Forces and Friction

El método, paso a paso

1. 1

   Identify every force acting

   weight down, normal contact force up, friction backwards, thrust forwards

   Drawing all the forces as labelled arrows is the first step in any question. Arrow length should represent size, so a diagram alone can show whether the forces are balanced.

2. 2

   Find the resultant force

   thrust 500 N − friction 300 N = 200 N forwards

   Forces in the same direction add and opposing forces subtract. The resultant is the single force that would have the same effect as all of them together, and it determines what the object does.

3. 3

   Apply the balanced or unbalanced rule

   balanced → constant velocity or stationary · unbalanced → acceleration

   Balanced forces do not mean stationary. An object moving at constant speed in a straight line has balanced forces too — a moving object needs no net force to keep moving, which contradicts everyday intuition.

4. 4

   Recognise friction as a contact force

   friction acts between touching surfaces, opposing relative motion

   Friction always opposes the direction of motion or attempted motion. It converts kinetic energy into heat, which is why brakes and rubbed hands warm up.

5. 5

   Explain terminal velocity through forces

   as speed rises, drag rises, until drag = weight → constant velocity

   A falling object accelerates, and air resistance grows with speed. When drag equals weight the forces balance and acceleration stops, leaving a constant speed — terminal velocity. Nothing pushes upward; the forces simply become equal.

Balanced forces do not mean stationary

The most persistent misconception in this topic is that a moving object must have a force pushing it along. It does not. An object moving at constant velocity has balanced forces, and it continues moving because nothing is changing its motion.

This is Newton's first law, and it contradicts everyday experience because friction is always present in ordinary life. Things stop when we stop pushing them because friction is unbalanced, not because motion requires force.

• Force measured in newtons
• Balanced → constant velocity, including zero
• Unbalanced → acceleration in the resultant's direction
• Friction opposes relative motion between surfaces
• Terminal velocity: drag grows until it equals weight

Friction is not simply a nuisance

Friction wastes energy as heat in engines and wears moving parts, which is why it is often reduced with lubricants and bearings. But walking, braking, gripping and writing all depend on it entirely.

Questions frequently ask for both useful and unhelpful examples, and students who have only met friction as a problem struggle with half the answer. Naming both from the start avoids that.

Terminal velocity, step by step

A skydiver leaves the aircraft with weight acting down and almost no air resistance, so acceleration is at its maximum. As speed increases, drag increases. Acceleration therefore decreases, though the diver is still speeding up.

Eventually drag equals weight, the resultant force is zero, and speed becomes constant. Opening a parachute increases drag suddenly, making the resultant upward so the diver decelerates to a new, lower terminal velocity. Describing this sequence in terms of forces is exactly what the mark scheme wants.

How we teach forces

We require a labelled force diagram before any calculation. Most errors in this topic are missing forces rather than wrong arithmetic, and a diagram makes an omission visible in a way that a list of numbers does not.

We tackle the moving-needs-force misconception directly rather than hoping it fades. Asking students to explain why a puck slides across ice for so long surfaces it immediately and gives us something concrete to work with.