Here's why:

* Newton's First Law of Motion: An object at rest stays at rest, and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

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

* Force causes acceleration: If a force is applied, the object will accelerate.

* More force = more acceleration: The greater the force, the greater the acceleration.

* More mass = less acceleration: The greater the mass of an object, the less it will accelerate for a given force.

Examples of external forces that cause acceleration:

* Pushing a box: You apply a force to the box, causing it to accelerate across the floor.

* Gravity: Earth's gravity pulls objects towards it, causing them to accelerate downwards.

* Friction: Friction opposes motion and can cause an object to slow down (decelerate) or change direction.

* Wind: Wind can push on an object, causing it to accelerate.

* Magnetic forces: Magnets can exert forces on objects made of certain materials, causing them to accelerate.

Important Note: A balanced force does not result in acceleration. This is because the forces cancel each other out.