Science question. First answer may get 10 points!
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Generally it is very useful, but if a child happens to fall from equipment or while running etc., friction is an enemy. It causes damage, the least of which would be skin loss.
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Yes. Without friction you would fall off the monkey bars, slide off the see-saw, and on slides you would just keep accelerating until you slammed into the ground at the bottom.
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without friction nuts and bolts would not stay tight, screws and nails would pull out and you couldn't actually stand up so it's pretty useful everywhere
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Yes, it is friction due to which, you are walking, running and playing but not slipping.