A.

Mrs Peters had two children. Sammy was seven years old, and his sister Annie was four. Sammy went to school, but Annie did not. When Sammy was at home, he often played with Annie while their mother was cooking or washing or cleaning, and he was usually very nice to his small sisster, and Mrs Peters was free to do her work quietly.

Khi xe lửa chạy trên đoạn đường vòng nó thực hiện một chuyển động tròn và đương nhiên cũng phải có tác dụng của lực hướng tâm. Lực hướng tâm ấy từ đâu mà ra?