DALLAS — President Barack Obama urged Americans rattled by a week of violence and protests to find "open hearts" and new empathy Tuesday in a speech that seesawed between honoring police officers for their bravery and decrying racial prejudice that can affect their work. Obama stood next to five empty chairs for the white police officers killed last week by a black man seeking vengeance for police killings. Behind him, underscoring his message of unity: Dallas police officers, a racially diverse church choir and local officials who ranged from black Police Chief David Brown to former President George W. Bush, a Dallas resident.