Solving a problem in physics section 22 point 80 mechan

80. The radius vector of a material point moving in the gravitational field of the Earth, described by the equation r = υ0ti - gt2 / 2j, where υ0 = 76 m / s, g - acceleration of gravity; i, j - the unit vectors of the coordinate axes x and y. Determine the time t1, after starting, when the velocity vector υ point at an angle α = 35 ° to the horizontal. What is the velocity υ at this time?