I am trying to solve a physic problem involving the velocity of earth and primitive earth. I have solved it using m1v1+m2v2=(m1+m2)v2 for inelastic equation and get a velocity of 29.65 km/s. Which I think is incorrect since the earth's rotational velocity today is 30 km/s. But I can't figure out what I am doing wrong. Any help on how to correctly solve this problem would be greatly appreciated. Problem: The leading theory for the origin of the Moon suggests that a large asteroid the size of Mars struck the Earth. Some of the asteroid material and some of the terrestrial material combined to form the Moon. Assume the combined mass today of the Earth and Moon equals the combined mass of the primitive Earth and the asteroid. Also assume that the asteroid struck the Earth at about 10 km/s. Assume the collision was inelastic. How much did the Earth's orbital speed change as a result of the collision Present Earth - Mass = 5.97 x 10^24 Velocity = 30 km/s Moon Mass = .0735 x 10^24 Velocity = 1 km/s Need velocity of primitive earth Thank you for any assistance in solving this problem.