Question

A 20kg cannonball is fired into a stationary 500kg boulder at on the edge of a 125m high cliff. If the cannonball remains in the boulder and together they land 140m horizontally from the edge of the cliff, what was the initial speed of the boulder?

Answer 1

Remember that this type of problem can be solved by applying the conservation of the linear amount of movement.
1: cannonball
2: boulder
m1v1i + m2v2i = m1v1f + m2v2f
v1f = v2f = vf (they fall together)
However, in this case we have:
"cannonball is fired into a stationary 500kg boulder"
Thus
v2i = 0
answer:
the initial speed of the boulder was 0 m / s (stationary 500kg boulder)