An engineer places a small sample of a material on a horizontal disk whose surface is made of another material and then rotates the disk from rest with a constant angular acceleration of 4 rad/s2. Assuming that the coefficient of static friction between the sample and disk is 0.2, what is the angular velocity of the disk at the time instant when the sample starts slipping on the disk

On solidification from a melt, the polymer polyethylene forms a semi-crystalline spherulite structure. (i) sketch an individual spherulite and label the amorphous and crystalline regions. (ii) draw a schematic to show the changes in specific volume with temperature during cooling from the melt, comparing it to the theoretical extremes of fully crystalline and fully amorphous (indicate tm and tg)