why does car speed increase and decrease?

Question

anonymous · Answer

a car has it's engine to supply power to move the entire system of assembly while the braking system is used for slowing down the car...

anonymous · Answer

Physically speed increases when more energy is produced by the engine of the car and it decreases whenever brake is applied.

gavlois1 · Answer

A change in speed is known as acceleration. We know from Newton's second law:

$$F=ma$$

That when a force is applied to a mass, it will experience acceleration. Speeding up will be positive acceleration, and slowing down will be a negative acceleration. 

Now, to answer your original question, the car speeds up due to a force applied to the car which allows it to speed up with positive acceleration. The friction between the tires and the road apply a force which applies negative acceleration, making it slow down.

anonymous · Answer

A car needs fuel to run. So when a car increases speed, its need for fuel increases too. The faster the car is going, the greater the need for fuel to burn will be. The same is true of when a car decreases speed. When a car decreases its speed, it does not have to burn as much fuel.
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The wind resistance against a car varies with the cube of its velocity. This means that the amount of power needed to overcome the wind resistance is 8 times as much when the speed is 2 times as much. At 50 or 55 miles per hour, most cars are putting over half the fuel they consume into overcome atmospheric resistance, where at 25 miles per hour it would be less than ten percent. This in turn means that the number of miles per gallon of gas is reduced by quite a lot as the car goes faster.