Friction is defined as the force that slows down object while sliding or rolling. that means that friction is only present when both bodies are in contact. by using ball bearing in fans reduces the friction between two bodies. by using lubricants like gresae or applying oil reduces the friction between two bodies
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1) Lubricants: A common way to reduce friction is to use lubricants. eg; oil, Greece.  these lubricants will place between two object to reduce the friction.

2) Using ball bearings: this technique is used  on machines which tiny steel balls roll over other which will reduce friction

3) Using streamlining : it help to cut air. The fluid current will help them to overcome  the fluid friction
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