The downward force acting on the suspended brick is its weight, caused by gravity. According to Newton's third law, the corresponding reaction force is the tension in the cord that pulls upward on the brick.
The question is concerned with the forces acting on a brick that is suspended from a rigid support by a cord. When answering the two parts of the question:
- Downward force acting on the brick: The downward force acting on the brick is its weight, which is the force due to gravity. Weight is determined by the mass of the brick (m) multiplied by the acceleration due to gravity (g), which is roughly 9.8 m/s2 on Earth.
- Action and reaction forces: Applying Newton's third law, which states that for every action, there is an equal and opposite reaction, we find that the reaction force to the weight of the brick (the action force) is the tension in the cord pulling upward on the brick. Therefore, if the downward force (weight of the brick) is the action force, the corresponding reaction force is the tension in the cord acting upward.
It's important to note that these action-reaction force pairs do not cancel each other out, as they act on different objects: the weight acts on the brick, and the tension acts on the cord.