Here's why:

* Velocity is a vector quantity, meaning it has both magnitude (speed) and direction.

* Force is an interaction that can cause a change in an object's motion.

* Newton's Second Law of Motion states that the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass.

Therefore, to change an object's velocity (either speed or direction), you need to apply a force that causes it to accelerate.

Here are some examples of how force changes velocity:

* Pushing a box: Applying a force to the box causes it to accelerate and change its velocity.

* Throwing a ball: The force of your throw accelerates the ball, changing its velocity from rest to a certain speed and direction.

* Braking a car: The force of friction from the brakes decelerates the car, reducing its speed.

* Turning a car: The force of friction from the tires turning on the road changes the car's direction, resulting in a change in velocity.

In summary, force is the key to changing velocity. Without a force acting on an object, its velocity will remain constant.