11. The car is traveling rightward and crashes into a brick wall. The brick wall acts as an unbalanced force and stops the car.
12. The truck stops when it crashes into the red car. But the ladder falls in front of the truck because the ladder was in motion with the truck but there is nothing stopping the ladder when the truck stops.
19. Newton’s Third Law As the man jumps off the boat, he exerts the force on the boat and the boat exerts the reaction force on the man. The man leaps forward onto the pier, while the boat moves away from the pier.
20. Newton’s Third Law Force exerted by the wheels Force exerted by the road