A plastic block released underwater rises to the surface because the buoyant force acting on it is greater than its weight due to the plastic’s lower density compared to water. This buoyant force, an upward push exerted by the water, is proportional to the volume of water displaced by the block. Since plastic is less dense, it displaces a larger volume of water for a given weight, leading to a stronger upward force. This results in a net upward force, causing the block to accelerate upwards and rise to the surface.