# 2008 AP Physics B Free Response MISTAKE?

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In summary, the question involves a situation where block A hits a wall and stops, while block B is still moving and causes the spring to compress. The conservation of energy equation set up in the rubric only includes the mass of block B because all of its kinetic energy is converted into potential energy of the spring. This is due to dynamic inertia, where block B continues to move even after block A has stopped. Therefore, the mass of block A is not included in the calculation.
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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

## 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

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!

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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."

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.

## 1. What was the mistake in the 2008 AP Physics B free response?

The mistake in the 2008 AP Physics B free response was an error in the calculation of the torque for one of the problems. This error caused the final answer to be incorrect.

## 2. How did the mistake impact the scoring of the exam?

The mistake did not impact the scoring of the exam. The College Board, which administers the AP exams, has a policy in place to ensure that students are not penalized for mistakes made by the exam writers. Therefore, students who attempted the problem and used the correct method to solve it were still able to receive full credit.

## 3. How was the mistake discovered?

The mistake was discovered by a group of teachers who were reviewing the exam after it was administered. They noticed that the answer key did not match the correct answer and brought it to the attention of the College Board.

## 4. Was the mistake corrected for future exams?

Yes, the mistake was corrected for future exams. The College Board issued a statement acknowledging the error and stated that it would not be included in future exams. They also provided a corrected version of the exam for students to use as practice.

## 5. Has there been any other major mistakes in past AP Physics B exams?

Yes, there have been a few other major mistakes in past AP Physics B exams. In 2015, there was a mistake in the wording of a question that caused confusion and a delay in scoring. In 2007, there was a mistake in the answer key for a multiple-choice question, which was also corrected for future exams.

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