Math and physics › Physics of motion
Forces: gravity, friction and drag
Know first: Motion: velocity, acceleration and the time step, Dot product: how much two arrows point the same way, Angles, sine and cosine: a point on a circle
A ball falls, a sled coasts to a stop, a boat slows down in the water, and a skydiver stops speeding up. One idea runs through all of it: a force changes velocity, and mass resists the change. That idea explains gravity, the push of the ground, friction and drag, and terminal velocity. It also explains the difference between a shove and a hit, and why you push a door at the handle.
Why this matters
The lesson on motion gave you acceleration as a ready number: gravity was 9.8 m/s² downward. Where does that number come from? Why does a heavy crate barely move when you kick it, while a light one flies away? A physics engine works with forces. You give it mass, friction, drag and pushes, and on every step it adds up the forces and changes the velocity. When a crate slides as if on ice, or a grenade flies off to the Moon, the cause is usually one of those settings.