First, 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.
But what exactly is meant by the phrase unbalanced force? One force - the Earth's gravitational pull - exerts a downward force. The other force - the push of the table on an object.
Since these two forces are of equal magnitude and in opposite directions, they balance each other. An object is said to be at equilibrium. There is no unbalanced force acting upon the object and thus the object maintains its state of motion. When all the forces acting upon an object balance each other, the object will be at equilibrium; it will not accelerate.
Consider another example involving balanced forces - a person standing on the floor. There are two forces acting upon the person. The force of gravity exerts a downward force. The floor exerts an upward force.
Since these two forces are of equal magnitude and in opposite directions, they balance each other. The person is at equilibrium. There is no unbalanced force acting upon the person and thus the person maintains its state of motion.
Unbalanced Forces
Now consider a book sliding from left to right across a tabletop. Sometime in the prior history of the book, it may have been given a shove and set in motion from a rest position. Or perhaps it acquired its motion by sliding down an incline from an elevated position. Whatever the case, our focus is not upon the history of the book but rather upon the current situation of a book sliding to the right across a tabletop. The book is in motion and at the moment there is no one pushing it to the right.
The force of gravity pulling downward and the force of the table pushing upwards on the book are of equal magnitude and opposite directions. These two forces balance each other. Yet there is no force present to balance the force of friction. As the book moves to the right, friction acts to the left to slow the book down. There is an unbalanced force; and as such, the book changes its state of motion. The book is not at equilibrium and subsequently accelerates. Unbalanced forces cause accelerations. In this case, the unbalanced force is directed opposite the book's motion and will cause it to slow down.
In conclusion
To determine if the forces acting upon an object are balanced or unbalanced, an analysis must first be conducted to determine what forces are acting upon the object and in what direction. If two individual forces are of equal magnitude and opposite direction, then the forces are said to be balanced. An object is said to be acted upon by an unbalanced force only when there is an individual force that is not being balanced by a force of equal magnitude and in the opposite direction.
Hope all this help you to understand the topic of balance and unbalance forces
