In classical mechanics, an object needs to accelerate even after reaching a certain velocity because the object may need to change its direction or overcome external forces acting upon it.
When an object moves at a constant velocity, it means that its speed and direction remain unchanged. However, if the object needs to change its direction, it must accelerate to do so. Acceleration is the rate of change of velocity, and it can involve increasing or decreasing the object's speed, changing its direction, or both.
Additionally, external forces can act on an object, such as friction, air resistance, or gravitational forces. These forces can oppose the object's motion or cause it to slow down. To counteract these external forces and maintain a desired velocity, the object must continue to accelerate.
Moreover, there may be practical limitations or constraints on the maximum velocity an object can achieve. For example, in the case of a car, there are speed limits on roads for safety reasons. In other situations, objects may have physical limitations that prevent them from achieving arbitrarily high velocities, such as structural integrity constraints or energy limitations.
So, while it is true that an object can theoretically move as fast as it wants, various factors like the need for directional changes, external forces, and practical limitations can necessitate ongoing acceleration even after reaching a certain velocity.