You push a book on a table so that its velocity is constant. If the force that you apply is 15N. what is the force due to sliding friction? If you accelerate, so that the velocity is no longer constant is the force you are applying to the book greater or smaller than the sliding force? (Please answer in 100 words)
If and only if the applied (push) force equals the opposing (friction) force, there is no acceleration. That is the case in part 1 of your question. You need to apply a larger force than friction to accelerate. That answers part 2.
If the book is moving with a constant velocity, then the force applied by pushing the book is equal in magnitude and opposite in direction to the force of sliding friction. Therefore, the force due to sliding friction is also 15N.
If the book starts to accelerate and its velocity is no longer constant, then the force you are applying to the book is greater than the sliding force. When the book accelerates, the force required to overcome the static friction between the book and the table increases. Once the static friction is overcome, the book starts sliding and experiences kinetic friction, which is usually smaller than static friction. Therefore, in order to accelerate the book, you need to apply a greater force to overcome the increased static friction.
To determine the force due to sliding friction, we need to apply Newton's second law, which states that force equals mass times acceleration (F = ma). Since the book's velocity is constant, its acceleration is zero. Therefore, the net force acting on the book must also be zero. We know that you are applying a force of 15N, so the force due to sliding friction must be -15N, equal in magnitude but opposite in direction.
If you accelerate the book, the velocity is no longer constant, which means there is a resultant force acting on it. This resultant force is equal to the mass of the book multiplied by its acceleration. Since there is a net force acting on the book now, the force you are applying is greater than the force due to sliding friction since it is required to provide the additional force to overcome the friction.