1. The problem statement, all variables and given/known data A projectile is thrown at angle θ with an inclined plane of inclination 45o . Find θ if projectile strikes the inclined the plane horizontal 2. Relevant equations Taking x-axis along the incline and y-axis perpendicular to incline. Vx=ucosθ - gsint(45)t Vy=usinθ - gcos(45)t These are the velocities after time t. Vx=Velocity along the plane after time t. Vy=Velocity perpendicular to plane after time t. 3. The attempt at a solution As at the time of horizontal collision with the incline. The projectile will make an angle of 45o with the incline and hence the velocity of projectile V=Vcos45i + Vsin45j which means the x and y components of velocity are equal (as sin45=cos45). And hence I applied the condition that Vx=Vy that gives us: ucosθ - gsint(45)t = usinθ - gcos(45)t => ucosθ-usinθ = gsin(45)t-gcos(45)t =>u(cosθ-sinθ) = gt(sin45-cos45) =>u(cosθ-sinθ)=0 (as sin45=cos45) =>cosθ-sinθ=0 =>cosθ=sinθ => Tanθ=1 which gives θ=45o But the answer is θ = tan-1(2) - 45o I know how the right solution came but I am more bothered about what was wrong in my attempt that I got it incorrect.