Force and Laws of Motion - Identify balanced and unbalanced forces and role of friction

Force is a push or pull that can change the state of rest, motion, or the shape of an object. Its SI unit is Newton ($N$).

Balanced Forces: If the resultant of all the forces acting on a body is zero, i.e., $\sum \vec{F} = 0$, the forces are called balanced forces.

Effects of Balanced Forces: They do not change the state of rest or uniform motion of an object, but they can change the shape or size of the object (e.g., compressing a spring from both sides).

Unbalanced Forces: If the resultant of all the forces acting on a body is non-zero, i.e., $\sum \vec{F} \neq 0$, the forces are called unbalanced forces.

Effects of Unbalanced Forces: They produce a change in the speed or direction of motion, meaning they cause acceleration ($a \neq 0$).

Frictional Force: To move a heavy object, the applied unbalanced force must be greater than the force of static friction ($f_s$) acting between the surfaces.