A Car at Steady Speed Has No Net Force Acting On It At All
Learn what a force can do to an object, how a spring balance measures it in newtons, how to add forces to find the net force, and where friction helps and where it hinders.
If a car is moving, must a force be acting on it?
Not a net force, no — and that surprises most students.
A car cruising along a straight road at a steady km/h has plenty of forces on it. The engine drives it forward; friction and air resistance push back; gravity pulls it down; the road pushes it up. But those forces cancel, and the net force is zero.
That is exactly why the speed is steady. A net force does not maintain motion — it changes motion. Zero net force means no change, which for a moving car means it keeps moving at the same speed in the same direction.
So the question to ask of any object is not is a force acting? but do the forces balance? Everything in this page turns on that distinction. It covers the first part of the CBSE Class 9 Science chapter on how forces affect motion.
A car cruising along a straight road at a steady km/h has plenty of forces on it. The engine drives it forward; friction and air resistance push back; gravity pulls it down; the road pushes it up. But those forces cancel, and the net force is zero.
That is exactly why the speed is steady. A net force does not maintain motion — it changes motion. Zero net force means no change, which for a moving car means it keeps moving at the same speed in the same direction.
So the question to ask of any object is not is a force acting? but do the forces balance? Everything in this page turns on that distinction. It covers the first part of the CBSE Class 9 Science chapter on how forces affect motion.
What can a force actually do to an object?
A force is a push or a pull, and it can produce five different effects.
- Make a stationary object move — kicking a football at rest
- Stop a moving object — a goalkeeper catching that football
- Change the speed of a moving object — pedalling a cycle harder
- Change the direction of motion — a batsman deflecting a ball down the leg side
- Change the shape or size of an object — pressing chapati dough flat, or squeezing a sponge
Force is a vector: it has both a magnitude and a direction, and the direction matters as much as the size. A push of N eastwards and a push of N westwards are quite different forces.
Everyday evidence. A potter shaping clay on a wheel is using force only for the fifth effect — the lump does not move anywhere, but its shape changes completely. A cyclist braking is using force for the second and third.
One force can produce more than one effect at the same time. Hitting a cricket ball with a bat changes its direction, changes its speed, and squashes it slightly for an instant — three effects from one blow. So a question asking what effect does this force have? may well have more than one correct answer, and listing all of them is what earns full marks.
A force always needs an agent. Something has to do the pushing or pulling — a hand, a magnet, the Earth's gravity, a stretched spring. Contact forces such as friction and muscular force require the objects to touch; non-contact forces such as gravity, magnetic and electrostatic forces act across a distance. If you cannot name what is exerting a force, it is worth asking whether there is one.
- Make a stationary object move — kicking a football at rest
- Stop a moving object — a goalkeeper catching that football
- Change the speed of a moving object — pedalling a cycle harder
- Change the direction of motion — a batsman deflecting a ball down the leg side
- Change the shape or size of an object — pressing chapati dough flat, or squeezing a sponge
Force is a vector: it has both a magnitude and a direction, and the direction matters as much as the size. A push of N eastwards and a push of N westwards are quite different forces.
Everyday evidence. A potter shaping clay on a wheel is using force only for the fifth effect — the lump does not move anywhere, but its shape changes completely. A cyclist braking is using force for the second and third.
One force can produce more than one effect at the same time. Hitting a cricket ball with a bat changes its direction, changes its speed, and squashes it slightly for an instant — three effects from one blow. So a question asking what effect does this force have? may well have more than one correct answer, and listing all of them is what earns full marks.
A force always needs an agent. Something has to do the pushing or pulling — a hand, a magnet, the Earth's gravity, a stretched spring. Contact forces such as friction and muscular force require the objects to touch; non-contact forces such as gravity, magnetic and electrostatic forces act across a distance. If you cannot name what is exerting a force, it is worth asking whether there is one.
How do you measure a force, and in what unit?
With a spring balance, in newtons.
A spring balance works because a spring stretches in proportion to the force pulling on it. Twice the force gives twice the extension, so a scale marked alongside the spring reads the force directly.
The SI unit of force is the newton, symbol N. One newton is the force that gives a mass of kg an acceleration of , so
Worked example 1. A kg mass hangs from a spring balance. What force does the balance read?
The force is the object's weight, so taking :
Worked example 2. A spring balance is marked in kilograms and reads kg. What force is it actually measuring?
With the answer is N. A balance marked in kilograms is a convenience for shoppers, not a statement that force is measured in kilograms.
Worked example 3. A force of N stretches a spring by cm. How far will N stretch it?
Since the extension is proportional to the force:
A spring balance measures force; a beam balance measures mass. That is the distinction this section exists for. Take both to the Moon and the spring balance reading changes, because the weight of the object is less there. The beam balance reading does not, because it compares the unknown mass against standard masses, and both are affected equally.
So mass and weight are different quantities, as the opening chapter of this course established: mass in kilograms does not change with location, while weight in newtons does. A question asking for a weight expects an answer in newtons, and giving it in kilograms is the commonest unit error in the whole of mechanics.
A spring balance works because a spring stretches in proportion to the force pulling on it. Twice the force gives twice the extension, so a scale marked alongside the spring reads the force directly.
The SI unit of force is the newton, symbol N. One newton is the force that gives a mass of kg an acceleration of , so
Worked example 1. A kg mass hangs from a spring balance. What force does the balance read?
The force is the object's weight, so taking :
Worked example 2. A spring balance is marked in kilograms and reads kg. What force is it actually measuring?
With the answer is N. A balance marked in kilograms is a convenience for shoppers, not a statement that force is measured in kilograms.
Worked example 3. A force of N stretches a spring by cm. How far will N stretch it?
Since the extension is proportional to the force:
A spring balance measures force; a beam balance measures mass. That is the distinction this section exists for. Take both to the Moon and the spring balance reading changes, because the weight of the object is less there. The beam balance reading does not, because it compares the unknown mass against standard masses, and both are affected equally.
So mass and weight are different quantities, as the opening chapter of this course established: mass in kilograms does not change with location, while weight in newtons does. A question asking for a weight expects an answer in newtons, and giving it in kilograms is the commonest unit error in the whole of mechanics.
Formula
How do you find the net force, and when are forces balanced?
Add the forces with their directions. Along one line:
- Balanced forces: . There is no change in motion — the object stays at rest, or keeps moving at constant velocity.
- Unbalanced forces: . The object accelerates in the direction of the net force.
Worked example 1 — same direction. Two boys push a box, one with N east and the other with N east.
Worked example 2 — opposite directions. One pushes with N east, the other with N west.
The box accelerates eastwards, towards the stronger push.
Worked example 3 — balanced. Both push with N in opposite directions.
The forces are balanced and the box does not start moving.
Worked example 4 — a tug of war. Team A pulls with N and team B with N.
A small net force from two large opposing ones, which is why a tug of war can look motionless and then move suddenly.
Worked example 5 — with friction. A box is pushed with N while friction opposes it with N.
The box accelerates. Reduce the push to N and the net force becomes zero, so the box either stays still or continues at constant speed — it does not stop of its own accord.
Worked example 6 — a box on a table. A box of weight N rests on a table, which pushes up with N.
Balanced, so the box stays at rest. If the table could push up with only N it would collapse, and the net N downward would accelerate the box towards the floor.
Balanced forces do not mean no motion — they mean no change in motion. The cruising car in the opening section has perfectly balanced forces and is travelling at km/h. A box at rest on a table also has balanced forces. Both are unchanging states, and that is what balance guarantees.
Only the net force produces acceleration. When a question gives several forces, add them first. Using one of the individual forces in a later calculation is the standard error, and it is the reason the net force earns a section of its own before any equation of motion appears.
- Balanced forces: . There is no change in motion — the object stays at rest, or keeps moving at constant velocity.
- Unbalanced forces: . The object accelerates in the direction of the net force.
Worked example 1 — same direction. Two boys push a box, one with N east and the other with N east.
Worked example 2 — opposite directions. One pushes with N east, the other with N west.
The box accelerates eastwards, towards the stronger push.
Worked example 3 — balanced. Both push with N in opposite directions.
The forces are balanced and the box does not start moving.
Worked example 4 — a tug of war. Team A pulls with N and team B with N.
A small net force from two large opposing ones, which is why a tug of war can look motionless and then move suddenly.
Worked example 5 — with friction. A box is pushed with N while friction opposes it with N.
The box accelerates. Reduce the push to N and the net force becomes zero, so the box either stays still or continues at constant speed — it does not stop of its own accord.
Worked example 6 — a box on a table. A box of weight N rests on a table, which pushes up with N.
Balanced, so the box stays at rest. If the table could push up with only N it would collapse, and the net N downward would accelerate the box towards the floor.
Balanced forces do not mean no motion — they mean no change in motion. The cruising car in the opening section has perfectly balanced forces and is travelling at km/h. A box at rest on a table also has balanced forces. Both are unchanging states, and that is what balance guarantees.
Only the net force produces acceleration. When a question gives several forces, add them first. Using one of the individual forces in a later calculation is the standard error, and it is the reason the net force earns a section of its own before any equation of motion appears.
Where is friction useful and where is it a nuisance?
Friction is the force that opposes relative motion between two surfaces in contact, and it acts along the surfaces, in the direction opposite to the motion or the attempted motion.
It arises because no surface is perfectly smooth. The irregularities of the two surfaces interlock, and moving one over the other means dragging those irregularities past each other.
What friction depends on:
- The nature and roughness of the two surfaces
- The force pressing them together — a heavier box is harder to push
Three kinds, in order of size.
- Static friction acts before motion begins, when a force is applied but the object has not yet moved
- Sliding friction acts while one surface slides over another
- Rolling friction acts when one surface rolls over another, and is the smallest of the three
So static friction is greater than sliding friction, which is greater than rolling friction. That order explains two familiar experiences: a heavy box is hardest to get started and easier to keep moving once it is going, and putting a load on wheels makes it far easier to move than dragging it.
Where friction is useful:
- Walking — without it your foot would slide back with every step
- Brakes on a cycle or car
- Writing with a pencil, and striking a match
- Tyres gripping the road, especially in a turn
- Holding a glass without it slipping, and nails staying in wood
Where friction is a nuisance:
- Wear on shoe soles, tyres and machine parts
- Heat generated in moving machinery
- Energy wasted in overcoming it, so an engine must do extra work
How it is changed on purpose. Friction is reduced by lubricants such as oil and grease, by ball bearings, and by polishing surfaces. It is increased by the treads cut into tyres, the grooves on shoe soles, and sand spread on a slippery road.
Everyday evidence. A cycle chain runs quietly and easily after oiling and grinds when dry. Walking across a wet marble floor is difficult precisely because the water has reduced the friction your feet depend on.
Friction is not simply undesirable. It is a necessary evil — the same force that wastes energy in an engine is what lets you walk, stop and hold things. And it can never be removed entirely; it can only be reduced. So a question asking whether friction is good or bad is asking for both sides, and an answer giving only one is incomplete.
It arises because no surface is perfectly smooth. The irregularities of the two surfaces interlock, and moving one over the other means dragging those irregularities past each other.
What friction depends on:
- The nature and roughness of the two surfaces
- The force pressing them together — a heavier box is harder to push
Three kinds, in order of size.
- Static friction acts before motion begins, when a force is applied but the object has not yet moved
- Sliding friction acts while one surface slides over another
- Rolling friction acts when one surface rolls over another, and is the smallest of the three
So static friction is greater than sliding friction, which is greater than rolling friction. That order explains two familiar experiences: a heavy box is hardest to get started and easier to keep moving once it is going, and putting a load on wheels makes it far easier to move than dragging it.
Where friction is useful:
- Walking — without it your foot would slide back with every step
- Brakes on a cycle or car
- Writing with a pencil, and striking a match
- Tyres gripping the road, especially in a turn
- Holding a glass without it slipping, and nails staying in wood
Where friction is a nuisance:
- Wear on shoe soles, tyres and machine parts
- Heat generated in moving machinery
- Energy wasted in overcoming it, so an engine must do extra work
How it is changed on purpose. Friction is reduced by lubricants such as oil and grease, by ball bearings, and by polishing surfaces. It is increased by the treads cut into tyres, the grooves on shoe soles, and sand spread on a slippery road.
Everyday evidence. A cycle chain runs quietly and easily after oiling and grinds when dry. Walking across a wet marble floor is difficult precisely because the water has reduced the friction your feet depend on.
Friction is not simply undesirable. It is a necessary evil — the same force that wastes energy in an engine is what lets you walk, stop and hold things. And it can never be removed entirely; it can only be reduced. So a question asking whether friction is good or bad is asking for both sides, and an answer giving only one is incomplete.
Exam tip
Exam tip: add the forces before you use any of them
Find the net force first. Add forces in the same direction and subtract opposing ones, then use that single value. Using an individual force where the net is needed is the standard error.
**Balanced means and no change in motion — the object may be at rest or moving at constant velocity. Unbalanced means acceleration.
State the direction of every force and of every net force. Force is a vector.
Force is measured in newtons**, with . A spring balance measures force; a beam balance measures mass.
Weight , so a kg mass weighs N with . Never give a weight in kilograms.
List all five effects of a force when asked — motion, stopping, speed, direction and shape — and remember one force may produce several at once.
Separate contact forces (friction, muscular) from non-contact forces (gravity, magnetic, electrostatic).
**Static sliding rolling friction — which explains why starting is harder than continuing, and why wheels are used.
Friction depends on the roughness and on the force pressing the surfaces together.
And when asked about friction, give both** its uses and its drawbacks — it is a necessary evil, not simply a nuisance.
**Balanced means and no change in motion — the object may be at rest or moving at constant velocity. Unbalanced means acceleration.
State the direction of every force and of every net force. Force is a vector.
Force is measured in newtons**, with . A spring balance measures force; a beam balance measures mass.
Weight , so a kg mass weighs N with . Never give a weight in kilograms.
List all five effects of a force when asked — motion, stopping, speed, direction and shape — and remember one force may produce several at once.
Separate contact forces (friction, muscular) from non-contact forces (gravity, magnetic, electrostatic).
**Static sliding rolling friction — which explains why starting is harder than continuing, and why wheels are used.
Friction depends on the roughness and on the force pressing the surfaces together.
And when asked about friction, give both** its uses and its drawbacks — it is a necessary evil, not simply a nuisance.
Did you know
Why a smooth road would be unusable
Friction is usually introduced as the thing that wastes energy and wears out machinery. So imagine it gone, and see what happens.
You could not walk. A step works because your foot pushes backwards against the ground and friction stops it sliding, so the push moves you forward instead. Remove friction and your foot simply slides back while you stay where you are — the experience of trying to walk on wet ice, taken to its limit.
A vehicle could neither start nor stop. Tyres drive a car forward by gripping the road, and brakes work by friction between pad and wheel and then between tyre and road. Without friction the wheels would spin freely and the car would stay put; if it were already moving, nothing would slow it.
Nails would fall out of wood, screws out of their threads, and knots would untie themselves. A pencil would leave no mark, since writing is the friction between graphite and paper wearing a little graphite off. A glass would slip from your fingers, because holding something means friction resisting its weight.
Even a nut on a bolt stays tight only because of friction — which is why the hexagonal nut met earlier in these notes needs a spanner to turn and does not simply unwind itself.
So the list of friction's disadvantages is real and the list of things that depend on it is longer. The engineering answer is never to remove friction but to place it well: reduce it inside a machine with oil and bearings, and increase it at exactly the surfaces where grip matters, with treads, grooves and rough finishes.
That is why necessary evil is the phrase the syllabus uses. The evil is the wasted energy; the necessity is everything else.
You could not walk. A step works because your foot pushes backwards against the ground and friction stops it sliding, so the push moves you forward instead. Remove friction and your foot simply slides back while you stay where you are — the experience of trying to walk on wet ice, taken to its limit.
A vehicle could neither start nor stop. Tyres drive a car forward by gripping the road, and brakes work by friction between pad and wheel and then between tyre and road. Without friction the wheels would spin freely and the car would stay put; if it were already moving, nothing would slow it.
Nails would fall out of wood, screws out of their threads, and knots would untie themselves. A pencil would leave no mark, since writing is the friction between graphite and paper wearing a little graphite off. A glass would slip from your fingers, because holding something means friction resisting its weight.
Even a nut on a bolt stays tight only because of friction — which is why the hexagonal nut met earlier in these notes needs a spanner to turn and does not simply unwind itself.
So the list of friction's disadvantages is real and the list of things that depend on it is longer. The engineering answer is never to remove friction but to place it well: reduce it inside a machine with oil and bearings, and increase it at exactly the surfaces where grip matters, with treads, grooves and rough finishes.
That is why necessary evil is the phrase the syllabus uses. The evil is the wasted energy; the necessity is everything else.
Exam relevance
How does the idea of net force feed into JEE and NEET Physics?
Because nearly every mechanics problem in the two years ahead begins by finding the net force, and this page is where that step is first taught as a step.
This is the foundation for the Class 11 Physics chapter Laws of Motion, examined in JEE Main and in NEET Physics. That chapter formalises the net force as the vector sum of all forces, introduces the free-body diagram as the standard way of finding it, and then applies to every situation from a block on a table to a lift in motion. The Class 9 habit of adding the forces before using any of them is exactly the habit a free-body diagram enforces.
Friction becomes a chapter section in its own right there. The Class 9 ordering — static greater than sliding greater than rolling — becomes the coefficient of friction, with limiting friction, the angle of repose and problems on inclined planes. JEE Main questions on friction are common, and the qualitative facts on this page are the ones those problems assume.
The spring balance leads into Hooke's law in Class 11 Mechanical Properties of Solids, where extension proportional to force becomes a formal law with a spring constant.
Mass against weight returns in Class 11 Gravitation, where the variation of with altitude and latitude makes the distinction quantitative.
What the questions look like. Numericals are the main form, and the first line of the solution is nearly always the net force. Assertion-reason items favour the counter-intuitive statements on this page: that a body moving at constant velocity has zero net force, and that friction is necessary for walking. Match-the-column questions pair situations with the type of friction acting. Diagram-based questions ask which of several force arrangements is balanced.
How board and competitive emphasis differ. A board paper asks you to define balanced forces or to list the effects of a force, and then sets a single-step net-force sum. A competitive paper embeds the net force inside a longer problem — a block on a rough surface pulled at an angle, where the friction itself depends on how hard the surface is pressed — so the reasoning about which forces act, and on what, matters more than the arithmetic.
The single trap that costs the most marks. Assuming that a moving object must have a net force on it. A cruising car, a lift descending at constant speed and a parachutist falling steadily all have zero net force. The clue in a question is the phrase constant velocity or uniform speed, and it always means the forces balance.
A second trap worth naming. Treating friction as a fixed quantity to be looked up. It depends on the force pressing the surfaces together, so loading a box changes the friction on it — which is precisely what the Class 11 coefficient-of-friction problems are built to test.
This is the foundation for the Class 11 Physics chapter Laws of Motion, examined in JEE Main and in NEET Physics. That chapter formalises the net force as the vector sum of all forces, introduces the free-body diagram as the standard way of finding it, and then applies to every situation from a block on a table to a lift in motion. The Class 9 habit of adding the forces before using any of them is exactly the habit a free-body diagram enforces.
Friction becomes a chapter section in its own right there. The Class 9 ordering — static greater than sliding greater than rolling — becomes the coefficient of friction, with limiting friction, the angle of repose and problems on inclined planes. JEE Main questions on friction are common, and the qualitative facts on this page are the ones those problems assume.
The spring balance leads into Hooke's law in Class 11 Mechanical Properties of Solids, where extension proportional to force becomes a formal law with a spring constant.
Mass against weight returns in Class 11 Gravitation, where the variation of with altitude and latitude makes the distinction quantitative.
What the questions look like. Numericals are the main form, and the first line of the solution is nearly always the net force. Assertion-reason items favour the counter-intuitive statements on this page: that a body moving at constant velocity has zero net force, and that friction is necessary for walking. Match-the-column questions pair situations with the type of friction acting. Diagram-based questions ask which of several force arrangements is balanced.
How board and competitive emphasis differ. A board paper asks you to define balanced forces or to list the effects of a force, and then sets a single-step net-force sum. A competitive paper embeds the net force inside a longer problem — a block on a rough surface pulled at an angle, where the friction itself depends on how hard the surface is pressed — so the reasoning about which forces act, and on what, matters more than the arithmetic.
The single trap that costs the most marks. Assuming that a moving object must have a net force on it. A cruising car, a lift descending at constant speed and a parachutist falling steadily all have zero net force. The clue in a question is the phrase constant velocity or uniform speed, and it always means the forces balance.
A second trap worth naming. Treating friction as a fixed quantity to be looked up. It depends on the force pressing the surfaces together, so loading a box changes the friction on it — which is precisely what the Class 11 coefficient-of-friction problems are built to test.
Key takeaways
Force, net force and friction: quick revision
- A force is a push or a pull, and it is a vector with magnitude and direction.
- Five effects: start motion, stop motion, change speed, change direction, change shape or size. One force may produce several at once.
- Contact forces (friction, muscular) need touching; non-contact forces (gravity, magnetic, electrostatic) act across a distance.
- Force is measured with a spring balance, in newtons, with .
- A kg mass weighs N with ; a balance reading kg is measuring N.
- Extension is proportional to force: N giving cm means N gives cm.
- A spring balance measures force; a beam balance measures mass. Only the spring balance reads differently on the Moon.
- Net force is the sum in one direction minus the sum in the other. N; N; .
- A tug of war at N against N gives a net of N.
- A N push against N of friction gives N net; equal push and friction give zero net, and the box keeps its current state.
- A N box on a table pushing back with N is balanced and stays at rest.
- Balanced forces mean no change in motion — at rest or at constant velocity. Unbalanced forces mean acceleration.
- Friction opposes relative motion, acts along the surfaces, and arises from interlocking irregularities.
- It depends on the roughness of the surfaces and the force pressing them together.
- **Static sliding rolling — so starting is harder than continuing, and wheels beat dragging.
- Useful: walking, brakes, writing, tyre grip, holding objects, nails in wood. A nuisance: wear, heat and wasted energy.
- Reduced by lubricants, ball bearings and polishing; increased by treads, grooves and sand.
- Friction is a necessary evil** — it can be reduced but never removed.
List every force acting on you as you sit reading this and add them up — if the total is not zero, explain why you are not accelerating.
- Five effects: start motion, stop motion, change speed, change direction, change shape or size. One force may produce several at once.
- Contact forces (friction, muscular) need touching; non-contact forces (gravity, magnetic, electrostatic) act across a distance.
- Force is measured with a spring balance, in newtons, with .
- A kg mass weighs N with ; a balance reading kg is measuring N.
- Extension is proportional to force: N giving cm means N gives cm.
- A spring balance measures force; a beam balance measures mass. Only the spring balance reads differently on the Moon.
- Net force is the sum in one direction minus the sum in the other. N; N; .
- A tug of war at N against N gives a net of N.
- A N push against N of friction gives N net; equal push and friction give zero net, and the box keeps its current state.
- A N box on a table pushing back with N is balanced and stays at rest.
- Balanced forces mean no change in motion — at rest or at constant velocity. Unbalanced forces mean acceleration.
- Friction opposes relative motion, acts along the surfaces, and arises from interlocking irregularities.
- It depends on the roughness of the surfaces and the force pressing them together.
- **Static sliding rolling — so starting is harder than continuing, and wheels beat dragging.
- Useful: walking, brakes, writing, tyre grip, holding objects, nails in wood. A nuisance: wear, heat and wasted energy.
- Reduced by lubricants, ball bearings and polishing; increased by treads, grooves and sand.
- Friction is a necessary evil** — it can be reduced but never removed.
List every force acting on you as you sit reading this and add them up — if the total is not zero, explain why you are not accelerating.