Every time you push a door open, catch a ball, or sit in a chair, forces are at work. But what exactly is a force? And how do multiple forces combine to produce motion — or prevent it?
Think about standing still on the ground.
Write down your answers before reading on:
The foundation of everything that moves — or stays still
A force is a push or a pull acting on an object. Forces are measured in newtons (N), named after Isaac Newton.
Every force has two important properties:
Because forces have both size and direction, they are vector quantities. A force of 10 N to the right is different from a force of 10 N to the left.
The forces you encounter every day
We encounter many different types of forces in everyday life:
Why some objects move and others do not
When all the forces acting on an object cancel out, the net force is zero. These are called balanced forces. An object with balanced forces either stays still or continues moving at a constant speed in a straight line.
When forces do not cancel out, there is an unbalanced force or net force. This causes the object to accelerate — it speeds up, slows down or changes direction.
Example: A book sitting on a table has two balanced forces — gravity pulls down, the normal force pushes up. The book does not move. If you push the book sideways and overcome friction, the forces become unbalanced and the book accelerates.
Visualising forces to predict motion
A force diagram (or free-body diagram) is a simple drawing showing all the forces acting on an object. Each force is represented by an arrow:
Drawing force diagrams helps us visualise whether forces are balanced or unbalanced, and predict what will happen to the object.
"Force is a type of energy." No — force is a push or pull, not energy itself. However, forces can transfer energy.
"If an object is moving, there must be a force keeping it moving." No — forces change motion; they do not maintain it. On Earth, friction is what slows moving objects.
Australian surf lifesaving: Surf lifesavers understand forces when using rescue boards and jet skis. They must balance thrust, drag and lift to move efficiently through water. The Royal Life Saving Society Australia trains thousands of lifeguards each year in water rescue techniques that rely on understanding how forces act on the human body in moving water.
Vehicle safety: Australian vehicle safety standards (ANCAP) test how cars respond to forces in crashes. Understanding balanced and unbalanced forces helps engineers design crumple zones, airbags and seatbelts that protect passengers.
Aboriginal boomerang: The returning boomerang is a remarkable application of aerodynamic forces. Its curved shape and angled wings create lift and torque forces that make it fly in an arc and return to the thrower — a technology developed by Aboriginal Peoples over thousands of years.
1. Which of the following is a vector quantity?
2. What happens when all forces on an object are balanced?
3. Which force prevents you from falling through the floor?
4. A car moves at constant speed on a straight road. What can you say about the forces on the car?
5. What is the SI unit of force?
1. Explain the difference between balanced and unbalanced forces. Use an example in your answer. 4 MARKS
2. Draw a force diagram for a book resting on a table. Label each force and explain why the book does not move. 4 MARKS
3. Describe two ways friction is helpful in everyday life and two ways it is a hindrance. 4 MARKS
Go back to your Think First answer. Has your understanding changed?
C — Force is a vector because it has both magnitude (size) and direction. Speed, distance and mass are scalar quantities.
D — When forces are balanced, the net force is zero. According to Newton's first law, the object will not accelerate — it stays at rest or continues at constant velocity.
C — The normal force is the force exerted by a surface to support the weight of an object resting on it. It acts perpendicular to the surface.
B — At constant speed on a straight road, the forward forces (engine) balance the backward forces (friction and air resistance), and the upward forces (normal) balance the downward forces (gravity).
C — The newton (N) is the SI unit of force. One newton is the force needed to accelerate a 1 kg mass at 1 m/s².
Model answer: Balanced forces are equal in size and opposite in direction, producing no change in motion. Unbalanced forces are not equal, causing acceleration. For example, a book on a table has balanced forces: gravity pulls down and the normal force pushes up with equal size. If you push the book sideways with a force greater than friction, the forces become unbalanced and the book accelerates across the table.
Model answer: A force diagram for a book on a table shows two arrows of equal length: one pointing down labelled "gravity (weight)" and one pointing up labelled "normal force." The book does not move because these two forces are balanced — they cancel each other out. There is no net force, so according to Newton's first law, the book remains at rest.
Model answer: Friction is helpful because it allows us to walk (our feet push backward, friction pushes us forward), grip objects, and stop vehicles safely. Friction is a hindrance because it causes wear on machine parts, reduces fuel efficiency in cars by opposing motion, and can generate unwanted heat. Engineers use lubricants and smooth surfaces to reduce harmful friction while maintaining beneficial friction where needed.
Navigate through force fields and test your knowledge! Dodge obstacles and answer force questions to boost your score.
Tick when you have finished all activities and checked your answers.