Calculate the impulse exerted after an inelastic collision

[marsupial]

Homework Statement

A block A of mass m traveling with speed v collides head-on with a stationary body B of mass 2m. The two bodies stick together. Calculate the impulse exerted by body B on body A during the collision.

Homework Equations

$J = p_{final} - p_{initial}$
$p_{initial} = p_{final}$

The Attempt at a Solution

I am not sure if this is correct. First I considered the system of the two blocks to calculate v':
mv + 0 = 3mv' -> v' = v/3

Then I calculated the initial and final momentum of block B on A using v' for final velocity:
$p_f - p_i = 2mv/3 - 0 = 2mv/3$

But I don't feel confident in the answer.

 

[TSny]

But I don't feel confident in the answer.

Newton's third law of motion implies that the blocks should experience "equal but opposite" impulses during the collision. How does the impulse delivered to A compare to the impulse delivered to B in your solution?

Does that boost confidence?

 

[marsupial]

It's -2mv/3, so it appears to be consistent. Thank you. Physics induces a great deal of uncertainty in me!

 

[TSny]

Physics induces a great deal of uncertainty in me!

You're not alone. (I think Heisenberg said something similar.)