PHYSICS · Multi-part · CBSE · Physics · Laws of Motion · Laws of Motion
Problem
A 1.30kg wood block rests on a horizontal concrete floor. A student attaches a spring scale to the block and slowly increases the horizontal pull until the block just begins to move. The largest reading observed immediately before motion starts is 4.2N.
After the block starts sliding, the student continues to pull so that the block moves across the floor at constant speed; the spring-scale reading during this motion is 3.5N. The spring scale has a calibration uncertainty of ±0.1N, and the block’s mass is known to within ±0.01kg. Take g=9.8m/s2.
(a) Calculate the coefficient of static friction between the wood and the concrete. Include the absolute uncertainty in your result.
(b) Calculate the coefficient of kinetic friction between the wood and the concrete. Include the absolute uncertainty in your result.
(c) Compare the numerical values obtained in parts (a) and (b) and explain, in terms of the microscopic origin of friction, why one coefficient is larger than the other.
(d) Identify one assumption made in your calculations that, if violated, would systematically affect both coefficients in the same direction. Briefly describe how the results would change if the assumption were not valid.