1. Gravity is measured by the acceleration that it gives to freely falling objects.
2. There are two main types of friction, static friction and kinetic friction. Static friction operates between two surfaces that aren't moving relative to each other, while kinetic friction acts between objects in motion.
3. The difference between mass and weight is that mass is the amount of matter in a material, while weight is a measure of how the force of gravity acts upon that mass.
4. Free fall is defined as the motion of an object where gravity is the only force acting upon it
