Balanced Force vs Unbalanced Force — Difference (with Table & FAQs)
Balanced Force vs Unbalanced Force compared side by side — a 6-row comparison table, 4 key points and 7 exam FAQs. Class 9 Physics, free and no sign-up.
TL;DR: Balanced Force vs Unbalanced Force compared side by side — a 6-row comparison table, 4 key points and 7 exam FAQs. Class 9 Physics, free and no sign-u…
Written & reviewed by the Syllab.in Academic Team (CBSE/NCERT subject experts) · Updated
The main difference between Balanced Force and Unbalanced Force: Balanced forces are equal forces acting in opposite directions that cancel out and produce no change in motion, while unbalanced forces are unequal forces
Balanced Force vs Unbalanced Force — Comparison Table
| Basis | Balanced Force | Unbalanced Force |
|---|---|---|
| Magnitude | Equal in magnitude | Not equal in magnitude |
| Direction | Opposite directions | Same or opposite directions |
| Net Force | Zero net force | Non-zero net force |
| Motion Change | No change in motion | Change in velocity or direction |
| Acceleration | Zero acceleration | Non-zero acceleration |
| Example | Tug-of-war with equal strength teams | Tug-of-war with unequal teams |
Key Points
- Balanced forces keep objects at rest or moving at constant velocity
- Unbalanced forces cause changes in motion according to Newton's second law
- The net force determines the acceleration of an object
- For an object to be in equilibrium, all forces must be balanced
Balanced forces — everything cancels
Forces acting on an object are balanced when they are equal in magnitude and opposite in direction, so the net (resultant) force is zero. The object's state of motion does not change: if it was at rest it stays at rest, and if it was moving at constant velocity it keeps moving at that same velocity in the same direction.
Balanced forces cannot make an object accelerate, but they can still do something — they can change its shape. Squeezing a rubber ball equally from both sides does not move it, yet the ball deforms. That is a favourite one-mark question.
Unbalanced forces — something is left over
Forces are unbalanced when they do not cancel, so the net force is not zero. An unbalanced force always produces acceleration: the object starts moving, speeds up, slows down or changes direction.
The crucial point students miss is that acceleration means any change in velocity, not just speeding up. A car braking is accelerating (negatively), and a car going round a bend at constant speed is accelerating too, because its direction is changing. Both need an unbalanced force.
Newton's three laws, and where each one fits
This topic is really an introduction to the laws of motion, so the laws are what the questions test.
- First law (law of inertia) — an object stays at rest or in uniform motion in a straight line unless acted upon by an unbalanced force. This law defines what force does; balanced forces are exactly the case where nothing changes.
- Second law — the rate of change of momentum is proportional to the applied unbalanced force and takes place in its direction. It gives the equation F = ma, which lets you calculate how much acceleration a given unbalanced force produces.
- Third law — to every action there is an equal and opposite reaction. Note carefully that these two forces act on different bodies, so they never cancel each other out. Confusing a third-law pair with balanced forces is the classic error in this chapter.
Inertia, and why mass is the measure of it
Inertia is the tendency of an object to resist any change in its state of rest or of uniform motion. The greater an object's mass, the greater its inertia — which is why mass is called the quantitative measure of inertia.
This is why a loaded truck is harder to start moving and harder to stop than an empty one, and why passengers lurch forward when a bus brakes suddenly: their bodies tend to keep moving even though the bus has slowed.
Friction: the force that hides in every question
A ball rolled across the ground eventually stops. Students often conclude that motion needs a continuous force, which is exactly backwards. The ball stops because friction acts on it as an unbalanced force opposing its motion.
Remove the friction and the ball would keep rolling forever, as Newton's first law says. Whenever a question asks why a moving object slows down on its own, friction (or air resistance) is the unbalanced force to name.
Balanced Force vs Unbalanced Force — Worked Examples
- A book resting on a table — Its weight acts downward and the table's normal reaction acts upward, equal and opposite. Balanced forces — the book stays at rest.
- A tug of war with neither side moving — Both teams pull equally hard in opposite directions. The net force is zero, so the rope does not accelerate.
- The moment one tug-of-war team pulls harder — The forces become unbalanced. There is now a net force, and the rope accelerates towards the stronger team.
- A car turning a corner at steady speed — Unbalanced force. The speed is constant but the direction is changing, so the car is accelerating and a net force must be acting.
Common Mistakes Students Make
- Thinking balanced forces mean the object must be stationary. An object moving at constant velocity also has balanced forces on it.
- Believing a moving object needs a continuous force to keep moving. It only appears that way because friction is constantly opposing it.
- Treating an action-reaction pair as balanced forces. They act on two different bodies, so they never cancel out.
- Assuming acceleration only means speeding up. Slowing down and changing direction are also acceleration.
- Forgetting that balanced forces can still change an object's shape.
Diagrams
Where This Shows Up in Real Life
Seat belts, headrests and inertia
When a car brakes hard, an unbalanced force stops the vehicle but nothing acts on the passenger, whose body continues forward by inertia of motion — which is what a seat belt is for. In a rear-end collision the opposite happens: the seat pushes the body forward while the head lags behind by inertia of rest, which is why headrests exist to prevent whiplash. Both are direct applications of Newton’s first law.
Why rockets work in empty space
A rocket does not push against air. It ejects hot gas backwards at high speed, and by Newton’s third law the gas pushes the rocket forwards with an equal and opposite force. Because this action-reaction pair acts on two different bodies, the forces do not cancel, and the rocket accelerates. That distinction — third-law pairs act on different bodies, balanced forces act on the same body — is the single most confused point in the chapter.
Friction: the force that is always in the answer
A ball rolled across a floor slows and stops, which tempts students to conclude that motion needs a continuous force. The truth is the reverse: friction acts as an unbalanced force opposing the motion. On a near-frictionless surface such as smooth ice the ball would travel far further, and with no friction at all it would never stop. Whenever a question asks why a moving object slows on its own, friction or air resistance is the force to name.
Why a loaded truck is harder to stop
Mass is the quantitative measure of inertia, so a heavily loaded truck resists changes in motion far more than an empty one. It needs a larger unbalanced force to start moving and a larger braking force to stop within the same distance. This is why heavy vehicles have longer stopping distances and stricter speed limits, and it is a practical statement of F = ma rearranged as a = F/m.
Solved Numericals — Step by Step
Problem 1. Two people push a box in opposite directions with forces of 60 N and 45 N. Find the net force and describe the motion. The box has a mass of 5 kg.
- The forces act in opposite directions, so subtract them.
- Net force = 60 − 45 = 15 N, in the direction of the 60 N push.
- The net force is not zero, so the forces are UNBALANCED.
- a = F ÷ m = 15 ÷ 5 = 3 m/s².
Answer: Net force 15 N; the forces are unbalanced and the box accelerates at 3 m/s² in the direction of the larger push.
Problem 2. A force of 25 N acts on a body of mass 10 kg initially at rest. Find its velocity after 4 s and the distance covered.
- a = F ÷ m = 25 ÷ 10 = 2.5 m/s².
- Using v = u + at with u = 0: v = 0 + 2.5 × 4 = 10 m/s.
- Using s = ut + ½at²: s = 0 + ½ × 2.5 × 4² = ½ × 2.5 × 16.
- s = 20 m.
Answer: Velocity 10 m/s after 4 s, having covered 20 m.
Problem 3. A car of mass 1000 kg travelling at 20 m/s is brought to rest in 5 s. Find the retarding force.
- a = (v − u) ÷ t = (0 − 20) ÷ 5 = −4 m/s².
- The negative sign indicates retardation.
- F = ma = 1000 × (−4) = −4000 N.
Answer: 4000 N acting opposite to the motion. The minus sign only shows direction, so quote the magnitude as 4000 N.
Board-Style Questions with Model Answers
Distinguish between balanced and unbalanced forces with one example of each. (3 marks)
Forces are balanced when they are equal in magnitude and opposite in direction, so the resultant or net force acting on the body is zero. A body under balanced forces does not change its state of motion: it stays at rest if it was at rest, or continues moving with the same uniform velocity in a straight line. An example is a book lying on a table, where its weight acting downward is exactly balanced by the normal reaction of the table acting upward. Forces are unbalanced when they do not cancel, so the net force is not zero, and the body accelerates — it starts moving, speeds up, slows down or changes direction. An example is a tug of war in which one team pulls harder than the other, so the rope accelerates towards the stronger team.
State Newton’s first law of motion and define inertia. What is the measure of inertia? (3 marks)
Newton’s first law states that a body continues in its state of rest, or of uniform motion in a straight line, unless it is acted upon by an unbalanced external force. Inertia is the natural tendency of a body to resist any change in its state of rest or of uniform motion. It is the property described by the first law, which is why that law is also called the law of inertia. The quantitative measure of inertia is mass: the greater the mass of a body, the greater its inertia and the harder it is to change its motion. This is why a loaded truck is much harder to start or stop than an empty one.
Explain why passengers fall backward when a bus starts suddenly and forward when it stops suddenly. (3 marks)
Both are consequences of inertia. When a stationary bus starts suddenly, the lower part of a passenger’s body in contact with the bus moves forward with it, but the upper part tends to remain at rest because of inertia of rest, so the passenger falls backward. When a moving bus stops suddenly, the lower part of the body stops with the bus, but the upper part tends to continue moving forward at the original speed because of inertia of motion, so the passenger lurches forward. In each case the unbalanced force acts on the bus, while the passenger’s body changes its state only when a force is applied to it.
A body is moving with a constant velocity. What can you say about the forces acting on it? (2 marks)
Constant velocity means there is no change in either speed or direction, so the acceleration is zero. By Newton’s second law, F = ma, a zero acceleration means the net force acting on the body must also be zero. This does not mean no forces are acting — several may be — but they must be balanced, that is equal and opposite so that they cancel out completely. A car moving at a steady speed on a straight road is a good example: the forward driving force is exactly balanced by friction and air resistance.
State Newton’s third law. Explain why action and reaction forces do not cancel each other. (3 marks)
Newton’s third law states that to every action there is an equal and opposite reaction. The two forces are equal in magnitude and opposite in direction. They do not cancel each other because they act on two different bodies, whereas forces can only cancel if they act on the same body. For example, when you push a wall, you exert a force on the wall and the wall exerts an equal force back on you — one force acts on the wall and the other on you, so neither is cancelled. Balanced forces, by contrast, act on the same body, which is exactly why they produce no acceleration.
Assertion–Reason Questions (CBSE Format)
For each question choose: (a) both A and R are true and R is the correct explanation of A; (b) both are true but R is not the correct explanation; (c) A is true but R is false; (d) A is false but R is true.
1. Assertion (A): A body moving with uniform velocity has no net force acting on it.
Reason (R): Uniform velocity means zero acceleration, and F = ma.
Answer: (a) — Both are true and the reason explains the assertion. Zero acceleration requires zero net force, although individual balanced forces may still be present.
2. Assertion (A): Action and reaction forces cancel each other out.
Reason (R): Action and reaction are equal in magnitude and opposite in direction.
Answer: (d) — The assertion is false — they act on two DIFFERENT bodies, so they never cancel. The reason is a true statement of the third law, which is precisely why this pairing catches so many students.
3. Assertion (A): A moving ball on the ground eventually comes to rest.
Reason (R): A body in motion needs a continuous force to keep moving.
Answer: (c) — The assertion is true but the reason is false and contradicts Newton’s first law. The ball stops because friction acts as an unbalanced force, not because motion requires continuous force.
4. Assertion (A): A heavier body is harder to accelerate than a lighter one under the same force.
Reason (R): Mass is the measure of inertia and a = F/m.
Answer: (a) — Both are true and correctly linked. For a fixed force, acceleration is inversely proportional to mass.
Quick Self-Check
Q1. An object moving at constant velocity in a straight line has:
- A. No forces acting on it
- B. Balanced forces acting on it
- C. An unbalanced force acting on it
- D. Zero inertia
Answer: B. Balanced forces acting on it — Constant velocity means no acceleration, so the net force is zero — the forces are balanced. Forces may still be acting; they simply cancel.
Q2. A ball rolling on the ground slows down and stops because of:
- A. Its own inertia
- B. Balanced forces
- C. Friction acting as an unbalanced force
- D. Gravity
Answer: C. Friction acting as an unbalanced force — Friction opposes the motion and is unbalanced, producing a negative acceleration until the ball stops.
Q3. Which is the correct measure of inertia?
- A. Velocity
- B. Mass
- C. Weight
- D. Acceleration
Answer: B. Mass — The greater the mass, the greater the resistance to a change in the state of motion, so mass is the quantitative measure of inertia.
Q4. A net force of 20 N acts on a 4 kg body. Its acceleration is:
- A. 80 m/s²
- B. 5 m/s²
- C. 0.2 m/s²
- D. 24 m/s²
Answer: B. 5 m/s² — From F = ma, a = F ÷ m = 20 ÷ 4 = 5 m/s².
Key Terms Glossary
- Force
- A push or pull that can change a body’s state of rest or motion, its direction, or its shape. SI unit newton (N).
- Balanced forces
- Forces on one body that are equal and opposite, giving zero net force and no change in motion.
- Unbalanced forces
- Forces that do not cancel, giving a non-zero net force and therefore acceleration.
- Net (resultant) force
- The single force equivalent to all forces acting on a body.
- Inertia
- The tendency of a body to resist a change in its state of rest or uniform motion.
- Mass
- The quantitative measure of inertia. Greater mass means greater resistance to a change in motion.
- Newton’s first law
- A body stays at rest or in uniform straight-line motion unless acted on by an unbalanced force. The law of inertia.
- Newton’s second law
- The rate of change of momentum is proportional to the applied force, giving F = ma.
- Newton’s third law
- To every action there is an equal and opposite reaction, acting on a DIFFERENT body.
- Friction
- The force opposing relative motion between surfaces in contact. Usually the unbalanced force that stops moving objects.
- Momentum
- The product of mass and velocity, p = mv. SI unit kg m/s.
- Retardation
- Negative acceleration — the velocity decreases with time, as during braking.
One-Minute Revision
- BALANCED = equal and opposite, net force ZERO, no change in motion (can still change SHAPE).
- UNBALANCED = net force NOT zero → ACCELERATION every time.
- Balanced forces do NOT mean stationary — uniform velocity also has zero net force.
- Acceleration means ANY change in velocity: speeding up, slowing down, OR changing direction.
- Action-reaction pairs act on DIFFERENT bodies, so they never cancel. Balanced forces act on the SAME body.
- A rolling ball stops because of FRICTION, not because motion needs continuous force.
- Mass is the measure of inertia. F = ma, so a = F/m — heavier means harder to accelerate.
- Newton 1 = inertia, Newton 2 = F = ma, Newton 3 = equal and opposite on two bodies.
Syllabus reference: NCERT Class 9 Science, Chapter 9 — Force and Laws of Motion.
Frequently Asked Questions
What is the difference between balanced and unbalanced forces?
Balanced forces are equal in size and opposite in direction, so they cancel out and do not change an object’s motion. Unbalanced forces do not cancel — the net force is non-zero, so the object’s speed or direction changes.
Do balanced forces cause motion?
No. Balanced forces keep an object at rest or moving at constant velocity; they do not cause acceleration.
What do unbalanced forces do to an object?
Unbalanced forces make an object accelerate — start moving, speed up, slow down or change direction.
Can balanced forces change the shape of an object?
Yes. Balanced forces cannot change an object's state of motion, but they can deform it. Pressing a rubber ball equally from both sides leaves it stationary while clearly changing its shape.
Why do passengers fall forward when a bus stops suddenly?
Because of inertia of motion. The bus is stopped by an unbalanced braking force, but the passengers' bodies tend to continue moving forward at the original speed until a force acts on them.
Are action and reaction forces balanced forces?
No, and this is the most common confusion in the chapter. Action and reaction are equal and opposite but act on two different bodies. Balanced forces act on the same body, which is why they cancel and produce no acceleration.
Does zero net force mean zero velocity?
No. Zero net force means zero acceleration — the velocity does not change. That velocity can be zero (at rest) or any constant value (uniform motion in a straight line).