That's **exactly** what I say, but Kaplan says this:

answer:

a) 0

explanation:

"An elastic collision means the kinetic energy of the system before the collision equals the kinetic energy after the collision. For all types of collision, both elastic and inelastic, momentum is conserved.

Consider the answer choices. Answer choice A says the velocity of the incoming ball is zero after the collision. The initial momentum is mv, so in order for momentum to be conserved, we require mv2 = mv, where v2 is the velocity of the other ball. This means v2=v.

Notice that this will also guarantee conservation of kinetic energy since the initial kinetic energy is 1/2 mv^2 and the final kinetic energy will now be 1/2mv2^2 = 1/2mv^2. Thus zero final velocity for the incoming ball results in conservation of momentum and conservation of kinetic energy, which means it's the correct value."

-WTF? It looks to me like the person who did the test questions did this problem by starting with the first choice, 0, which does work, but so could other answers. And the explanation is crappy. They set only half of each of the equations - momentum and KE - equal to half of the variables on the other side. Where are the other parts of the eqn???

I'm not trying to be a brat, I'm really annoyed bc I looked over this question a million times trying to see how 0 could be the only answer. I heard the OAT tests a lot on concepts, not using numbers, so if you can't trust Kaplan's test answers that really sucks. Not to mention, I paid for these tests. So, even if their answer is right, their crappy explanation could be much better.