Speed may not always affect soccer in my opinion. It is really just about stamina and not even that much because you can always be subbed-out if you get tired and have subs. Sure, speed helps trick other players and get the ball up the field faster but if you don't like to run then goalie would probably be the best position for you (but you still have to run in goal if you're on a serious team).
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You do not necessarily need speed.
If you are quick, however, it becomes much easier for you to quickly accelerate past other players, or catch up with them. I
f you are a centre back you need strength to be able to tackle players.
If you are a full back or a winger, you will need speed to run up and down the wings.
If you are a centre midfielder, you need stamina to run up and down the pitch to help with both attacking and defending.
If you are a striker, you need to be fast and strong to make good runs, get past players, and shoulder barge defenders away.
The footballer runs with the ball in a straight line ( His speed). He then has to change direction because another footballer trys to block him so he changes direction (Velocity) i.e a change in direction. Velocity is speed with a direction. Say 5 mph in a NE direction.
Answer
The above answer is correct. Soccer is all about velocity. Rarely do you see a footballer run for long in a straight line. He uses the field in every direction to reach his goal, well the opposites sides goal.
You need speed to lose your marker, get to a loose ball, dribble away from a tackler and chase your mark if they get away.
Acceleration affects soccer by allowing your muscles to be strengthened and your speed to be increased. It also allows you to become more physically fit.