Q.Two different unknown masses $A$ and $B$ collide. $A$ is initially at rest when $B$ has a speed $v$. After the collision $B$ has a speed $v/2$ and moves at right angles to its original direction of motion. Find the direction in which $A$ moves after the collision.

Two-Dimensional Collision: The Intuition

Imagine two pool balls on a table. You hit one, it slides across the felt, and then it strikes the other ball at an angle — not dead centre, but off to the side. The balls don't just move along the same straight line after the hit; they scatter in different directions. That's a two-dimensional collision.

In one dimension, everything happens along a line — think of two trains bumping on a track. But in two dimensions, the collision is oblique: the objects approach each other at some angle, and after the collision they fly off in directions that are not along the original line of motion. The key insight is that momentum is a vector, and it is conserved component by component.

The Core Idea: Momentum Conservation in Two Axes

Momentum is a vector — it has both magnitude and direction. When two objects collide, the total momentum vector before the collision equals the total momentum vector after the collision. That single vector equation breaks into two independent scalar equations, one for each perpendicular direction.

We choose two perpendicular axes — typically the $x$-axis and $y$-axis — that lie in the plane of motion. Then:

• The total $x$-component of momentum before the collision equals the total $x$-component after.
• The total $y$-component of momentum before the collision equals the total $y$-component after.

These two equations are completely independent. Nothing that happens along $x$ affects the $y$-momentum conservation, and vice versa. This is the entire mathematical engine of two-dimensional collision analysis.

Here $\vec{u}$ are velocities before collision, $\vec{v}$ are velocities after, and $m$ are masses.

What About Energy?

Momentum is always conserved in any collision (provided no external forces act). Energy is a separate story.

• Elastic collision: Kinetic energy is also conserved. This gives you a third equation (scalar, not vector) that relates the speeds.
• Inelastic collision: Kinetic energy is not conserved — some is lost to heat, sound, or deformation. You only have the two momentum equations.

The Typical Problem Setup

Here is how a standard two-dimensional collision problem looks:

1. Object 1 (mass $m_1$) moves with known velocity $\vec{u}_1$ along the $x$-axis.
2. Object 2 (mass $m_2$) is initially at rest ($\vec{u}_2 = 0$).
3. They collide obliquely. After collision, object 1 moves at an angle $\theta_1$ to the $x$-axis with speed $v_1$, and object 2 moves at an angle $\theta_2$ to the $x$-axis with speed $v_2$.

You are typically asked to find some of these unknowns. The momentum equations give:

The minus sign in the $y$-equation appears because the two objects usually scatter to opposite sides of the $x$-axis — one with positive $y$, the other with negative $y$.

Why Two Axes Are Enough

You might wonder: why only $x$ and $y$? Because the collision happens in a plane — two dimensions. Any vector in that plane can be fully described by its components along two perpendicular axes. There is no third independent direction. So two equations from momentum conservation, plus possibly one from energy conservation, give you up to three equations to solve for unknowns.

A Concrete Example …