Theory:

Imagine a block is placed on the table as shown in the below image. Two strings \(X\) and \(Y\) are tied to the two opposite surfaces of the block, as shown. If we apply a force on the block by pulling the string \(X\), it begins to move towards the right. Similarly, if we pull the string along \(Y\), the block moves to the left. But, if the block is pulled from both sides with the same forces, the block will not move. Such forces are called balanced forces, and they do not change the state of rest or motion of an object.
Let us consider a situation in which two opposite forces of various magnitudes act on the block. In this case, the block would begin to travel in the direction of the greater force. Thus, the two forces are not balanced, and the unbalanced force acts in the direction the block moves.
 
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A block placed on the table 
 
What happens when you try to push a box on a rough floor?
 
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Pushing a box on a rough floor
 
If we push the box with a small force, it does not move as the friction opposes the push.

The friction force arises between two surfaces that in contact; in this case, the bottom of the box and the rough surface of the floor. It counteracts the pushing force, and therefore the box does not move. When you push the box harder and the box still does not move, it denotes that the frictional force still balances the pushing force. The box will move only if the applied force is greater than the frictional force. When the applied force is greater than the frictional force, an unbalanced force acts on the box. Hence, the box starts moving.
What happens when we ride a bicycle?
 
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A man riding bicycle
When we stop pedalling the bicycle, it begins to slow down. This occurs due to the frictional forces acting opposite to the direction of motion. To retain the bicycle moving, we have to start pedalling it again. You may think that an unbalanced force is always required to maintain the motion of an object. However, it is quite wrong.
 
If the forces acting on the object are balanced, and there is no net external force acting on it, the object moves with a uniform velocity.
 
If an unbalanced force is applied to the object, there will be a change either in its speed or the direction of motion. Thus, to accelerate the motion of an object, an unbalanced force should act on it. And, the change in its speed (or in the direction of motion) would continue as long as this unbalanced force is applied. However, if this force is removed completely, the object would continue to move with the velocity it has acquired till then.