Here's why:

* Newton's First Law of Motion (Law of Inertia): An object at rest stays at rest, and an object in motion stays in motion at a constant velocity, unless acted upon by a net force.

* Velocity: Velocity has both speed and direction. So, a change in velocity can be a change in speed, a change in direction, or both.

* Net Force: The net force is the overall force acting on an object after considering all the individual forces. If there is a net force, the object will accelerate (change its velocity).

Examples:

* Pushing a box: When you push a box across the floor, you apply a force that causes it to accelerate.

* Throwing a ball: When you throw a ball, your hand applies a force that changes its velocity from rest to a high speed.

* Friction: Friction acts as a force that opposes motion, causing a change in velocity.

Important Note: It's not necessarily just one force that causes a change in velocity. It could be multiple forces acting together to create a net force that causes the object to accelerate.