Question

How does the application of brakes on a car affects it's Velocity?what is the sign of acceleration

Answer 1

If there was no velocity there would be little point in applying the brakes!

Once the brakes are applied then the velocity should decrease - unless there is an equal or greater force acting in the opposite direction to the braking force.

Accelleration is defined as the rate of change of velocity, so as velocity decreases accelleration will be negative.