This question is asked in a famous competition in my country: "Approximately calculate the diameter of the lens which this picture is taken with.(for measurements consider lens as a flat object) "
People in the past thought that a city which the length of the visible arc of a star's path in that city is longer, that city is the luck city; Now, for a city with a latitude of 35° how much should be the star's declination to be the luck city?
Consider a pendulum in it's balance point hanging from ceiling. It can swing in all the directions in the space. The pendulum can only swing in a sphere(the string can't bend). Now, is it possible to release the pendulum in a particular height and with a initial condition that in the first...