Conservation of linear momentum
During a direct impact, the two bodies exchange momentum. Each body can gain or lose momentum, but the total signed momentum of the chosen system remains constant under the collision model.
A 4 kg particle moving right at 6 m s⁻¹ hits a stationary 2 kg particle. After the impact, the first particle moves right at 1.5 m s⁻¹. Find the second particle's velocity.
The positive result means 9 m s⁻¹ to the right.
In one-dimensional motion, momentum conservation supplies one number equation. It can determine one unknown directly. If two quantities are unknown, the question must provide another relationship, such as one body being stationary, both bodies having equal speed, or one velocity being a multiple of another.
A 3 kg particle moving right at 5 m s⁻¹ collides with a stationary 2 kg particle. Afterwards the first moves left with speed and the second moves right with speed .