when a truck pulls a trailer the trailer and truck accelerate forward even though the action and reaction forces are the same size but are in oppsoite direction. Why don't these forces balance each other?
too late now for that answer!
The reason the forces don't balance each other in this scenario is because the forces act on different objects. According to Newton's third law of motion, for every action, there is an equal and opposite reaction. When the truck pulls the trailer, the action force is the force exerted by the truck on the trailer. As a reaction, there is an equal and opposite force exerted by the trailer on the truck.
However, it's important to note that these forces act on different objects and therefore do not cancel each other out. According to Newton's second law of motion, the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. In this scenario, the truck and the trailer have different masses.
When the truck pulls the trailer, the force being applied to both the truck and the trailer results in an acceleration for each. Due to the difference in mass between the truck and the trailer, their accelerations will be different. The truck, having a larger mass, will experience a smaller acceleration compared to the trailer.
In other words, the force exerted by the truck on the trailer causes the two objects to move forward together, but their individual mass differences result in different accelerations. Therefore, the combined system of the truck and trailer accelerates forward, even though the action and reaction forces are of the same size and in opposite directions.