PHYSICS · Multi-part · CBSE · Physics · Laws of Motion · Laws of Motion
Problem
Two identical wooden blocks, each of mass 1.5kg, are stacked one on top of the other on a nearly friction-free horizontal table. Friction between the blocks is characterized by coefficients μs=0.60 and μk=0.45. A horizontal pulling force of variable magnitude is applied to the lower block only. Gravity and air resistance may be considered uniform and constant.
(a) Draw a clearly labeled free–body diagram for the upper block just before any slipping occurs.
(b) Explain, without using numerical calculations, why the two blocks share a common acceleration as long as they do not slip relative to each other.
(c) Describe qualitatively how the magnitude of the static friction force exerted on the upper block changes as the applied pulling force on the lower block is increased smoothly from zero up to the threshold of slipping. Sketch the expected trend of friction magnitude versus applied force and justify your reasoning.
(d) Once the applied force exceeds the maximum static friction value, the upper block begins to slide. Explain what happens to (i) the direction and (ii) the magnitude of the friction force on the upper block at the instant sliding starts and during the subsequent motion.