Why Momentum? The Big Picture
Imagine trying to stop a slowly rolling bowling ball versus a fast-moving tennis ball. Even though the tennis ball moves faster, the bowling ball is much harder to stop , it has more momentum. Now picture trying to stop a truck moving at the same speed as the bowling ball: even harder still.
Momentum captures both mass and velocity together, giving us a single quantity that describes how difficult it is to change an object's motion. It also turns out to be the key to understanding collisions, explosions, and any situation where forces act over time.
The familiar form is actually a special case of a deeper, more universal law expressed in terms of momentum. Momentum is also a conserved quantity , in an isolated system, the total momentum never changes. These two ideas underpin almost every question in A.2.
Defining Momentum
Momentum: The momentum of an object is the product of its mass and velocity:
Momentum is a vector quantity , it has both magnitude and direction, always pointing in the same direction as the velocity.
Here:
- = momentum
- = mass (kg)
- = velocity (m s⁻¹)
Units of Momentum
The SI unit of momentum is . This is equivalent to (Newton-seconds), since:
Both units ( and ) are equally acceptable in IB exams.
Because momentum is a vector, you must always assign a positive direction before solving problems. Any quantity in the opposite direction gets a negative sign. Draw a quick sketch showing before and after velocities with arrows , this makes it much harder to assign the wrong sign.