Kick 1 of 5. Drag from the ball to where you want it to go. Drag further for more power.
Why does the ball bend?
Every kick you just took, from the one that hit the wall to the one that found the corner, followed the same three rules. None of them needs an equation.
Nothing in the air goes straight
Gravity starts pulling the ball down the moment it leaves your boot, so even a shot that looks flat is falling the whole way. Hit it hard and the fall is gentle: the ball seems to fly straight, then dips late. Hit it softly and it has to climb to get there at all, so it loops.
Look for it: water from a garden hose. The jet makes exactly this shape, an arch that climbs and then falls faster and faster. Every thrown, kicked or launched thing draws the same curve. It has a name, a parabola, and it is also the shape of the cables on a suspension bridge.
Spin makes the air push sideways
Kick across the ball and it spins. One side of the ball is now moving with the air rushing past it, the other side against it. The air slips more easily round one side than the other, and that difference gives the ball a small, steady nudge sideways, for the whole flight. In the game that nudge is the curl slider.
Look for it: a tennis player's topspin, which drags the ball down over the net, or a table tennis player's sidespin serve. The nudge is called the Magnus effect, after a German scientist who was trying to work out why cannonballs drifted.
Why the wall never sees it coming
This is the part that makes free kicks unfair on defenders. The sideways nudge is tiny, but it keeps nudging, and the drift builds up. Double the flying time and the drift does not double, it quadruples. So for the first stretch of the flight the ball has barely moved off its line and looks dead straight. The bend arrives late, after the wall, when there is nothing anyone can do about it.
Look for it: Roberto Carlos for Brazil against France in 1997. He aimed what looked like a yard outside the post. The ball boy behind the goal ducked. Then it swerved back in. Your shots in the game do the same thing: look at the drawn path and notice how straight the first half is.
Next time you watch a free kick, watch where the taker is aiming. It is almost never the goal.