2008 AP Physics B Free Response MISTAKE?

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2008 AP Physics B Free Response MISTAKE?

Homework Statement

Question number two can be found here...
http://apcentral.collegeboard.com/apc/public/repository/ap08_physics_b_frq.pdf

Homework Equations

The Attempt at a Solution

Ok I was working out question number two and was doing (e). How come when they solved it in the rubric

http://www.collegeboard.com/prod_downloads/ap/students/physics/ap08_physics_b_sgs_rev.pdf

they solved it this way

.5 (mass of block b) v^2 = .5 kx^2

how come they only included the mass of block b and not the mass of the whole system?

The wording of the question states specifically

In a new situation, the blocks and spring are moving together...

this would allow one to conclude that the whole system was moving which would mean block A and block B but yet they only include the mass of block b in their calculation...

Can you please explain why they chose only the mass of B in this calculation and didn't include the other block thanks!

 

[cepheid]

The question says that block A hits the wall and sticks there. If it stops, all of its kinetic energy is gone. Because block B is still moving and block A has stopped, the spring will begin to compress. All of the kinetic energy of the still moving block B will be tranformed into elastic potential energy of the spring. This is the conservation of energy equation that has been set up in your "rubric."

 

[rl.bhat]

After impact block A comes to rest. But due to dynamic inertia, block B continues to move with the velocity V. Before impact the spring is in un stretched position. Kinetic energy of the block B is converted into potential energy of the spring by compressing it.