A solid sphere of mass 0.750 kg and radius 5.00 cm is spun up to an angular velocity 10.0 rad/s, spinning clockwise. The sphere is held just barely above a flat platform of mass 5.00 kg, then released onto the platform. The sphere begins to skid and moves along the platform until it rolls without slipping. The coefficient of kinetic friction between the sphere and the platform is 0.70. The coefficient of friction between the platform and the ground is 0.05, and the platform begins to slide across the ground. Find an expression for the horizontal position of the center of mass of the sphere as a function of time (hint—it’s piecewise!), and note the times at which the sphere’s motion changes.

Astudent is performing an experiment comparing sound and light waves. the student gathers the following data. what conclusion does the student most likely make based on this data? light waves always travel the same speed; however, the speed of sound is determined by the medium that it travels through. all sound waves always have the same energy, so the temperature of the medium does not affect wave speed. light needs to vibrate particles, so it travels fastest in tightly packed solids, while sound does not need a medium, so it travels fastest in a gas. tightly packed particles in solids slow down the light waves; however, sound waves make particles bounce into each other, so they travel faster in solids.

You have two photos of a person walking. one shows the person at the corner of third and main streets, the other shows the person at the corner of tenth and main streets. there are lampposts at every corner in this town, and the first picture shows it to be 10: 32: 00 exactly. the second picture shows it to be 10: 49: 30. you know three facts: (1) all of the clocks are synchronized; (2) there are exactly 12 equal-sized blocks per kilometer in this town; and (3) the streets that cross main in this area are numbered consecutively, with no interruptions. what is the person’s average speed in kilometers per hour?

How many excess electrons must be present on each sphere if the magnitude of the force of repulsion between them is 4.57×10−21 newtons?