Why Do Pro Soccer Fields Have Wet Grass Over Dry Grass? by Andrew Cooke

If you have been living under a rock then you may not know that the World Cup is currently going on in Qatar right now. The World Cup is the most popular soccer tournament in the world where 32 of the best national teams compete to see who is the best, and it is held only once every 4 years. If you have watched any of these games lately you’ll notice a common trend. The sprinkler system in the field will always spray the field with water before warms-up, kick off, and during halftime. You may think this seems excessive to keeping the field healthy and you would be right, the sprinklers have no use on the health of the grass but instead on how the players like the game to be played. Any soccer player will tell you that a damp, not overly wet, field is the best to play on because of the speed that the ball travels across the grass. It’s fast, consistent, and keeps the spin that is applied to the pass across the ground. This makes for a better soccer game but how does the wet grass do this?

The answer is because the wet grass has less friction compared to dry grass, which can allow the ball to travel faster and do all these other things, but how is this possible from a basic physics level? So on a microscopic level, friction occurs because of irregularities between 2 materials. These irregularities can interlock with each other and cause a resistive force in the opposite direction of the object that is moving or having force applied to it. This is the basis of what causes friction force. When looking at a soccer ball on grass, there are irregularities in the ball and each individual grass it touches which slows the linear and centripetal velocities of the ball as it moves over them. Water, however, is liquid, and has no irregularities, so when it is on the grass/ball in small amounts it can fill in some of these irregularities and allow for less contact to occur. This means that the ball feels like friction force so the linear and centripetal velocities after it is kicked will decelerate at a low rate then if it was dry.

1. https://www.sciencedirect.com/topics/materials-science/lubricant#:~:text=Lubricant%20is%20widely%20used%20when,between%20the%20interfaces%20is%20reduced. 

2. https://www.semanticscholar.org/paper/Biomechanical-Analysis-%3A-Different-Phases-Of-Motion-Peter-Whiston/928e8e59c86265b47a80cb98e2fd95973ba2ba58