to gravity on the blackboard erased themselves and were replaced by 9.8 m/s2
"What does this mean about using the levitation charm somewhere other than the Earth? On the moon, for example?" Melinda asked.