There are six books in a stack, and each book weighs 5N. The coefficient of friction between the books is 0.2. With what horizontal force must one push to start sliding the top five books off the bottom one? For the five books: m * a_max = u_s * m * g a_max = u_s * g a_max = 0.2 * 10 m/s^2 = 2 m/s^2 For the system: F_max = M * a_max F_max = 3 kg * 2 m/s^2 = 6N Was there a mistake in the approach I took? Apparently, the correct answer is 5N, since you only need to find F_s between the five books and the sixth book which is u_s * m * g. However, this would mean each book gets an applied force of 5/6 N. With a mass of 0.5 kg each, each book would only accelerate at 1.67 m/s^2, which is not equal to a_max (2 m/s^2). Any thoughts?