Forces and Their Effects
In 2022, Engineers Australia found the Sydney Harbour Bridge supports 1.15 million tonnes, its steel arches balancing gravitational and compression forces every second.
A book is resting on a table. List all the forces you think are acting on the book. Are they balanced or unbalanced? How do you know?
Two people push a heavy box from opposite sides with equal force. The box does not move. Does this mean no forces are acting on it? Explain why balanced forces can still exist without causing movement.
● Know
- What a force is and how to describe it
- The difference between balanced and unbalanced forces
- How to identify different types of forces
● Understand
- Why forces are vectors (have direction)
- How net force determines motion
- That forces always occur in pairs
● Can do
- Identify forces acting on an object
- Determine whether forces are balanced or unbalanced
- Draw simple force diagrams
Kick a football resting on the grass and it shoots forward; the same kick applied to a 10 kg medicine ball barely moves it. Push a shopping trolley with a broken wheel and it veers sideways instead of going straight. Every one of those outcomes is caused by a forceany interaction that, when unopposed, changes the motion of an object. Forces can start things moving, stop them, speed them up, slow them down, change their direction, or deform them by stretching, compressing, or twisting.
Forces are vectors - they have both magnitude (how strong) and direction (which way). A 10 N force upward is different from a 10 N force downward. The unit of force is the newton (N), defined as the force needed to accelerate 1 kg at 1 m/s².
Contact forces require physical contact: friction, normal force, tension, applied force.
Non-contact forces act at a distance: gravity, magnetism, electrostatic force.
Balanced forces occur when forces cancel out, producing no acceleration. Unbalanced forces produce acceleration in the direction of the net force.
A book on a table experiences two forces: gravity pulls downward (weight = mg), and the table pushes upward (normal force). These forces are equal and opposite, so they balance. The book does not accelerate - it remains stationary. But the forces are NOT an action-reaction pair (those act on different objects). The reaction to the table upward force on the book is the book downward force on the table. The reaction to gravity pulling the book down is the book pulling Earth up (immeasurably small). Understanding that balanced forces are not action-reaction pairs is a crucial distinction.
Australian engineering standards: Australian engineers use the International System of Units (SI), with forces measured in newtons. The Standards Australia organisation publishes codes for structural engineering that specify how buildings and bridges must withstand forces from wind, earthquakes, and traffic. The Sydney Harbour Bridge was designed to withstand forces from its own weight, traffic loads, wind, and temperature changes. Modern Australian infrastructure uses computer modelling to predict how forces distribute through complex structures, ensuring safety while minimising material use.
If an object is not moving, no forces are acting on it. This is false. The book on the table has multiple forces acting on it - gravity downward and the normal force upward. These forces balance to produce zero net force, so the book does not accelerate. But forces are definitely present. Similarly, a car moving at constant velocity has balanced forces: engine thrust equals air resistance and friction. Motion does not require unbalanced forces; acceleration does. This is Newton first law in action.
Classify each situation as having balanced or unbalanced forces.
Gravity is the attractive force between any two objects with mass. Near Earth surface, gravitational field strength (g) is approximately 9.8 N/kg (often rounded to 10 N/kg for calculations).
Mass is the amount of matter in an object, measured in kilograms. Mass is invariant - it does not change with location.
Weight is the gravitational force on an object: W = mg. Weight depends on the local gravitational field strength. On the Moon, g ≈ 1.6 N/kg, so a 70 kg astronaut weighs 112 N instead of 686 N on Earth. In deep space, far from any massive object, weight is nearly zero (weightlessness), but mass remains 70 kg.
Weight vs mass confusion: In everyday language, we say "I weigh 70 kg." This is technically incorrect - we should say "My mass is 70 kg" or "I weigh 686 N." But the kilogram is so commonly used for weight that the distinction is often ignored in casual conversation. In physics, the distinction is essential.
A 1,000 kg satellite in orbit around Earth has the same mass as on the surface, but its weight is different. In low Earth orbit (400 km altitude), g ≈ 8.7 N/kg, so the satellite weighs 8,700 N instead of 9,800 N on the surface. Astronauts in orbit feel weightless not because there is no gravity (g is still about 90% of surface value), but because they and their spacecraft are in free fall together. The satellite is constantly falling toward Earth, but its horizontal speed is so high that it falls around Earth rather than into it. This continuous free fall creates the sensation of weightlessness.
Australian space research: Australia emerging space industry includes companies developing satellite technologies and launch services. The Australian Space Agency coordinates national space activities. Understanding weight, mass, and gravity is essential for spacecraft design. Australian researchers study how reduced gravity affects plant growth, fluid behaviour, and human physiology - relevant for future long-duration space missions. The Woomera Test Range in South Australia has been used for rocket testing since the 1950s and remains an important space facility.
Weight and mass are the same thing. This is false, though the confusion is understandable because we use the same units incorrectly in everyday life. Mass is an intrinsic property of matter; weight is a force that depends on gravity. Your mass is the same on Earth, the Moon, or in space. Your weight changes. A scale actually measures the normal force supporting you, which equals your weight in equilibrium. In an accelerating elevator, the scale reading changes even though your mass is constant. Understanding this distinction is fundamental to physics.
Several important contact forces appear in everyday situations:
Friction: The force that opposes relative motion between surfaces in contact. Static friction prevents stationary objects from moving; kinetic friction acts on moving objects. Friction depends on the nature of the surfaces and the normal force pressing them together, not on contact area. Friction is essential for walking, driving, and holding objects.
Normal force: The perpendicular force exerted by a surface on an object in contact with it. It prevents objects from passing through surfaces. On a horizontal surface, normal force equals weight. On a slope, normal force is less than weight because only the perpendicular component of weight is balanced.
Air resistance (drag): The force opposing motion through air. It increases with speed and depends on object shape and cross-sectional area. At low speeds, drag is approximately proportional to speed. At high speeds, it is proportional to speed squared.
A skydiver initially accelerates at nearly g because air resistance is small at low speed. As speed increases, air resistance grows. Eventually, air resistance equals weight, and the skydiver reaches terminal velocity - about 200 km/h in a spread-eagle position, or 300 km/h in a head-down dive. Opening the parachute dramatically increases drag area, reducing terminal velocity to about 20 km/h - slow enough for a safe landing. The physics is straightforward: terminal velocity occurs when drag force equals weight. A larger drag area or higher drag coefficient means lower terminal velocity.
Australian sports aerodynamics: Australian Institute of Sport (AIS) researchers study drag and aerodynamics for cycling, swimming, and athletics. Cyclists in time trials adopt extreme positions to minimise drag area, sometimes using disk wheels and aero helmets. Swimmers wear full-body suits designed to reduce drag and improve flow over the body. In AFL, the Sherrin football shape creates complex aerodynamics that allow players to kick torpedo punts that travel further than spherical balls. Understanding fluid forces gives Australian athletes competitive advantages at international events.
Friction depends on contact area. This is false for dry friction between solid surfaces. A brick sliding on a table has the same friction whether standing on end or lying flat, because while contact area changes, pressure (normal force per area) changes inversely, and the product (normal force) stays the same. Friction = μN, where μ is the coefficient of friction and N is normal force. Area does not appear in this equation. However, for rolling friction, fluid drag, and some special cases, area does matter. The independence of area is a specific property of Amontons-Coulomb dry friction.
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:
- The length of the arrow shows the size of the force
- The direction of the arrow shows which way the force acts
Drawing force diagrams helps us visualise whether forces are balanced or unbalanced, and predict what will happen to the object.
On a force diagram, what does the length of an arrow represent?
Wrong: "Force is a type of energy." No, force is a push or pull, not energy itself. However, forces can transfer energy.
Right: Force and energy are different physical quantities. Force is a push or pull (measured in newtons); energy is the capacity to do work (measured in joules). Forces can do work and transfer energy, but the two concepts are not the same thing.
Wrong: "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.
Right: Forces cause changes in motion (acceleration), but an object moving at constant speed needs no net force to keep moving, Newton's first law. On Earth, friction acts as an unbalanced force that slows things down; without it, they would continue moving indefinitely.
Wrong: "Forces always cause motion." No, balanced forces produce no motion at all. A book on a table has forces but does not move.
Right: Forces only cause motion when they are unbalanced. When all forces on an object are balanced (equal in size and opposite in direction), the net force is zero and the object stays still or keeps moving at the same speed. A book on a table is held in place by balanced gravity and normal force.
Forces in Australian Context
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.
✍ Copy Into Your Books
▾What Is a Force?
- A push or pull acting on an object
- Measured in newtons (N)
- Has magnitude and direction
Types of Forces
- Gravity, normal force, friction, tension, applied force, air resistance
- Each acts in a specific direction
- Forces can be contact or non-contact
Balanced vs Unbalanced
- Balanced: net force = 0, no acceleration
- Unbalanced: net force ≠ 0, acceleration occurs
- Force diagrams show all forces on an object
Force Identification
Balanced and Unbalanced Forces Scenarios
At the start of this lesson you were shown a book sitting on a desk being pulled down by gravity with about 10 N and pushed back up with exactly 10 N by the desk, four forces acting at once while nothing moves.
Now that you've worked through the lesson, how has your thinking shifted? Can you explain that hook idea more precisely using what you've learned today?
Q1. 1. Explain the difference between balanced and unbalanced forces. Use an example in your answer. 4 MARKS
Q2. 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
Q3. 3. Describe two ways friction is helpful in everyday life and two ways it is a hindrance. 4 MARKS
Revisit Your Thinking
Go back to your Think First answer. Has your understanding changed?
- Can you now list all the forces acting on a stationary object and explain why it does not move?
- How would you explain to a friend why a moving object does not need a force to keep it moving?
Model answers (click to reveal)
Answers
▾MCQ 1
CForce is a vector because it has both magnitude (size) and direction. Speed, distance and mass are scalar quantities.
MCQ 2
DWhen 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.
MCQ 3
CThe 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.
MCQ 4
BAt 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).
MCQ 5
CThe newton (N) is the SI unit of force. One newton is the force needed to accelerate a 1 kg mass at 1 m/s².
Short Answer 1
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.
Short Answer 2
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.
Short Answer 3
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.