Two bodies move towards each other. The mass of the first body is 2 kg, the mass of the second body is 0.1 kg.

Two bodies move towards each other. The mass of the first body is 2 kg, the mass of the second body is 0.1 kg. The speed of the first body is 5 m / s and the speed of the second is 15 m / s. The collision is elastic. The speed of the first after impact is 4 m / s. You need to find the speed of the second after the collision

M1v01 + m1v02 = m2v1 + m2v2 – momentum conservation law.
In projections on the X axis, you can write:
m1v01 – m1v02 = m2v1 + m2v2;
2 * 5 – 0.1 * 15 = 2 * 4 + 0.1 * v2;
10 – 1.5 = 8 + 0.1 * v2;
10 – 1.5 = 8 + 0.1 * v2;
0.5 = 0.1 * v2;
v2 = 5 m / s
Answer: 5 m / s