What must be true about two objects if heat is flowing between them? 1. the objects must be different temperatures. 2. the objects must be the same temperature. 3. the objects must both be over room temperature. 4. the objects must both be under room temperature.

2. The objects must be the same temperature.  

1968, the United States suffered a national nervous breakdown. And one of the most shocking events during those tempestuous 12 months was the assassination of presidential candidate Robert F. Kennedy just as he was gaining momentum in his bid to follow his martyred brother, John, into the White House.

"Bobby," as his family and friends called him, had just won a crucial victory in the California Democratic primary when the assassin struck on June 5 – 47 years ago today. At 12:50 a.m., as a jubilant but weary Kennedy made his way from a stage at the Ambassador Hotel in Los Angeles, where he had claimed victory in a room filled with cheering supporters, he was gunned down by a 22-year-old Palestinian sympathizer named Sirhan Sirhan. The killer later said he murdered Kennedy because he believed the senator played a key role in oppressing the Palestinian people.

Sirhan was carrying a .22-caliber handgun rolled up in a campaign poster. As the candidate made a last-moment detour through a kitchen, Sirhan shot Kennedy three times, with one sending a lethal bullet to his head. The senator lay immobile for several agonizing minutes, his blood staining the floor, in a horrendous image captured by news photographers and transmitted around the world. Five bystanders were also wounded. Kennedy died the next day. He was 42, even younger than his brother when John F. Kennedy, 46, was slain in November 1963.

The killing, which followed the assassination of civil rights leader Martin Luther King Jr. by only two months, deepened America's self-doubt. Many concluded that violence had become a toxic and permanent virus infecting American society, that something had gone profoundly wrong in the country and that the road to peaceful change had become blocked by madmen, evildoers and fanatics. Optimism, that most American of virtues, plummeted.

"Perhaps if Robert Kennedy's murder hadn't occurred so hard on the heels of Dr. King, the sense of national desperation would not have been so acute," says Rutgers University political scientist Ross Baker. "Here were two men committed to change within the system and who were killed for their effort. The two killings seemed to make a compelling argument to many that the peaceful path was a dead end and that the resort to violence was now acceptable. I trace the most violent phase of this turbulent period in our history to Kennedy's assassination. The hopefulness of the early 1960s was replaced by a pervasive cynicism and a conviction that change was impossible within the bounds of normal politics."

Adds historian Robert Dallek: "RFK's killing was a terrible blow to America's self-image. The idea of American immunity to political assassinations took a terrible blow from the killings of John F. Kennedy, Martin Luther King Jr. and Robert F. Kennedy." And the absence of Robert Kennedy from the presidential campaign had serious consequences. "If RFK had lived, I think he would have won the presidency and we would have seen an earlier exit from Vietnam than occurred in 1975," saving many lives and perhaps reducing the rancor in American life, Dallek says.

Many Americans bemoan the polarization in politics and culture today, but as the Kennedy assassination showed, things were much worse in the late 1960s. "It felt like American society was coming apart at the seams," says political scientist Bill Galston of the Brookings Institution, a former White House adviser to President Bill Clinton.

Bobby Kennedy's assassination was part of a broader picture of a nation unhinged, with ever-worsening divisions based on race, wealth, age, culture, gender, values and ideology. Throughout 1968, the U.S. lurched from one crisis to another.

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