- Identify common contact and non-contact forces.
- Draw clear free-body diagrams.
- Use resultants and F = ma.
- Apply Newton’s first and third laws correctly.
6.1 Types of force
Forces are interactions that can change an object’s speed, direction or shape. Contact forces include normal contact force, friction, drag, tension and thrust. Non-contact forces include gravitational, electrostatic and magnetic forces. Weight is the gravitational force on an object; it is not the same as mass.
A normal contact force acts perpendicular to a surface. Tension acts along a stretched string or cable. Friction opposes relative motion or attempted motion between surfaces. Drag opposes motion through a fluid such as air or water.
6.2 Free-body diagrams
A free-body diagram isolates one object and shows only the forces acting on that object. Draw the object as a simple box or point, with labelled arrows starting at the object. Do not include forces that the object exerts on other bodies.
Arrow lengths can show relative magnitudes. Balanced vertical forces do not imply balanced horizontal forces. A car moving at constant speed on a level road has zero resultant force: driving force balances resistance, and normal force balances weight.
6.3 Resultant force and Newton’s first law
The resultant is the single force with the same effect as all forces combined. Along one straight line, choose a positive direction and add signed forces. Newton’s first law states that an object remains at rest or moves in a straight line at constant speed unless acted on by a resultant force.
This law links zero resultant force with zero acceleration, not necessarily zero velocity. A spacecraft far from significant forces can continue moving without a continuous driving force.
6.4 Newton’s second-law equation
For constant mass, resultant force F = ma. The acceleration has the same direction as the resultant force. If the same force acts on a larger mass, acceleration is smaller. If the same mass experiences a larger force, acceleration is larger.
Use the resultant force, not one individual force. If an engine provides 5000 N forward and resistive forces total 1800 N backward, the resultant is 3200 N forward.
6.5 Newton’s third law
When object A exerts a force on object B, object B exerts an equal and opposite force on object A. The two forces are the same type, act simultaneously and act on different objects. Because they act on different objects, they do not cancel on one free-body diagram.
A book on a table provides a useful contrast. The table’s upward force and the book’s weight can balance, but they are not a third-law pair because both act on the book and are different types. The third-law partner of the book’s weight is the gravitational force of the book on Earth.
Worked examples
Driving force = 4200 N, resistance = 1700 N, mass = 1000 kg. Resultant = 2500 N, so a = F/m = 2.5 m s⁻².
An 800 kg lift moves upward at constant speed. Resultant force is zero, so cable tension equals weight = 800×9.8 = 7840 N.
A swimmer pushes water backward; the water pushes the swimmer forward with an equal force. These forces act on different objects.
Practical focus
Use a dynamics trolley and hanging mass. Keep total mass constant while changing the pulling force, then keep force constant while changing total mass. Discuss how the measured acceleration supports F ∝ a and a ∝ 1/m.
Examination guidance
- Begin with a force diagram; many calculation errors come from using the wrong resultant.
- Third-law pairs act on different objects. Balanced forces act on the same object.
- Constant speed in a straight line means zero resultant force even when several forces are present.
Check your understanding
- A 12 N force acts right and a 7 N force acts left. Resultant?
- What is the acceleration when resultant force is zero?
- Why do action and reaction not cancel?
- 5 N right.
- Zero.
- They act on different objects.
