   deplacement in 2D space
  Hi guys,I'm writing a bawling game for the iPhone and I'm preparing my own "physics" engine because I don't need much.Is really simple what I have to do, I need to launch a ball from the bottom of the screen to the upper part. I prepare a system that give a point in the screen(UITouch) it calculate the angle between the y-axis and the vector is created. (I use atan to calculate this angle and I think I'm ok with this. I have negative angle when I touch on the right side of my ball and positive on the left.) The angle are right I checked and they seems the right ones.My problem is when I have to launch the ball and calculate the new x and y coordinates.When I draw the new position of the ball it's like my launch is rotated 45 degree counterclockwise. (If I touch upper right it goes up. If I touch right it goes upper right..so on)I'm really upset with this I can't figure out what's the problem. I tried playing with the angle(adding PI/2) to rotate all the stuff but I'm doing something wrong.Thank you in advance for your help.- (void) applyForceToObject:(int)force withDirectionVectorX:(int)vectorX vectorY:(int)vectorY { double x1 = (vectorX - self.x);double y1 = (vectorY - self.y); trajectory =  atan(x1 / y1);NSLog(@"Trajectory:%f rad, %f degree.", trajectory, trajectory*180/PI); speed = force;}- (void) updateWithTimeInterval:(NSTimeInterval)timeInterval { timeSinceLastUpdate = timeInterval;timeSinceFirstUpdate += timeSinceLastUpdate;  self.x += timeInterval/100*speed*cos((trajectory + (PI / 2)));self.y -= timeInterval/100*speed*sin((trajectory + (PI / 2)));   // I sottract this because the y axis is inverted. (0 up, 320 down)}   