Georgia State Patrol spokesman Gordy Wright said the truck was headed east when parts of its transmission came off and went across the median, striking the west-bound Chevrolet Blazer on Wednesday night.
Wright tells The Atlanta Journal-Constitution that the debris went through the windshield, striking driver Jameka French of Decatur and her son, Cameron Mcilwain, who was in the back seat.
Authorities say French was able to bring her vehicle to a stop near the median wall. The 29-year-old French was taken to Grady Memorial Hospital, where her arm was amputated below the elbow.
Initials reports from the accident mistakenly identified the child as a girl.