You're asking for the formula to calculate momentum, but you need to provide more context for me to give you a specific answer. Here's why:

Momentum depends on both mass and velocity.

* Formula: Momentum (p) = mass (m) × velocity (v)

To calculate momentum for each ball before and after the collision, you need:

1. The initial velocities of each ball before the collision.

2. The final velocities of each ball after the collision.

Here's how you'd apply the formula:

* Before the collision:

* Ball 1: p₁ = m₁ × v₁₁ (where v₁₁ is the initial velocity of ball 1)

* Ball 2: p₂ = m₂ × v₂₁ (where v₂₁ is the initial velocity of ball 2)

* After the collision:

* Ball 1: p₁' = m₁ × v₁₂ (where v₁₂ is the final velocity of ball 1)

* Ball 2: p₂' = m₂ × v₂₂ (where v₂₂ is the final velocity of ball 2)

Important Note: You mentioned the mass of each ball is 0.4 kg. This information is useful, but you need to know the velocities of each ball to calculate the momentum.

Please provide the velocities of the balls before and after the collision, and I can help you calculate the momentum.