1. The problem statement, all variables and given/known data A projectile with initial velocity u making an angle A with the horizontal. Find the time t when the velocity of the projectile becomes perpendicular to its original velocity. 2. Relevant equations v = u + at s = ut + 1/2at^2 v^2 - u^2 = 2as 3. The attempt at a solution I have an intuitive idea of how to solve it. When the velocity becomes perpendicular to the original velocity, their dot product will be zero. I don't know if this is the right way. If this the right way, I don't know how to solve for it.