Today was one of the hardest lessons of the year: calculating gravity. Students practiced using Newton’s equation (g=m/r2) to calculate the gravity of various planets in the solar system. They plugged in the mass (in kilograms) and divided by the radius (in meters) squared. Then they got their answer, but it was in some very strange units. So they multiplied by Newton’s gravitational constant (6.67×10-11) in order to convert the units into the more useful form: m/s2. It was difficult day for students. But after a full period of practice, most of them were getting it.