when a truck pulls a trailer the trailer and truck accelerate forward even though the action and reaction forces are the same size but in opposite direction why don't these forces balance each other out

The two action-reaction forces act on separate bodies.

When a truck pulls a trailer, the truck and trailer accelerate forward together, even though the action and reaction forces are equal in magnitude and opposite in direction. This might seem puzzling at first, but it can be explained by considering Newton's third law of motion and the concept of the net force.

According to Newton's third law, for every action, there is an equal and opposite reaction. In this scenario, the action force is the force exerted by the truck on the trailer, and the reaction force is the force exerted by the trailer back on the truck. These forces are of the same magnitude but act in opposite directions: the truck pulls the trailer forward, and the trailer pushes the truck backward.

Now, let's consider the net force. The net force acting on an object is the vector sum of all the forces acting on it. In this case, the action and reaction forces are the only forces acting on the truck and trailer system. Since these forces are equal in magnitude but in opposite directions, they cancel each other out when looking at the net force.

However, it's important to note that the net force is not the only factor determining the acceleration of an object. The net force acting on an object is related to its acceleration through Newton's second law, which states that the net force on an object is equal to the mass of the object multiplied by its acceleration (F = ma). In this case, since the trailer has its own mass, it resists being accelerated forward by the truck's action force. As a result, the truck and trailer system will accelerate forward, but the truck will have a greater acceleration compared to the trailer due to its larger mass.

In summary, while the action and reaction forces in the truck-trailer system are equal in magnitude and opposite in direction, they do not cancel each other out completely because the net force also takes into account the mass of the objects involved.