You are a passenger in a car and not wearing your seat belt. Without increasing of decreasing its speed, the car makes a sharp left turn and you find yourself colliding with the right-hand door. Which is the correct analysis of the situation?

Green: Before and after the collision, there is a rightward force pushing you into the door.

Blue: Starting at the time of collision, the door exerts a leftward force on you.

Pink: Both of the above.

Purple: Neither of the above.

Answer: Blue. To change your direction of motion, your velocity vector must change. That means that you were accelerated. Therefore, a net force was applied, in this case, by the door, in the direction leftward.