A small car bumps into a large truck.
Compare the forces that the truck exerts on the car (F_truck on car) and the car exerts on the truck (F_car on truck) if before the collision the truck was stationary and the car was moving.
Newton's 3rd Law tells is that the forces of an action/reaction pair are equal and opposite. Since F_truck on car and F_car on truck are an action/reaction pair, they are equal no matter their state of movement before the collision.
