Examples and common pitfalls

Pitfall 1: confusing constant speed with the absence of force

A common misconception is to believe that if an object moves at constant speed, no force acts on it. This is false: several forces can act as long as they cancel out. For example, a car traveling at 90 km/h on a highway (constant speed) experiences the driving force of the engine as well as air and road friction, but these effects exactly cancel each other out.

Pitfall 2: the impact with no apparent force

When a car brakes sharply, the passengers are thrown forward. No forward force acted on them: they simply continue their straight-line motion by inertia, while the car itself decelerates. It is the seatbelt that exerts the force needed to slow them down along with the vehicle.

Pitfall 3: a motionless object means no force at all

A book resting on a table is motionless, but it does experience two forces: its weight (downward) and the reaction from the support (upward), which cancel out exactly.

Summary table of pitfalls

Pitfall	Common error	Correction
Constant speed	"No force is acting"	The forces cancel out
Sudden braking	"A force pushes forward"	Inertia of the body, no forward force
Motionless object	"Nothing acts on it"	Weight and reaction cancel out

Everyday application

Wearing a seatbelt, headrests, or sliding on the floor when a bus brakes suddenly are all explained by the principle of inertia.