Final answer:
The constant acceleration of block B under the force P is zero when it moves with a constant velocity, as the applied force is balanced by the frictional force, leading to a net force of zero.
Step-by-step explanation:
Under the action of force P, the constant acceleration of block B depends on the net force acting on it as per Newton's second law of motion, which states that acceleration (a) is directly proportional to the net external force (Fnet) and inversely proportional to the mass (m) of the block, as represented by the equation a = Fnet / m. When a block is moving at constant velocity, it implies that there is no net force acting in the direction of motion, hence no acceleration. This is because the applied force is balanced by the frictional force. For example, if the applied force by P is exactly equal to the frictional force (which is the product of the coefficient of kinetic friction and the normal force), the net force will be zero, resulting in zero acceleration for block B.