Answer:
Step-by-step explanation:
When you push your sister in the swing, you are applying Newton's third law, which refers to two objects that use equal and opposing forces but since there is no other object on the other side of the swing, your sister is pushed and starts swinging. She continues accelerating forward and upward until gravity becomes too much and decelerates the swing until it stops at a certain point in the air. At this point Newton's second law applies, gravity begins to accelerate the swing downward which is multiplied by the weight of your sister in the swing to cause it to accelerate rapidly in the opposite direction she was initially going.