1.   A 20kg traffic light hangs midway on a cable between two poles 40 meters apart. If the sag in the cable is 0.4 m, what is the tension in each side of the cable?

2.   Say, I have a glass tube and connect it to a vacuum pump to remove the air from the tube, would the glass tube shatter if air was removed from it?

3.   Please explain how a bicycle is able to remain upright when moving?