Final answer:
An unbalanced force leads to a change in an object's speed as per Newton's second law of motion. It causes acceleration if no other forces balance it and can increase or decrease the object's speed or alter its direction. Friction, for example, can act as a balancing force to eventually stop a moving object.
Step-by-step explanation:
An unbalanced force causes a change in the speed of an object according to Newton's second law of motion. This law states that the acceleration of an object depends on the net force acting upon the object and inversely on the object's mass. When a force is applied to an object and no other equal and opposite forces are acting to balance it, this unbalanced or net force will result in the object's acceleration.
For example, if you push a skateboard forwards, the force of your push is initially unbalanced by other forces (like friction or air resistance). It's this unbalanced force that changes the skateboard's speed - if you push harder (increase the force), the skateboard accelerates more and its speed increases. If the force is removed, the skateboard would eventually come to a stop due to the balancing force of friction.
This principle doesn't just apply to increasing speed; unbalanced forces can also cause objects to decrease speed or change direction. In essence, any change in velocity (which includes both speed and direction) indicates the presence of an unbalanced force acting on an object.