1. The problem statement, all variables and given/known data Finding equation allowing to calculate time of zero-g (weightlessness) in a parabolic flight using non-powered glider (no thrust). 2. Relevant equations I have found a few ways to calculate the time (here is one example which is basing on aircraft's velocity only - https://www.nasa.gov/pdf/466762main_AP_ED_WeightlessWonder.pdf). Here it's basically the same equation where U is initial speed and sin0 is a sinus of the initial angle: 3. The attempt at a solution I tried to draw the situation using vectors and forces but there is still something wrong. It is obvious that the vertical acceleration of the glider (acting downward?) should be equal to g=9,81m/s^2 (and cancel it). But how to combine it with other forces?