A Bus Brakes and Your Head Keeps Going
Learn what a force can do to a body and how to sort forces into contact and non-contact kinds, state the first law and use it on everyday observations, tell the three kinds of inertia apart, and spot balanced and unbalanced forces.
Why do you lurch forward when a bus brakes suddenly?
You are standing in a crowded bus moving at a steady speed. The driver brakes hard, and your body pitches forward.
Nothing pushed you forward. Whatever happened, it was not a shove from behind.
What actually happened is that the bus stopped and you did not. Your feet were held by friction with the floor, so the lower half of you was brought to rest with the bus. The upper half had nothing acting on it to stop it, so it carried on moving at the speed it already had — and pitched over your stationary feet.
That unwillingness to change what it is already doing is called inertia, and it is the property every body has. A body at rest stays at rest; a body in motion keeps moving in a straight line at the same speed. Something external has to intervene, and that something is a force.
So the everyday question what makes things move turns out to be the wrong question. Moving needs no explanation at all — changing the motion is what needs a cause. That reversal is the content of Newton's first law, and it is what this page is about.
This page covers the first part of the ICSE Class 9 Physics chapter on the laws of motion — the effects of a force, contact and non-contact forces, the first law, inertia, and balanced forces.
Nothing pushed you forward. Whatever happened, it was not a shove from behind.
What actually happened is that the bus stopped and you did not. Your feet were held by friction with the floor, so the lower half of you was brought to rest with the bus. The upper half had nothing acting on it to stop it, so it carried on moving at the speed it already had — and pitched over your stationary feet.
That unwillingness to change what it is already doing is called inertia, and it is the property every body has. A body at rest stays at rest; a body in motion keeps moving in a straight line at the same speed. Something external has to intervene, and that something is a force.
So the everyday question what makes things move turns out to be the wrong question. Moving needs no explanation at all — changing the motion is what needs a cause. That reversal is the content of Newton's first law, and it is what this page is about.
This page covers the first part of the ICSE Class 9 Physics chapter on the laws of motion — the effects of a force, contact and non-contact forces, the first law, inertia, and balanced forces.
What can a force actually do to a body?
A force can change a body's state of rest or motion, its speed, its direction, or its shape and size — and often several at once.
The four effects, with examples.
- Set a body in motion, or bring it to rest. A push starts a stationary football rolling; brakes bring a cycle to a stop
- Change the speed. Pedalling harder speeds the cycle up; friction slows it down
- Change the direction. A batsman's stroke sends the ball back the way it came; turning the handlebars curves the cycle's path
- Change the shape or size. Squeezing a rubber ball flattens it; stretching a rubber band lengthens it; a hammer flattens hot iron
Contact forces act only when the two bodies touch:
- Muscular force — pushing a door, lifting a bag
- Friction — the road on a tyre, a brake pad on a wheel rim
- Normal reaction — a table pushing up on a book
- Tension — a rope pulling a bucket from a well
- Air resistance — the drag on a moving cycle
Non-contact forces, also called field forces, act across a gap with nothing in between:
- Gravitational force — the Earth pulling a stone, holding the Moon in orbit
- Electrostatic force — a rubbed comb lifting small pieces of paper
- Magnetic force — a magnet attracting iron filings without touching them
Worked classification.
- A bucket hanging from a rope — tension, a contact force
- A mango falling from a tree — gravitational, non-contact
- A cycle slowing down after pedalling stops — friction and air resistance, both contact
- A magnet moving a paper clip across a table — magnetic, non-contact
- A book resting on a shelf — weight (non-contact) balanced by normal reaction (contact)
Weight is a non-contact force even though the body is resting on something. A book on a table experiences its weight — the Earth's pull, acting with no contact — and separately the table's upward push, which is a contact force. Two different forces, two different kinds, and the book question is a favourite because it contains one of each.
A force can change the shape of a body without moving it at all. Press a lump of dough with both hands and it flattens where it lies. So "force causes motion" is too narrow a statement — the effect on shape is a separate outcome, and the next section is about why the first law is careful to talk about change of state rather than about motion.
The four effects, with examples.
- Set a body in motion, or bring it to rest. A push starts a stationary football rolling; brakes bring a cycle to a stop
- Change the speed. Pedalling harder speeds the cycle up; friction slows it down
- Change the direction. A batsman's stroke sends the ball back the way it came; turning the handlebars curves the cycle's path
- Change the shape or size. Squeezing a rubber ball flattens it; stretching a rubber band lengthens it; a hammer flattens hot iron
Contact forces act only when the two bodies touch:
- Muscular force — pushing a door, lifting a bag
- Friction — the road on a tyre, a brake pad on a wheel rim
- Normal reaction — a table pushing up on a book
- Tension — a rope pulling a bucket from a well
- Air resistance — the drag on a moving cycle
Non-contact forces, also called field forces, act across a gap with nothing in between:
- Gravitational force — the Earth pulling a stone, holding the Moon in orbit
- Electrostatic force — a rubbed comb lifting small pieces of paper
- Magnetic force — a magnet attracting iron filings without touching them
Worked classification.
- A bucket hanging from a rope — tension, a contact force
- A mango falling from a tree — gravitational, non-contact
- A cycle slowing down after pedalling stops — friction and air resistance, both contact
- A magnet moving a paper clip across a table — magnetic, non-contact
- A book resting on a shelf — weight (non-contact) balanced by normal reaction (contact)
Weight is a non-contact force even though the body is resting on something. A book on a table experiences its weight — the Earth's pull, acting with no contact — and separately the table's upward push, which is a contact force. Two different forces, two different kinds, and the book question is a favourite because it contains one of each.
A force can change the shape of a body without moving it at all. Press a lump of dough with both hands and it flattens where it lies. So "force causes motion" is too narrow a statement — the effect on shape is a separate outcome, and the next section is about why the first law is careful to talk about change of state rather than about motion.
What does Newton's first law actually say?
A body continues in its state of rest, or of uniform motion in a straight line, unless acted upon by an external unbalanced force.
Every phrase in that sentence is doing work.
- "State of rest, or of uniform motion in a straight line" — these are the two things a body does when left alone. Notice that steady motion is just as natural a state as standing still
- "In a straight line" — a body going round a bend at a steady speed is not being left alone, because its direction is changing
- "External" — a force from outside the body. Internal pushes between its own parts cannot change its motion as a whole
- "Unbalanced" — forces that cancel out change nothing, which is the subject of the last section
Worked explanation 1 — a bus braking. The passenger's feet are brought to rest by friction with the floor; the upper body has no force stopping it and continues forward at the old speed, so the passenger lurches forward.
Worked explanation 2 — a bus starting. The floor drags the feet forward, while the upper body is still at rest and stays at rest, so the passenger falls backward.
Worked explanation 3 — a bus turning left. The passenger's body continues in its original straight line while the bus curves away, so the passenger appears to be thrown outward, to the right.
Worked explanation 4 — dust on a carpet. Beating a hanging carpet sets the carpet in motion suddenly. The dust particles are at rest and stay at rest, so the carpet leaves them behind and they fall clear.
Worked explanation 5 — a coin on a card. Rest a card on a glass with a coin on top, then flick the card away sharply. The coin stays where it was and drops straight into the glass, because the flick acted on the card and barely on the coin.
Worked explanation 6 — an athlete's run-up. A long jumper runs before jumping so that the body is already in motion at take-off, and that motion continues through the air, carrying the jumper further.
The law also defines what a force IS. Read it backwards: whenever a body's state of rest or of uniform straight-line motion does change, an external unbalanced force must be acting. So a force is that external agency which changes, or tends to change, a body's state of rest or of uniform motion. The first law tells you what a force does, and therefore what it is — the second law, in the next part of this chapter, tells you how to measure it.
"Tends to change" matters in that definition. Push a heavy almirah and it may not budge, yet a force certainly acted — friction cancelled it. A force that produces no motion is still a force, which is why the definition says changes or tends to change rather than simply changes.
Every phrase in that sentence is doing work.
- "State of rest, or of uniform motion in a straight line" — these are the two things a body does when left alone. Notice that steady motion is just as natural a state as standing still
- "In a straight line" — a body going round a bend at a steady speed is not being left alone, because its direction is changing
- "External" — a force from outside the body. Internal pushes between its own parts cannot change its motion as a whole
- "Unbalanced" — forces that cancel out change nothing, which is the subject of the last section
Worked explanation 1 — a bus braking. The passenger's feet are brought to rest by friction with the floor; the upper body has no force stopping it and continues forward at the old speed, so the passenger lurches forward.
Worked explanation 2 — a bus starting. The floor drags the feet forward, while the upper body is still at rest and stays at rest, so the passenger falls backward.
Worked explanation 3 — a bus turning left. The passenger's body continues in its original straight line while the bus curves away, so the passenger appears to be thrown outward, to the right.
Worked explanation 4 — dust on a carpet. Beating a hanging carpet sets the carpet in motion suddenly. The dust particles are at rest and stay at rest, so the carpet leaves them behind and they fall clear.
Worked explanation 5 — a coin on a card. Rest a card on a glass with a coin on top, then flick the card away sharply. The coin stays where it was and drops straight into the glass, because the flick acted on the card and barely on the coin.
Worked explanation 6 — an athlete's run-up. A long jumper runs before jumping so that the body is already in motion at take-off, and that motion continues through the air, carrying the jumper further.
The law also defines what a force IS. Read it backwards: whenever a body's state of rest or of uniform straight-line motion does change, an external unbalanced force must be acting. So a force is that external agency which changes, or tends to change, a body's state of rest or of uniform motion. The first law tells you what a force does, and therefore what it is — the second law, in the next part of this chapter, tells you how to measure it.
"Tends to change" matters in that definition. Push a heavy almirah and it may not budge, yet a force certainly acted — friction cancelled it. A force that produces no motion is still a force, which is why the definition says changes or tends to change rather than simply changes.
How do the three kinds of inertia differ?
Inertia is a body's resistance to any change in its state of rest or motion, and it comes in three kinds according to what is being resisted.
- Inertia of rest — resistance to being set in motion
- Inertia of motion — resistance to being brought to rest, or slowed
- Inertia of direction — resistance to a change of direction
Inertia depends only on mass. The greater the mass, the greater the inertia, and mass is the quantitative measure of inertia. That is why a loaded truck is harder to start and harder to stop than an empty one — the same property shows up in both.
Worked examples of inertia of rest.
- Dust flies off when a carpet is beaten — the dust stays at rest while the carpet moves
- A coin on a flicked card falls into the glass below
- A passenger falls backward when a bus starts
- Fruit falls when a branch is shaken sharply
Worked examples of inertia of motion.
- A passenger lurches forward when a bus brakes
- An athlete runs before a long jump
- A moving cycle keeps going for a while after pedalling stops
- A person jumping from a moving bus must run forward on landing, because the body is still moving
Worked examples of inertia of direction.
- Mud flies off tangentially from the wheel of a moving cycle, in a straight line rather than around the curve
- A stone whirled on a string flies off along a tangent the moment the string is released
- A passenger leans outward when a bus takes a turn
- Water in a spinning wet umbrella leaves along straight lines
Worked comparison of masses. Push an empty shopping trolley and a fully loaded one with the same effort. The empty one starts easily and the loaded one hardly moves. Now get both rolling at the same speed and try to stop them: the loaded one takes far more effort. Same mass difference, opposite situations, and the same explanation.
Inertia is not a force. It is a property, like mass or density — it does not push or pull anything and it is not measured in newtons. The passenger in the braking bus is not pushed forward by inertia; the passenger continues forward because of inertia, in the absence of a force. Writing "the force of inertia pushed him forward" is a marked error, and it is the commonest way this topic is mis-answered.
And inertia does not depend on speed. A truck at rest and the same truck at have identical inertia, because they have identical mass. What differs is the momentum, which is the topic of the next part of this chapter — and momentum does depend on speed.
- Inertia of rest — resistance to being set in motion
- Inertia of motion — resistance to being brought to rest, or slowed
- Inertia of direction — resistance to a change of direction
Inertia depends only on mass. The greater the mass, the greater the inertia, and mass is the quantitative measure of inertia. That is why a loaded truck is harder to start and harder to stop than an empty one — the same property shows up in both.
Worked examples of inertia of rest.
- Dust flies off when a carpet is beaten — the dust stays at rest while the carpet moves
- A coin on a flicked card falls into the glass below
- A passenger falls backward when a bus starts
- Fruit falls when a branch is shaken sharply
Worked examples of inertia of motion.
- A passenger lurches forward when a bus brakes
- An athlete runs before a long jump
- A moving cycle keeps going for a while after pedalling stops
- A person jumping from a moving bus must run forward on landing, because the body is still moving
Worked examples of inertia of direction.
- Mud flies off tangentially from the wheel of a moving cycle, in a straight line rather than around the curve
- A stone whirled on a string flies off along a tangent the moment the string is released
- A passenger leans outward when a bus takes a turn
- Water in a spinning wet umbrella leaves along straight lines
Worked comparison of masses. Push an empty shopping trolley and a fully loaded one with the same effort. The empty one starts easily and the loaded one hardly moves. Now get both rolling at the same speed and try to stop them: the loaded one takes far more effort. Same mass difference, opposite situations, and the same explanation.
Inertia is not a force. It is a property, like mass or density — it does not push or pull anything and it is not measured in newtons. The passenger in the braking bus is not pushed forward by inertia; the passenger continues forward because of inertia, in the absence of a force. Writing "the force of inertia pushed him forward" is a marked error, and it is the commonest way this topic is mis-answered.
And inertia does not depend on speed. A truck at rest and the same truck at have identical inertia, because they have identical mass. What differs is the momentum, which is the topic of the next part of this chapter — and momentum does depend on speed.
How do you tell balanced forces from unbalanced ones?
Add the forces with their directions. If they cancel to zero the forces are balanced; if anything is left over they are unbalanced and the body accelerates.
Worked example 1 — a book on a table. A book of mass kg rests on a table. Its weight is
and the table pushes up with a normal reaction of N. Taking upward as positive:
Balanced, so the book stays at rest.
Worked example 2 — a tug of war. Two teams pull with N each in opposite directions:
Balanced, so the rope does not move — though it is certainly being stretched, which is the shape-changing effect from the second section.
Worked example 3 — a box being pushed. A box is pushed with N against a friction of N:
Unbalanced, so the box accelerates.
Worked example 4 — a box that will not move. The same box pushed with only N against a friction that can reach N:
Balanced, and the box stays put. Friction adjusted itself to match the push, which is why pushing a heavy cupboard gently achieves nothing at all.
Worked example 5 — a lift going up at a steady speed. A person of kg stands in a lift rising at a constant . Weight is N downward, and the floor pushes up with N:
Balanced — even though the lift is moving.
Balanced forces do not mean the body is at rest. They mean the body's motion is not changing, so it is either at rest or moving with uniform velocity in a straight line. That is exactly the pair of states the first law names. The lift in worked example 5 is moving steadily upward with zero net force on the passenger, and a question asking whether the forces are balanced is not asking whether anything is moving.
Worked example 6 — a body moving at constant velocity through air. A parachutist falling at a steady speed has weight downward and air resistance upward, and the two are equal. The parachutist is moving quickly and accelerating not at all.
"No force" and "balanced forces" produce the same motion. A body drifting in empty space with nothing acting on it and a book sitting on a table with two forces cancelling both obey the first law identically. So you cannot tell from the motion whether forces are absent or merely balanced — which is why the law says unbalanced rather than simply a force, and why the net force is what must always be computed.
Worked example 1 — a book on a table. A book of mass kg rests on a table. Its weight is
and the table pushes up with a normal reaction of N. Taking upward as positive:
Balanced, so the book stays at rest.
Worked example 2 — a tug of war. Two teams pull with N each in opposite directions:
Balanced, so the rope does not move — though it is certainly being stretched, which is the shape-changing effect from the second section.
Worked example 3 — a box being pushed. A box is pushed with N against a friction of N:
Unbalanced, so the box accelerates.
Worked example 4 — a box that will not move. The same box pushed with only N against a friction that can reach N:
Balanced, and the box stays put. Friction adjusted itself to match the push, which is why pushing a heavy cupboard gently achieves nothing at all.
Worked example 5 — a lift going up at a steady speed. A person of kg stands in a lift rising at a constant . Weight is N downward, and the floor pushes up with N:
Balanced — even though the lift is moving.
Balanced forces do not mean the body is at rest. They mean the body's motion is not changing, so it is either at rest or moving with uniform velocity in a straight line. That is exactly the pair of states the first law names. The lift in worked example 5 is moving steadily upward with zero net force on the passenger, and a question asking whether the forces are balanced is not asking whether anything is moving.
Worked example 6 — a body moving at constant velocity through air. A parachutist falling at a steady speed has weight downward and air resistance upward, and the two are equal. The parachutist is moving quickly and accelerating not at all.
"No force" and "balanced forces" produce the same motion. A body drifting in empty space with nothing acting on it and a book sitting on a table with two forces cancelling both obey the first law identically. So you cannot tell from the motion whether forces are absent or merely balanced — which is why the law says unbalanced rather than simply a force, and why the net force is what must always be computed.
Exam tip
Exam tip: name which inertia, and never call inertia a force
Name the kind of inertia explicitly — inertia of rest, inertia of motion or inertia of direction — and then give the reason. Naming without explaining earns half the marks.
Inertia is a property, not a force. Never write "the force of inertia". Write the upper body continued moving because of its inertia, as no force acted on it.
Explain a bus observation in two halves: what happened to the part in contact (feet held by friction) and what happened to the rest (continued at the old speed).
Quote the first law in full, including external, unbalanced and in a straight line — each word carries a mark.
Inertia depends on mass alone, not on speed. A truck at rest and the same truck moving have equal inertia.
Sort forces by contact: muscular, friction, normal reaction, tension and air resistance are contact; gravitational, electrostatic and magnetic are non-contact.
Weight is a non-contact force even for a body resting on a surface.
Compute the net force with signs before calling forces balanced: N is unbalanced, is balanced.
Balanced forces allow uniform motion, not only rest — a lift rising steadily has zero net force on its passenger.
**Use with ** and give the answer in newtons: kg weighs N.
And list all four effects of a force when asked — state of motion, speed, direction, and shape or size. The last one is the one most often left out.
Inertia is a property, not a force. Never write "the force of inertia". Write the upper body continued moving because of its inertia, as no force acted on it.
Explain a bus observation in two halves: what happened to the part in contact (feet held by friction) and what happened to the rest (continued at the old speed).
Quote the first law in full, including external, unbalanced and in a straight line — each word carries a mark.
Inertia depends on mass alone, not on speed. A truck at rest and the same truck moving have equal inertia.
Sort forces by contact: muscular, friction, normal reaction, tension and air resistance are contact; gravitational, electrostatic and magnetic are non-contact.
Weight is a non-contact force even for a body resting on a surface.
Compute the net force with signs before calling forces balanced: N is unbalanced, is balanced.
Balanced forces allow uniform motion, not only rest — a lift rising steadily has zero net force on its passenger.
**Use with ** and give the answer in newtons: kg weighs N.
And list all four effects of a force when asked — state of motion, speed, direction, and shape or size. The last one is the one most often left out.
Did you know
Why a coin drops into the glass instead of flying off with the card
Balance a stiff card on the mouth of a glass and put a coin in the middle of it. Now flick the card away sideways, hard and fast.
The card shoots off. The coin drops straight down into the glass.
What makes this work is not that the coin is heavy or that the card is smooth — it is the time the push lasts. The only thing that could drag the coin sideways is friction between the coin and the card, and that friction acts for only as long as the card is under the coin. A hard flick means the card is gone in a tiny fraction of a second, so the sideways nudge on the coin has almost no time to build up any speed.
Pull the card away slowly and the trick fails completely — the coin travels along with it and falls off the edge. Same coin, same card, same friction; only the duration changed.
That is a hint at something the next part of this chapter makes precise. What changes a body's motion is not force alone but force acting for a time, and a large force for a very short time can do less than a small force applied patiently.
The same reasoning explains a cricketer catching a fast ball. A fielder who holds the hands rigid stops the ball in a very short time and feels a large force; one who draws the hands back stretches the stopping out over a longer time and feels much less. The ball's change of motion is the same in both cases — only the time over which it happens differs, and so the force differs.
And it explains why the carpet has to be beaten sharply. A gentle shake gives the dust time to be carried along; a sharp blow moves the carpet out from under it before friction can act.
The card shoots off. The coin drops straight down into the glass.
What makes this work is not that the coin is heavy or that the card is smooth — it is the time the push lasts. The only thing that could drag the coin sideways is friction between the coin and the card, and that friction acts for only as long as the card is under the coin. A hard flick means the card is gone in a tiny fraction of a second, so the sideways nudge on the coin has almost no time to build up any speed.
Pull the card away slowly and the trick fails completely — the coin travels along with it and falls off the edge. Same coin, same card, same friction; only the duration changed.
That is a hint at something the next part of this chapter makes precise. What changes a body's motion is not force alone but force acting for a time, and a large force for a very short time can do less than a small force applied patiently.
The same reasoning explains a cricketer catching a fast ball. A fielder who holds the hands rigid stops the ball in a very short time and feels a large force; one who draws the hands back stretches the stopping out over a longer time and feels much less. The ball's change of motion is the same in both cases — only the time over which it happens differs, and so the force differs.
And it explains why the carpet has to be beaten sharply. A gentle shake gives the dust time to be carried along; a sharp blow moves the carpet out from under it before friction can act.
Exam relevance
How does Newton's first law feed into JEE Main and NEET?
Because every mechanics problem begins by identifying the forces on a body, and the first law is what turns that list into an equation.
This is the foundation for Class 11 Physics Laws of Motion, examined in JEE Main and NEET. The first law becomes the condition for equilibrium: a body at rest or in uniform motion has
and in two dimensions that single vector statement splits into two equations, one for each direction. Almost every statics problem in Class 11 is that pair of equations, applied to a block on an incline, a hanging sign, or a body held by two strings.
The free-body diagram is the tool built on this page. Class 11 formalises the habit of listing every force on one chosen body — weight, normal reaction, friction, tension — and the contact-against-non-contact classification here is what makes the list complete. The single most common failure in Class 11 mechanics is a missing force in the diagram, and the fix is the systematic sort used in the second section.
Inertial and non-inertial frames are the conceptual extension. The lurching passenger is explained on this page from the road's point of view, where no force acts on the upper body. Class 11 adds the bus's point of view, where an apparent pseudo-force has to be introduced to make the first law work — and the reason it must be introduced is precisely that the bus is accelerating. Assertion-reason questions on pseudo-forces and on inertia appear in both exams, and they rest on the distinction drawn here between a property and a force.
Friction becomes its own topic, and worked example 4 of the last section — where friction adjusted itself to match a push — is static friction, which Class 11 treats with a maximum value and a coefficient. Numericals on whether a body moves at all begin with exactly that comparison.
Where the equilibrium idea reappears. Class 11 Systems of Particles and Class 11 Mechanical Properties of Solids both use balanced forces, the second one for the shape-changing effect listed in the second section — stress and strain are what a balanced pair of forces does to a body that cannot move.
What the questions look like. For board work, expect state the first law, explain a bus, carpet or coin observation naming the kind of inertia, classify forces as contact or non-contact, give the four effects of a force, and decide whether forces on a given body are balanced. These are reasoning questions, and the marks are in the wording. For JEE Main and NEET, expect free-body diagrams, equilibrium numericals, friction problems, and conceptual items on inertia and frames of reference.
How board and competitive emphasis differ. A board paper rewards the complete statement of the law and a clearly reasoned explanation in words. A competitive paper assumes the law and tests whether you can draw the force diagram correctly and resolve it — the explanation is never asked, only its consequences.
The single trap that costs the most marks. Treating inertia as a force. The passenger is not pushed forward by inertia; the passenger continues forward because no force acted to stop the upper body. The defence is to phrase every explanation as an absence rather than a presence — nothing acted on the upper body, so it kept its velocity — which is both correct and exactly what the first law says.
This is the foundation for Class 11 Physics Laws of Motion, examined in JEE Main and NEET. The first law becomes the condition for equilibrium: a body at rest or in uniform motion has
and in two dimensions that single vector statement splits into two equations, one for each direction. Almost every statics problem in Class 11 is that pair of equations, applied to a block on an incline, a hanging sign, or a body held by two strings.
The free-body diagram is the tool built on this page. Class 11 formalises the habit of listing every force on one chosen body — weight, normal reaction, friction, tension — and the contact-against-non-contact classification here is what makes the list complete. The single most common failure in Class 11 mechanics is a missing force in the diagram, and the fix is the systematic sort used in the second section.
Inertial and non-inertial frames are the conceptual extension. The lurching passenger is explained on this page from the road's point of view, where no force acts on the upper body. Class 11 adds the bus's point of view, where an apparent pseudo-force has to be introduced to make the first law work — and the reason it must be introduced is precisely that the bus is accelerating. Assertion-reason questions on pseudo-forces and on inertia appear in both exams, and they rest on the distinction drawn here between a property and a force.
Friction becomes its own topic, and worked example 4 of the last section — where friction adjusted itself to match a push — is static friction, which Class 11 treats with a maximum value and a coefficient. Numericals on whether a body moves at all begin with exactly that comparison.
Where the equilibrium idea reappears. Class 11 Systems of Particles and Class 11 Mechanical Properties of Solids both use balanced forces, the second one for the shape-changing effect listed in the second section — stress and strain are what a balanced pair of forces does to a body that cannot move.
What the questions look like. For board work, expect state the first law, explain a bus, carpet or coin observation naming the kind of inertia, classify forces as contact or non-contact, give the four effects of a force, and decide whether forces on a given body are balanced. These are reasoning questions, and the marks are in the wording. For JEE Main and NEET, expect free-body diagrams, equilibrium numericals, friction problems, and conceptual items on inertia and frames of reference.
How board and competitive emphasis differ. A board paper rewards the complete statement of the law and a clearly reasoned explanation in words. A competitive paper assumes the law and tests whether you can draw the force diagram correctly and resolve it — the explanation is never asked, only its consequences.
The single trap that costs the most marks. Treating inertia as a force. The passenger is not pushed forward by inertia; the passenger continues forward because no force acted to stop the upper body. The defence is to phrase every explanation as an absence rather than a presence — nothing acted on the upper body, so it kept its velocity — which is both correct and exactly what the first law says.
Key takeaways
Force, the first law, inertia and balanced forces: quick revision
- A force can set a body in motion or stop it, change its speed, change its direction, or change its shape and size. All four count.
- Contact forces: muscular, friction, normal reaction, tension, air resistance.
- Non-contact (field) forces: gravitational, electrostatic, magnetic.
- Weight is non-contact even for a body resting on a surface; the surface's push is a separate contact force.
- A force can change shape without producing motion — pressing dough, stretching a rubber band.
- Newton's first law: a body continues in its state of rest, or of uniform motion in a straight line, unless acted upon by an external unbalanced force.
- Bus braking — the feet are held by friction, the upper body continues, so the passenger lurches forward.
- Bus starting — the feet are dragged forward, the upper body stays at rest, so the passenger falls backward.
- Bus turning — the body continues in its straight line, so the passenger is thrown outward.
- Beaten carpet: the dust stays at rest. Flicked card: the coin drops into the glass. Long jump: the run-up puts the body in motion first.
- The first law also defines force — that external agency which changes or tends to change a body's state of rest or of uniform motion. The "tends to" covers a push that friction cancels.
- Inertia is resistance to a change of state, and it comes in three kinds: rest, motion and direction.
- Inertia of rest: dust off a carpet, a coin on a card, a passenger falling back.
- Inertia of motion: lurching forward, an athlete's run-up, a cycle coasting on.
- Inertia of direction: mud flying tangentially off a wheel, a whirled stone released along a tangent.
- Mass is the measure of inertia — a loaded truck is harder both to start and to stop.
- Inertia is a property, never a force — nothing is measured in newtons here.
- Inertia does not depend on speed; momentum does, and that is the next part of this chapter.
- Balanced forces sum to zero and change nothing; unbalanced forces leave a net force and cause acceleration.
- A kg book weighs N and the table pushes up N, so the net force is zero.
- Tug of war at N each side: balanced. A N push against N friction: net ** N**, unbalanced.
- A N push against a friction that can reach N: balanced, and the box stays put.
- Balanced forces allow uniform motion, not only rest — a lift rising at a steady has zero net force on its passenger, and so does a parachutist falling steadily.
- "No force" and "balanced forces" give the same motion, which is why the law says unbalanced.
Stand in a moving bus the next time it brakes, and work out which part of you was acted on and which part simply kept going — then say which kind of inertia you just felt.
- Contact forces: muscular, friction, normal reaction, tension, air resistance.
- Non-contact (field) forces: gravitational, electrostatic, magnetic.
- Weight is non-contact even for a body resting on a surface; the surface's push is a separate contact force.
- A force can change shape without producing motion — pressing dough, stretching a rubber band.
- Newton's first law: a body continues in its state of rest, or of uniform motion in a straight line, unless acted upon by an external unbalanced force.
- Bus braking — the feet are held by friction, the upper body continues, so the passenger lurches forward.
- Bus starting — the feet are dragged forward, the upper body stays at rest, so the passenger falls backward.
- Bus turning — the body continues in its straight line, so the passenger is thrown outward.
- Beaten carpet: the dust stays at rest. Flicked card: the coin drops into the glass. Long jump: the run-up puts the body in motion first.
- The first law also defines force — that external agency which changes or tends to change a body's state of rest or of uniform motion. The "tends to" covers a push that friction cancels.
- Inertia is resistance to a change of state, and it comes in three kinds: rest, motion and direction.
- Inertia of rest: dust off a carpet, a coin on a card, a passenger falling back.
- Inertia of motion: lurching forward, an athlete's run-up, a cycle coasting on.
- Inertia of direction: mud flying tangentially off a wheel, a whirled stone released along a tangent.
- Mass is the measure of inertia — a loaded truck is harder both to start and to stop.
- Inertia is a property, never a force — nothing is measured in newtons here.
- Inertia does not depend on speed; momentum does, and that is the next part of this chapter.
- Balanced forces sum to zero and change nothing; unbalanced forces leave a net force and cause acceleration.
- A kg book weighs N and the table pushes up N, so the net force is zero.
- Tug of war at N each side: balanced. A N push against N friction: net ** N**, unbalanced.
- A N push against a friction that can reach N: balanced, and the box stays put.
- Balanced forces allow uniform motion, not only rest — a lift rising at a steady has zero net force on its passenger, and so does a parachutist falling steadily.
- "No force" and "balanced forces" give the same motion, which is why the law says unbalanced.
Stand in a moving bus the next time it brakes, and work out which part of you was acted on and which part simply kept going — then say which kind of inertia you just felt.