A force which can change the state of an object by changing it's speed, position or direction of motion is known as an unbalanced force. It is the resultant of all forces which are acting on a body and causing a change in the state of rest of motion of the body.
Assume a box is resting on the floor. The forces acting on the box in the rest are Normal (by the ground on the box) in upward direction and gravitational force Mg (due to gravity). Although these two forces are acting on the box, it is still in rest because these forces are balanced to each other. When we pull or push the box by applying appropriate force, the box will start to move because the force applied by us is unbalanced and there is no any force in the opposite direction to make this force balanced.
Metals consist of giant structures of atoms arranged in a regular pattern. The electrons from the outer shells of the metal atoms are delocalised , and are free to move through the whole structure. This sharing of delocalised electrons results in strong metallic bonding .
