Frictional Forces

A force is a push or pull.
  
Try this on line experiment 

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 When the surface of one object is moved against another, e.g. these gears, the second object applies a force that acts against the the force causing the movement. 

This movement-opposing force is called friction.  
You can measure the size of the friction between an object and a surface by measuring how much force it takes to just get the object moving on that surface. 
The size of the frictional force depends on the what the two surfaces are like. 

  • If the surface you are moving the object on is smooth, the force of friction will be small. 
  • If the surface you are moving the object on is rough, the force of friction will be large.
You can reduce friction by 
  1. making the surface smooth
  2. making the surface area of contact less
  3. adding a a slippery liquid called a lubricant
How do you think ball bearings work? 
How would you walk without friction?