Friction is a force that opposes the motion of an object when it is in contact with a surface. When considering the effect of friction on acceleration, it's important to distinguish between two types of friction: static friction and kinetic (or sliding) friction.
Static Friction: Static friction is the force that prevents an object from starting to move when a force is applied to it. It acts in the direction opposite to the force trying to move the object. If you apply a force to an object but the force of static friction is greater, the object will not move. In this case, the acceleration is zero because there is no change in velocity.
Kinetic Friction: Kinetic friction comes into play when an object is already in motion. Once the object is moving, the force of kinetic friction opposes the direction of motion and acts to slow it down. Kinetic friction is generally weaker than static friction. The magnitude of the kinetic friction force can be calculated using the equation F_k = μ_k * N, where F_k is the force of kinetic friction, μ_k is the coefficient of kinetic friction, and N is the normal force.
Regarding the change in the direction of the force vector from being parallel to velocity to being opposite to velocity, this happens because friction always acts in the direction that opposes motion. When an object is not moving, the force of static friction acts in a direction that balances the applied force. However, when the object starts moving, the force of kinetic friction acts opposite to the direction of motion. This is due to the microscopic interactions between the object and the surface, which generate forces that resist the relative motion between them.
In summary, friction affects acceleration by opposing motion and reducing the net force acting on an object. It changes the direction of the force vector from being parallel to velocity to being opposite to velocity because its purpose is to resist the motion of the object.