What is a Force?
A force is a push or pull. Forces are measured in Newtons (N) and have both size and direction (they are vectors).
Balanced Forces
Forces are balanced when they cancel each other out (equal size, opposite directions). The net (resultant) force is zero.
Result: the object stays still OR keeps moving at constant speed.
Balanced — no motion
Unbalanced Forces
When forces are unbalanced, there is a net force and the object accelerates.
A book on a table
Weight (down) = 10 N. Normal force from table (up) = 10 N.
Net force = 0 → book stays still. Forces are balanced.
A rocket launching
Thrust (up) = 50,000 N. Weight (down) = 30,000 N.
Net force = 20,000 N upwards → rocket accelerates upward.
Resultant Force
Real world: car cruising at constant speed
A car on a motorway travels at 100 km/h without speeding up or slowing down. The engine provides a forward driving force of 600 N; air resistance and road friction together produce a backward force of 600 N.
Net force = 600 − 600 = 0 N → balanced → constant velocity. The driver keeps the accelerator pressed only to maintain that balance against resistance forces, not to accelerate further.
Real world: hovering hot-air balloon
A hot-air balloon floats at a fixed altitude. The upward buoyancy force from the surrounding air exactly equals the downward weight of the balloon, basket, and passengers.
Net force = 0 → balanced → the balloon stays at the same height. To climb, the pilot heats the air inside (reducing overall density and thus weight); to descend, they open a vent to release hot air.
Real world: bridge in equilibrium
A pedestrian bridge carries a total downward load of 20 000 N from its own weight and the people on it. Two support pillars each push upward with 10 000 N.
Total upward reaction = 20 000 N = total downward load → net force = 0 → the bridge stays in place. Structural engineers calculate these balanced forces to ensure a bridge never accelerates (i.e. collapses).
Where do you see balanced forces? A book on a shelf, a bridge standing still, a plane flying at constant altitude and speed, a boat floating, or a car at steady speed on a motorway — all involve forces that balance to zero net force.
Practice Problems
Two forces act on a box: 40 N right and 25 N left. What is the resultant force and in which direction?
Show answer
A parachutist weighs 800 N downward. Air resistance is 800 N upward. What happens to their speed?