* Velocity is a vector: It has both magnitude (speed) and direction. Changing either the speed or the direction of an object changes its velocity.
* Forces cause changes in velocity: The fundamental principle is Newton's Second Law of Motion: Force = mass * acceleration. Acceleration is the rate of change of velocity. So, an object can only change its velocity if a force acts upon it.
* Many things can exert forces: This could be:
* Gravity: Always pulling objects toward each other.
* Friction: Opposes motion between surfaces in contact.
* Air resistance: A force opposing motion through the air.
* Electromagnetism: Forces between charged particles.
* Contact forces: Pushing or pulling directly on an object.
Example:
Imagine a ball rolling across a table.
* Friction from the table will constantly slow the ball down, changing its velocity.
* Gravity will also be pulling the ball down, though this effect may not be noticeable on a flat table.
Therefore, the key is not a specific object, but a force acting on an object. Any object capable of exerting a force can potentially change the velocity of another object.
