A ball suspended by a thread swings in a vertical plane, so that its acceleration in the extreme position and lowest position are equal. The angle $\theta$ of thread deflection in the extreme position will be

A cone filled with water is revolved in a vertical circle of radius $$4 \mathrm{~m}$$ and the water does not fall down. What must be the maximum period of revolution?

A train has to negotiate a curve of radius $$400 \mathrm{~m}$$. By how much should the outer rail be raised with respect to the inner rail for a speed of $$48 \mathrm{~km} / \mathrm{h}$$ ? The distance between the rails is $$1 \mathrm{~m}$$.