Balanced and Unbalanced Forces
In 2011, engineers at Boeing showed the 777X wing flexes 7 metres before breaking — perfectly balanced lift and gravity forces keep it level at 39,000 feet.
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A book sits on a table and does not move. Is the book experiencing any forces? Explain your thinking.
When you walk, your foot pushes backward on the ground. What pushes you forward? Why does this happen?
● Know
- Balanced forces produce no change in motion.
- Unbalanced forces produce acceleration.
- Free-body diagrams show all forces acting on an object.
● Understand
- The net force is the vector sum of all forces acting on an object.
● Can do
- Draw simple free-body diagrams and determine whether forces are balanced or unbalanced.
Place a textbook flat on your desk and press down on it firmly: you feel the desk pushing back up against your hand with exactly as much force as you apply. That push-back from the desk surface is always there — even when you remove your hand, the book still presses down due to gravity and the desk still pushes up with an equal and opposite normal force. Because both forces are equal in magnitude, the net force is zero, and according to Newton's First Law, zero net force means zero acceleration — the book stays exactly where it is.
Students often ask: if the forces are equal and opposite, why do they not cancel each other completely and make the book weightless? The answer is that forces only cancel when they act on the same object. Gravity acts on the book. The normal force also acts on the book. Because both act on the same object, they do cancel in terms of net force. But the book still has mass and would accelerate downward if the table were removed.
Newton Third Law action-reaction pairs are different from balanced forces. The table pushes up on the book (normal force), and the book pushes down on the table (also a normal force). These are equal and opposite, but they act on different objects, so they do not cancel. They are a Third Law pair, not balanced forces on a single object.
When you stand on a bathroom scale, gravity pulls you down with force mg. The scale pushes up on you with a normal force. If you are stationary, these forces are balanced and the scale reads your weight (mg). If you crouch down and then suddenly stand up, you briefly accelerate upward. During that acceleration, the normal force exceeds your weight, and the scale reads more than your actual weight. If you jump, the scale reads zero during the brief moment when you are not in contact with it. These changes demonstrate that normal force is not always equal to weight - it adjusts to produce whatever acceleration the situation requires.
Australian structural engineering: Engineers Australia certifies structural engineers who design buildings, bridges, and towers. Every structural element must withstand the forces acting on it, including gravity, wind loads, earthquake forces, and live loads from occupants and vehicles. The Sydney Harbour Bridge, for example, must support its own weight (gravity), the weight of trains and cars (live load), and horizontal forces from wind. The bridge design ensures that at every point, internal forces balance external forces, preventing collapse.
If forces are balanced, nothing is happening. This is false. Balanced forces mean no acceleration, but motion can still occur. A plane flying at constant altitude and constant speed has balanced forces (lift = weight, thrust = drag), but it is clearly moving. A book sliding across a frictionless ice rink at constant velocity has balanced forces (zero horizontal force), but it is in motion. Balanced forces mean constant velocity, not necessarily rest.
Place a heavy book on a table. The book exerts a downward force on the table due to gravity. Yet the book does not accelerate downward. Predict: what force balances gravity, and where does it come from?
The table exerts an upward normal force equal in magnitude to the book weight. These forces are balanced, so the net force is zero and the book remains at rest.
Use these terms in your explanation: normal force · balanced · net force · gravity
Newton Third Law of Motion states: For every action, there is an equal and opposite reaction. This is one of the most counterintuitive laws in physics because the action and reaction forces never act on the same object, so they never cancel each other.
When you walk, your foot pushes backward on the ground (action). The ground pushes forward on your foot with equal force (reaction). It is the reaction force that propels you forward, not the action force. The action force acts on the ground, so it cannot move you.
When a rocket launches, it pushes exhaust gases downward at high speed (action). The gases push the rocket upward with equal force (reaction). Rockets do not push against the ground or the air - they work in the vacuum of space by the same principle.
When a book rests on a table, the book pushes down on the table (action), and the table pushes up on the book (reaction). These are equal and opposite, but they act on different objects, so they do not cancel.
A swimmer pushes water backward with their hands and feet (action). The water pushes the swimmer forward with equal force (reaction). A skilled swimmer maximises this reaction force by catching as much water as possible and pushing it backward efficiently. Competitive swimmers use techniques like the dolphin kick and high-elbow catch to optimise the action-reaction interaction with the water. Australian swimmers like Ian Thorpe and Ariarne Titmus mastered these techniques to generate extraordinary propulsive forces.
Australian space technology: Equatorial Launch Australia operates the Arnhem Space Centre near Nhulunbuy in the Northern Territory - Australia first commercial spaceport. Rocket launches there exploit Newton Third Law on a massive scale, expelling tonnes of exhaust gases to achieve orbit. The site proximity to the equator gives rockets an additional speed boost from Earth rotation, reducing fuel requirements. Understanding action-reaction dynamics is fundamental to rocket engineering.
Action-reaction forces cancel each other out. This is false because they act on different objects. The action force acts on Object B; the reaction force acts on Object A. Since they act on different objects, they cannot cancel. If they acted on the same object, they would cancel, but then neither object would accelerate and no motion would ever change. The fact that action-reaction pairs act on different objects is what makes motion possible.
Newton Second Law is the quantitative relationship between force and motion: F_net = ma. This simple equation connects three of the most important quantities in mechanics: force (F), mass (m), and acceleration (a).
The law tells us that acceleration is directly proportional to net force and inversely proportional to mass. Doubling the net force on an object doubles its acceleration. Doubling the mass of an object halves its acceleration for the same force.
This equation is a vector equation, meaning direction matters. If forces act in different directions, you must add them as vectors (considering direction) to find the net force before calculating acceleration.
The unit of force, the newton (N), is defined using this law: 1 N is the force required to accelerate 1 kg at 1 m/s2. This makes the equation dimensionally consistent: N = kg * m/s2.
A sprinter of mass 70 kg accelerates from rest to 10 m/s in 2 seconds. Average acceleration = (10 - 0) / 2 = 5 m/s2. Average net force = 70 * 5 = 350 N. This force comes from the reaction force of the track pushing against the sprinter feet. Elite sprinters like Usain Bolt generated peak forces of over 1,000 N during acceleration. For comparison, the sprinter weight (gravity force) is about 686 N. The propulsive force from the track is about half their body weight - a remarkable feat of muscular power.
Australian sports science: The Australian Institute of Sport (AIS) uses force plates to measure the ground reaction forces generated by elite athletes. These measurements quantify how effectively athletes apply Newton Second Law. Force plate data helps coaches optimise technique in sports like athletics, weightlifting, and volleyball. Australian swim coaches also use force sensors on starting blocks and underwater cameras to analyse the forces swimmers generate during starts and turns.
F = ma means force causes mass times acceleration. This is a subtle linguistic trap. The equation is a relationship, not a causal statement in that direction. In many situations, acceleration is the given quantity and force is what we calculate. For example, in circular motion, the acceleration (toward the centre) is determined by the geometry and speed, and the force (tension, gravity, or normal force) adjusts to provide exactly that acceleration. The equation describes a constraint, not a one-directional causality.
1. Draw a free-body diagram for a stationary book on a table, labelling all forces and showing that they are balanced.
2. A car accelerates forward. Explain what this tells you about the forces acting on the car.
- Forgetting to include all forces in a free-body diagram. — Always consider gravity, normal force, friction, air resistance, and any applied or tension forces. Missing one force can lead to incorrect conclusions.
- Assuming that equal forces in opposite directions on different objects are balanced forces. — Balanced forces must act on the SAME object. Action-reaction pairs (Newton's third law) act on different objects and are not balanced forces.
📓 Copy Into Your Books
▼Balanced Forces
Balanced forces are equal and opposite. Net force = 0. No change in motion occurs.
Unbalanced Forces
Unbalanced forces do not cancel out. Net force is not zero. The object accelerates in the direction of the net force.
Free-Body Diagrams
A free-body diagram shows all forces acting on an object as arrows. The length of the arrow represents the size of the force.
Net Force
Net force (or resultant force) is the vector sum of all forces acting on an object.
You learned that balanced forces cause no change in motion, while unbalanced forces cause acceleration.
A parachutist falls at a constant speed after their parachute opens. Are the forces on them balanced or unbalanced? Explain.
The hook described a surfer at constant speed: gravity and the wave's upward push perfectly balanced. The moment those forces become unbalanced — the surfer accelerates, turns, or wipes out.
Now that you can draw and analyse force diagrams, how would you sketch the forces on that surfer in each situation: gliding steadily, accelerating down the wave, and wiping out? Did the surfing example make balanced and unbalanced forces easier to picture?
1. What is the net force when forces are balanced?
2. An object moving at constant velocity must have:
3. Which force opposes the motion of objects through air?
4. A skydiver reaches terminal velocity when:
5. In a free-body diagram, what does the length of an arrow represent?
Draw a free-body diagram for a car driving at constant speed on a level road. Label all forces and explain why the car does not accelerate. (3 marks)
Hint: Consider forward driving force, air resistance, friction, gravity, and normal force.
Explain the difference between balanced and unbalanced forces, and describe what happens to an object's motion in each case. (3 marks)
Hint: Use the terms net force and acceleration in your answer.
A student claims that if an object is moving, there must be a force keeping it moving. Explain why this statement is incorrect. (3 marks)
Hint: Consider what happens when forces are balanced and the object is already in motion.