Part a: what is president obama's purpose in this speech? to give his opinion on the trayvon martin case and condemn the jury’s verdict of “not guilty” to contextualize the trayvon martin case and reconcile people's reactions to the ruling to talk about his own experiences with prejudice and how adversity gave him opportunities to grow to contextualize the trayvon martin case and condemn the jury’s verdict of “not guilty”

The correct answer is: to contextualize the Trayvon Martin case and condemn the jury’s verdict of “not guilty”, because President Obama uses his own experiences as an African American man to show the stigmatization to which people of color are subjected simply because of the color of their skin, assuming they are delinquents and / or dangerous.  And therefore, expressing his disagreement with the jury’s verdict taking into account Trayvon Martin wasn’t armed and didn’t represent any real threat to the community, besides being a member of the community itself.  

"the sandy walls seemed to collapse in on themselves"

and

"the bloodshot waters seemed to have no bottom"

explanation:

it refers to the definition of full-contact sports, where the condition is that the sport must have athletes must "purposely collide with each other