Balanced and Unbalanced Forces

When forces act on an object, the key question is whether they are balanced or unbalanced.

Balanced forces (resultant = 0)

When forces are balanced:

• The object remains stationary, OR
• The object continues moving at constant velocity in a straight line (Newton’s first law)

Examples: a book on a table (weight balanced by normal force), a car cruising at constant speed (driving force balanced by drag).

Unbalanced forces (resultant $\neq$ 0)

When forces are unbalanced, there is a non-zero resultant force. The object accelerates in the direction of the resultant force.

$F_{\text{resultant}} = ma$

Examples: a ball being thrown (hand pushes, ball accelerates), a car braking (friction is greater than driving force, car decelerates).

Important distinction

An object moving at constant velocity is NOT being pushed by a net force. Many students wrongly think a constant forward force is needed to maintain constant velocity. In reality, constant velocity means balanced forces (Newton’s first law).

Acceleration does not mean “going fast.” It means “changing velocity.” An object can accelerate while moving slowly (just after release) or move fast without accelerating (terminal velocity).

Common errors and how to correct them

• Thinking that an object needs a net force to keep moving. Without friction, an object on a flat surface would keep moving forever (Newton’s first law).
• Confusing acceleration with velocity. An object decelerates when the net force opposes its motion.

How to apply this in an exam

Draw a free-body diagram. Add all forces as labelled arrows. Compare sizes to determine if balanced or unbalanced. State the consequence (constant velocity or acceleration).