The coffee stays in motion because the stopping force acts only on the car. The coffee stays in motion because a force from the cup holder pulls on the coffee. The stopping force on the car causes an equal and opposite reaction from the coffee. The stopping force causes the motion of the car to slow down and the motion of the coffee to speed up.
I gave my students this multiple choice question: A teacher puts a cup of coffee into the cup holder of a car. When the car stops suddenly some of the coffee spills out of the cup. Which statement best explains why the coffee spills?

Only one answer is correct among these 4 choices:
- The coffee stays in motion because the stopping force acts only on the car.
- The coffee stays in motion because a force from the cup holder pulls on the coffee.
- The stopping force on the car causes an equal and opposite reaction from the coffee.
- The stopping force causes the motion of the car to slow down and the motion of the coffee to speed up.

Could you tell me which one is correct?

The coffee stays in motion because the stopping force acts only on the car.