The laws that apply to negligence cases against the federal government are the same laws that apply in the state where the accident occurs. When lawmakers passed the Federal Tort Claims Act, they had to decide what rules apply to the case. They chose to use the laws of each state, depending on where the accident occurred.
For example, if the accident occurs in Tampa, that state laws of Florida apply. The United States defers to the state law even if this means that cases are not decided in the same way throughout the United States.