We can reduce friction by adding oil between the parts.
Friction depends of the mass.
     Skating is one of the fastest sports.

1. What happens between the blades of the skater and the ice?
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2. Why oil is added into the car engine?
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3. Why you fall down easily on the wet floor?
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4.Why is it hard to move a box on the sidewalk?
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How can friction be a problem?
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