A force changes the speed of an object when it is not balanced by another force . Aqui está o porquê:

* Newton's First Law of Motion: Um objeto em repouso permanece em repouso, e um objeto em movimento permanece em movimento com a mesma velocidade e na mesma direção, a menos que seja acionado por uma força desequilibrada.
* 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.
* Acceleration means a change in speed or direction.
* Net force is the sum of all forces acting on an object.

Examples:

* Pushing a box: When you push a box, you exert a force on it. If there is no friction or other opposing force, the box will accelerate (change speed).
* Throwing a ball: When you throw a ball, you exert a force on it, causing it to accelerate and gain speed.
* Braking a car: The brakes exert a force that opposes the car's motion, causing it to slow down.

Key point: A force alone doesn't always change speed. If forces are balanced, they cancel each other out, and the object's speed remains constant.