Question

what happens if the frictional force is reduced

Answer 1

When we reduce frictional force by polishing the surface the frictional force is less then in our life there was a huge distruction is held FOR EXAMPLE------------
1-if we runing our vechels if we want to stop them we push the breck but due to low friction force the vechels are stop but after going very far(due to gravitational force) . So we not stop any vechels sudenly

Answer 2

Firstly, I would like to explain friction. Friction is produced when two surfaces rub against each other. Friction slows down a moving object. 
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There is more friction if a surface is rough. Friction can be reduced by using lubricants like oil, wax or grease to make the surface smooth. Balls or wheels are the most common reality of reducing friction; they roll to help reduce friction. Another possible way to reduce friction is to change the types of materials in contact with one another.
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Lets experiment. Take the example of a heavy box without wheels. Suppose, you have to push it through the rough ground. It would be very hard to push and it will slow down the box and also cause some damage. Now, lets see what happens if we slowly reduce the friction. 

1. If we use wheels, then it would be much easier to carry the heavy box and result in swifter movement of the box. 

2. If we push the box after spreading oil or wax on the ground, the surface will become more slippery than smooth, and it will result into different accidents and crashes because the box will become uncontrollable. 

3. Let's change the surface on we are going to push the box. Suppose, instead of the ground, if we push the box through slippery ice, it would be extremely uneasy to push the box. It is quite the minimum degree of friction. 
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Now, we have understood the effects that take place when we reduce friction. As too much friction is not good, too less friction is not good either. But, if we reduce friction by using wheels, it results in perfect movement; so, I guess, the best method for reducing friction to a perfect extent is using wheels. In this way, I briefly conclude that friction is strongly important for motion to an extent; neither too less or too much is good.