Mistakes Students Make in IGCSE Physics Forces Questions

Forces and motion is the largest section of the IGCSE Physics syllabus. These are the errors that cost marks repeatedly.

Mistake 1: Net force means no force

Students say “there is no force acting on the object” when it moves at constant velocity. The correct statement is “the resultant (net) force is zero.” Forces still act on the object; they balance each other.

Mistake 2: Force is needed to keep moving

Newton’s first law: an object continues at constant velocity unless a resultant force acts. Students often state that a force is needed to maintain constant speed. In reality, a force is only needed to overcome friction or air resistance.

Mistake 3: Confusing mass and weight

Mass is the amount of matter, measured in kg. Weight is the gravitational force, measured in N. Weight = mass x gravitational field strength. Mass does not change with location; weight does.

Mistake 4: Action-reaction pairs on the same object

Newton’s third law states that forces come in pairs that act on two different objects. Students often give both forces acting on the same object (e.g., weight and normal contact force on a book). The correct pair for the weight of the book is the gravitational pull of the book on the Earth.

Mistake 5: Area of the velocity-time graph is speed

The area under a velocity-time graph is distance (or displacement), not speed. Students sometimes state that the area gives speed.

Mistake 6: Acceleration means going fast

Acceleration is the rate of change of velocity, not the same as high speed. An object can have a high speed but zero acceleration (constant velocity), or a low speed but high acceleration (just starting from rest).

Mistake 7: Moment = force x distance (not perpendicular distance)

The correct formula is moment = force x perpendicular distance from the pivot. If the force is not perpendicular to the distance line, you must resolve the force or use the perpendicular component. Using the straight-line distance gives the wrong answer.

Mistake 8: Forgetting to convert units

Calculating speed in m/s when the data is given in km and hours. Convert everything to SI units before substituting: km to m, hours to seconds.