Two objects collide elastically. The first has a mass of 5.00kg and a velocity of 8.00 m/s. The second has a mass of 2.50 kg and a velocity
of -4.00m/s. If the velocity of object 1 after the collision is -4.00m/s,
what is the velocity of object 2?

Explanation:

General formula

m1 * vi + m2*v2 = m1*v3 + m2*v4

Givens

m1 = 5

m2 = 2.5

v1 = 8 m/s

v2 = - 4 m/s

v3 = -4 m/s

v4 = x

Solution

5 * 8 - 2.5 * 4 = 5 * -4 + 2.5*x

40 - 10 = -20 + 2.5x

30 = - 20 + 2.5x

50 = 2.5x

x = 50/2.5

x = 20 m/s in the positive direction

Remark

Does this answer make sense? It should. You have 5 kg moving 8m/s in the plus direction. That's a lot of momentum. In addition after the collision, it turns around which is more momentum needed.

It has to give up that extra momentum to the 2.5 kg mass.

find the sum of the two horizontal vectors and the sum of the two vertical vectors

isoceles right angled triangle