Physics ,friction

Q: In pushing a car , explain briefly why it is more difficult to start the
motion , but once the car is moving it is relatively easier to maintain
its motion.

To start a car from rest, one needs to overcome limiting static friction. But after the car has started moving, only dynamic friction needs to overcome. Since dynamic friction is always smaller than static friction, this is the reason why a smaller force is required to maintain the motion of the car.
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Q: In the design of the breaking system in bicycles , what parameters
should be maximized to achieve a larger frictionsl force?

I presume you refer to the "braking" system of bicycles.

Bicycles brakes work on friction. The important parameters to increase frictional force is the material nature of the surfaces and the force applied to press two surfaces together.

Hence, bicycles brakes need to use materials of high coefficient of friction, with a design so as to increase the force in pressing the brake pad onto the wheel.

to set a car in motion, we have to overcome the friction and "accelerate" the car from stationary. To maintain a car in motion, all we need is to overcome the deceleration by friction.

in bicycles, the pressure that the break exert on the wheel affects the frictional force between the break and the wheel.