How Do You Calculate the Original Velocity of a Cannonball in a Collision?

[sweedeljoseph]

Homework Statement

A 30 kg cannonball strikes a 150 kg stationary wooden block and embeds itself in the block. The block and cannonball move together at 4 m/s. What was the original velocity of the cannonball?

Homework Equations

m₁v₁=m₂v₂

The Attempt at a Solution

using the equation i gave i got:

30v=150(4)
v=20

the answer my teacher my teacher gave me was 24 m/s. and i was thinking if you add the '4 m/s' to the velocity i just found it would be '24 m/s'. but i don't know if that was right. i was thinking it might be right because they both moved together after the ball hit the wood for 4 m/s. so is that right? or am i supposed to do something else?

thank you!
sweedeljoseph

 

[Physics197]

The mass after the collision would be the masses of both objects added together since they are now considered to be one object moving at the same velocity.

m1V1 + m2V2 (equals 0) = v'(m1 + m2)

Solve for V1 before

 

[sweedeljoseph]

so i was right? you add the '4 m/s' to the velocity that i found?