Final answer:
When force is applied to a wooden block on a rough surface, the frictional force increases with the force until the block starts moving. Once in motion, the frictional force becomes constant as kinetic friction.
option a is the correct
Step-by-step explanation:
When a wooden block is placed on a rough surface and force is applied to it, the behavior of the frictional force is quite specific. Initially, as you start to increase the applied force, the frictional force will increase as well. This is because of static friction, which acts to resist the initial motion and will increase to match the applied force up to its maximum value.
Once the maximum static friction force is exceeded, the block will start to move. At this point, the frictional force will become kinetic friction, which is generally less than the maximum static friction. As you continue to apply the force, the kinetic friction will act in the opposite direction to the movement of the block but unlike static friction, it will remain relatively constant regardless of any further increases in the applied force, assuming all other conditions are constant and the force does not exceed the limit where the surfaces begin to degrade or melt due to excessive forces or heat.
Thus, the correct answer to the question is (b) - the frictional force will increase proportionally to the applied force, but only up to the point of impending motion. After which it will reach a maximum and then remain constant as kinetic friction when the block is in motion.