P. 4003. On a running training the runners run at a speed of v=5 m/s along a straight path, forming an L=35 m long column. Their coach runs towards them at a speed of u=2 m/s. The first runner turns back and runs in the same direction as the coach, still at a speed of v, when he meets the coach, and so do all the runners when they meet their coach.
a) How much time elapses between the turn of the first and the last runner?
b) How long is the column when they all run in the same direction as the coach?
P. 4011. Below the fixed end of the insulating horizontal thread, at a distance equal to the length of the thread there is a fixed charge, which has the same charge as the charge of the small body of mass m at the end of the thread as shown in the figure. The body is released and when it is at the lowest position the angle between the thread and the horizontal is 30o.
a) Find the mass of the moving body.
b) Find the tension at the lowest position.
c) By what factor is the acceleration of the body at the lowest position is greater than the initial acceleration?