momentum
Momentum is the "oomph" a moving object carries — how much motion it has, packed into one number. It depends on two things: how heavy something is, and how fast it's going. A heavy truck and a bicycle can roll at the same speed, but the truck has far more momentum, which is exactly why it's so much harder to stop.
Physicists write it as a simple recipe: p = mv — momentum equals mass times velocity. Double the weight, or double the speed, and you double the momentum. Because velocity has a direction, momentum does too: a ball thrown left and the same ball thrown right carry the same amount, but pointed opposite ways.
The magic is that momentum is conserved: in a collision, the total momentum of everything involved is the same before and after. When two billiard balls click together, whatever motion one loses, the other gains — the books always balance. That single rule lets us predict crashes, rocket launches, and pool shots alike.
Don't confuse momentum with inertia or kinetic energy. Inertia is just an object's reluctance to change motion (tied to mass alone); kinetic energy grows with the square of speed. Momentum sits in between — it scales straight with both mass and speed, and crucially, it has a direction.