A Fielder Pulls His Hands Back, and the Physics Explains Why
Learn Newton's first law and inertia, why mass measures inertia, how to use F = ma in calculations, and how momentum explains why catching softly hurts less.
Why does a cricket fielder pull his hands back while catching?
To make the catch take longer, because a longer time means a smaller force.
A ball of mass kg arriving at m/s has to be brought to rest, so its momentum must change by
Stop it in s with stiff hands and the force needed is
Draw the hands back so the catch takes s, and the same change in momentum needs only
One fifth of the force, for exactly the same catch. The change in momentum was fixed by the ball; the only thing the fielder controls is the time — and the force follows.
That relationship is Newton's second law, and it is the heart of this page. It covers the second part of the CBSE Class 9 Science chapter on how forces affect motion.
A ball of mass kg arriving at m/s has to be brought to rest, so its momentum must change by
Stop it in s with stiff hands and the force needed is
Draw the hands back so the catch takes s, and the same change in momentum needs only
One fifth of the force, for exactly the same catch. The change in momentum was fixed by the ball; the only thing the fielder controls is the time — and the force follows.
That relationship is Newton's second law, and it is the heart of this page. It covers the second part of the CBSE Class 9 Science chapter on how forces affect motion.
What does Newton's first law say about a moving object?
A body at rest stays at rest, and a body in motion keeps moving at the same speed in the same straight line, unless an unbalanced external force acts on it.
This is also called the law of inertia, because inertia is the name for the tendency of a body to resist any change in its state of rest or of motion.
Inertia of rest — the tendency to stay at rest.
- When a bus starts suddenly, you jerk backwards. Your feet move with the bus while the rest of you tends to remain where it was.
- A coin resting on a card over a glass falls straight into the glass when the card is flicked away.
- Dust is beaten out of a carpet because the carpet moves and the dust tends to stay.
Inertia of motion — the tendency to keep moving.
- When a bus stops suddenly, you lurch forwards, because your body continues moving after the bus has stopped. Seat belts exist for this reason.
- A long-jumper runs up before the jump, gaining motion the body will keep during the leap.
- Passengers hold the rail in a moving bus in anticipation of exactly this.
Inertia of direction — the tendency to keep moving in a straight line. Water flies off an umbrella spun round, and mud flies off a cycle wheel tangentially.
The law does not say a force is needed to keep something moving. It says a force is needed to change motion. A ball rolling on a perfectly smooth surface would roll for ever. Real balls stop because friction acts on them, not because their motion runs out — and once friction is understood, as it was in the previous part of this chapter, the law stops sounding strange.
Notice what the first law really defines. It tells you what happens when the net force is zero, which is the balanced case from the previous page. Rest and uniform motion are the same state as far as this law is concerned — both are unchanging, and neither needs a force to maintain it.
This is also called the law of inertia, because inertia is the name for the tendency of a body to resist any change in its state of rest or of motion.
Inertia of rest — the tendency to stay at rest.
- When a bus starts suddenly, you jerk backwards. Your feet move with the bus while the rest of you tends to remain where it was.
- A coin resting on a card over a glass falls straight into the glass when the card is flicked away.
- Dust is beaten out of a carpet because the carpet moves and the dust tends to stay.
Inertia of motion — the tendency to keep moving.
- When a bus stops suddenly, you lurch forwards, because your body continues moving after the bus has stopped. Seat belts exist for this reason.
- A long-jumper runs up before the jump, gaining motion the body will keep during the leap.
- Passengers hold the rail in a moving bus in anticipation of exactly this.
Inertia of direction — the tendency to keep moving in a straight line. Water flies off an umbrella spun round, and mud flies off a cycle wheel tangentially.
The law does not say a force is needed to keep something moving. It says a force is needed to change motion. A ball rolling on a perfectly smooth surface would roll for ever. Real balls stop because friction acts on them, not because their motion runs out — and once friction is understood, as it was in the previous part of this chapter, the law stops sounding strange.
Notice what the first law really defines. It tells you what happens when the net force is zero, which is the balanced case from the previous page. Rest and uniform motion are the same state as far as this law is concerned — both are unchanging, and neither needs a force to maintain it.
Which object has more inertia, and how do you decide?
The one with more mass. Mass is the measure of inertia.
The greater a body's mass, the harder it is to start it moving, to stop it, or to change its direction. Nothing else about the body matters — not its size, its shape or its material, only its mass.
Worked comparison 1. A stone of kg and a stone of kg. The kg stone has five times the inertia, so it takes five times the force to give it the same acceleration.
Worked comparison 2. A cricket ball and a tennis ball of the same size. The cricket ball is heavier, so it has more inertia — which is why it stings to catch and why it is harder to stop.
Worked comparison 3. A loaded truck and an empty one. Both have the same brakes, and the loaded truck needs a far longer distance to stop. Its greater mass means greater inertia, so the same braking force produces a smaller deceleration.
Everyday evidence. Pushing an empty shopping trolley into motion takes a light shove; a full one needs a real effort, and once moving it is much harder to turn at the end of an aisle. Nothing changed except the mass.
Inertia is a property, not a force. This is the misconception worth fixing early. When a bus stops and you lurch forward, nothing pushed you. No force acted in the forward direction at all. You simply continued moving, which is what you were already doing, while the bus stopped. Writing inertia pushed me forward describes the feeling accurately and the physics wrongly.
Inertia has no unit of its own, because it is not a separate quantity — it is the mass, in kilograms, looked at from the point of view of resisting change. That is why a question asking which has more inertia? is answered simply by comparing masses, and needs no calculation at all.
The greater a body's mass, the harder it is to start it moving, to stop it, or to change its direction. Nothing else about the body matters — not its size, its shape or its material, only its mass.
Worked comparison 1. A stone of kg and a stone of kg. The kg stone has five times the inertia, so it takes five times the force to give it the same acceleration.
Worked comparison 2. A cricket ball and a tennis ball of the same size. The cricket ball is heavier, so it has more inertia — which is why it stings to catch and why it is harder to stop.
Worked comparison 3. A loaded truck and an empty one. Both have the same brakes, and the loaded truck needs a far longer distance to stop. Its greater mass means greater inertia, so the same braking force produces a smaller deceleration.
Everyday evidence. Pushing an empty shopping trolley into motion takes a light shove; a full one needs a real effort, and once moving it is much harder to turn at the end of an aisle. Nothing changed except the mass.
Inertia is a property, not a force. This is the misconception worth fixing early. When a bus stops and you lurch forward, nothing pushed you. No force acted in the forward direction at all. You simply continued moving, which is what you were already doing, while the bus stopped. Writing inertia pushed me forward describes the feeling accurately and the physics wrongly.
Inertia has no unit of its own, because it is not a separate quantity — it is the mass, in kilograms, looked at from the point of view of resisting change. That is why a question asking which has more inertia? is answered simply by comparing masses, and needs no calculation at all.
Formula
How do you use F = ma to calculate force, mass or acceleration?
Newton's second law states that the rate of change of momentum of a body is directly proportional to the applied unbalanced force, and takes place in the direction of that force. For constant mass this gives
with in newtons, in kilograms and in . The unit follows directly: .
Worked example 1 — finding acceleration. A force of N acts on a mass of kg.
Worked example 2 — finding force. A kg object accelerates at .
Worked example 3 — a car. A car of mass kg accelerates from rest to m/s in s.
Worked example 4 — a retarding force. A ball of mass kg moving at m/s is brought to rest in s.
The negative sign means the force opposes the motion, so it is a retarding force of N.
Worked example 5 — combining with the equations of motion. A force of N acts on a kg body at rest for s. Find its final velocity and the distance covered.
Check: , so m/s. The two routes agree.
For the same force, acceleration is inversely proportional to mass. A N force gives a kg body and a kg body only . That inverse relationship is the statement that mass measures inertia, now written as an equation.
**The in the formula is the net force.** If a box is pushed with N while friction opposes with N, the acceleration comes from the net N, not from the N. Substituting the applied force while friction is also acting is the single commonest error in second-law numericals, and the fix is to find the net force on its own line first, as the previous part of this chapter insisted.
with in newtons, in kilograms and in . The unit follows directly: .
Worked example 1 — finding acceleration. A force of N acts on a mass of kg.
Worked example 2 — finding force. A kg object accelerates at .
Worked example 3 — a car. A car of mass kg accelerates from rest to m/s in s.
Worked example 4 — a retarding force. A ball of mass kg moving at m/s is brought to rest in s.
The negative sign means the force opposes the motion, so it is a retarding force of N.
Worked example 5 — combining with the equations of motion. A force of N acts on a kg body at rest for s. Find its final velocity and the distance covered.
Check: , so m/s. The two routes agree.
For the same force, acceleration is inversely proportional to mass. A N force gives a kg body and a kg body only . That inverse relationship is the statement that mass measures inertia, now written as an equation.
**The in the formula is the net force.** If a box is pushed with N while friction opposes with N, the acceleration comes from the net N, not from the N. Substituting the applied force while friction is also acting is the single commonest error in second-law numericals, and the fix is to find the net force on its own line first, as the previous part of this chapter insisted.
What is momentum, and how does it give the force needed?
Momentum is mass times velocity, and it measures how hard a moving body is to stop.
Its unit is , and it is a vector pointing in the direction of the velocity.
The second law, in its original form, says that force is the rate of change of momentum:
Worked example 1 — calculating momentum. A kg object moves at m/s.
Worked example 2 — heavy and slow against light and fast. A truck of kg at m/s, and a car of kg at m/s.
Identical. A slow heavy body can carry exactly as much momentum as a fast light one, and both are equally hard to stop.
Worked example 3 — the force from a momentum change. The kg ball at m/s stopped in s.
Exactly the answer worked example 4 of the previous section gave through — the two forms of the second law are the same law.
Worked example 4 — the catch, in full. A ball of kg at m/s is caught.
- Stopped in s: N
- Stopped in s: N
Worked example 5 — a reversal doubles the change. A kg ball travelling at m/s is struck and returns at m/s the other way.
That is twice the change involved in merely stopping it, so a bat that returns a ball must supply twice the momentum change of a glove that simply catches it. Getting the sign of the returning velocity right is what this example tests.
Everyday consequences of the time factor. Crash barriers and vehicle bumpers are designed to crumple, stretching the collision over a longer time so the force on the occupants is smaller. A gymnast lands on a thick mat for the same reason, and a person jumping from a height bends the knees on landing. In every case the change in momentum is fixed and only the time is under control.
Momentum is not the same as inertia, and not the same as force. Inertia depends on mass alone; momentum depends on mass and velocity, so a stationary truck has huge inertia and zero momentum. And momentum has units of while force has newtons — a quantity and its rate of change are never the same thing.
Its unit is , and it is a vector pointing in the direction of the velocity.
The second law, in its original form, says that force is the rate of change of momentum:
Worked example 1 — calculating momentum. A kg object moves at m/s.
Worked example 2 — heavy and slow against light and fast. A truck of kg at m/s, and a car of kg at m/s.
Identical. A slow heavy body can carry exactly as much momentum as a fast light one, and both are equally hard to stop.
Worked example 3 — the force from a momentum change. The kg ball at m/s stopped in s.
Exactly the answer worked example 4 of the previous section gave through — the two forms of the second law are the same law.
Worked example 4 — the catch, in full. A ball of kg at m/s is caught.
- Stopped in s: N
- Stopped in s: N
Worked example 5 — a reversal doubles the change. A kg ball travelling at m/s is struck and returns at m/s the other way.
That is twice the change involved in merely stopping it, so a bat that returns a ball must supply twice the momentum change of a glove that simply catches it. Getting the sign of the returning velocity right is what this example tests.
Everyday consequences of the time factor. Crash barriers and vehicle bumpers are designed to crumple, stretching the collision over a longer time so the force on the occupants is smaller. A gymnast lands on a thick mat for the same reason, and a person jumping from a height bends the knees on landing. In every case the change in momentum is fixed and only the time is under control.
Momentum is not the same as inertia, and not the same as force. Inertia depends on mass alone; momentum depends on mass and velocity, so a stationary truck has huge inertia and zero momentum. And momentum has units of while force has newtons — a quantity and its rate of change are never the same thing.
Exam tip
Exam tip: use the net force, and watch the sign of a reversal
** takes the net force.** A N push against N of friction accelerates the body with N, not N.
Write the unit on each answer: N for force, for acceleration, for momentum.
A negative force or acceleration means it opposes the motion — report it as a *retarding force of N*, and keep the sign in the working.
For a reversal, subtract with signs: a ball at m/s returning at m/s changes velocity by m/s, so the momentum change is doubled.
and are the same law — use whichever the question's data fits, and check with the other.
The change in momentum is fixed; only the time is adjustable. That is the answer to every question about soft hands, crumple zones, mats and bending knees.
Mass measures inertia, so a question asking which body has more inertia is answered by comparing masses.
Inertia is a property, not a force — nothing pushes you forward when a bus stops.
Name the type of inertia when asked: rest, motion or direction.
And remember the first law describes the zero net force case, where rest and uniform motion are the same state.
Write the unit on each answer: N for force, for acceleration, for momentum.
A negative force or acceleration means it opposes the motion — report it as a *retarding force of N*, and keep the sign in the working.
For a reversal, subtract with signs: a ball at m/s returning at m/s changes velocity by m/s, so the momentum change is doubled.
and are the same law — use whichever the question's data fits, and check with the other.
The change in momentum is fixed; only the time is adjustable. That is the answer to every question about soft hands, crumple zones, mats and bending knees.
Mass measures inertia, so a question asking which body has more inertia is answered by comparing masses.
Inertia is a property, not a force — nothing pushes you forward when a bus stops.
Name the type of inertia when asked: rest, motion or direction.
And remember the first law describes the zero net force case, where rest and uniform motion are the same state.
Did you know
Why a truck and a bullet can be equally hard to stop
A rifle bullet has a tiny mass and an enormous speed. A loaded truck has an enormous mass and a modest speed. Momentum, being the product of the two, can come out the same — and when it does, the two are equally difficult to bring to rest.
That product is why momentum is a more useful quantity than either mass or speed alone for describing a collision. Asking how fast is it moving? does not tell you how dangerous it is, and neither does asking how heavy is it?
But notice what the two situations do not share. Stopping the bullet happens in a very short distance and a very short time, so the force is enormous. Stopping the truck with its brakes takes many seconds and a long distance, so the force is modest even though the momentum change is identical.
That is the whole content of the second law read in its momentum form. The same change in momentum, spread over a longer time, needs a smaller force — and it is the reason a bullet does damage that a slow truck does not.
Engineers use the same arithmetic deliberately and in the opposite direction. A helmet's inner lining crushes, a car's bonnet folds, a cricketer's glove is padded, a jumper's mat is thick. None of these reduces the momentum that has to be removed. Every one of them stretches the time over which it is removed, and the force falls in exact proportion.
So the fielder's soft hands, the crumple zone and the gymnastics mat are not three separate pieces of design sense. They are one equation, applied three times.
That product is why momentum is a more useful quantity than either mass or speed alone for describing a collision. Asking how fast is it moving? does not tell you how dangerous it is, and neither does asking how heavy is it?
But notice what the two situations do not share. Stopping the bullet happens in a very short distance and a very short time, so the force is enormous. Stopping the truck with its brakes takes many seconds and a long distance, so the force is modest even though the momentum change is identical.
That is the whole content of the second law read in its momentum form. The same change in momentum, spread over a longer time, needs a smaller force — and it is the reason a bullet does damage that a slow truck does not.
Engineers use the same arithmetic deliberately and in the opposite direction. A helmet's inner lining crushes, a car's bonnet folds, a cricketer's glove is padded, a jumper's mat is thick. None of these reduces the momentum that has to be removed. Every one of them stretches the time over which it is removed, and the force falls in exact proportion.
So the fielder's soft hands, the crumple zone and the gymnastics mat are not three separate pieces of design sense. They are one equation, applied three times.
Exam relevance
How are Newton's laws and momentum tested in JEE Main and NEET?
This page is the foundation for the Class 11 Physics chapter Laws of Motion, which is examined in JEE Main and in NEET Physics, and almost every idea here is used there without being re-explained.
The second law becomes the central tool of that chapter. It is combined with free-body diagrams to handle blocks on tables, blocks on inclines, connected bodies over pulleys, and a person in a lift. In every one of those, the first step is the net force — which is why the Class 9 warning about substituting the applied force instead of the net force matters far beyond this page.
Momentum is developed in two directions. In Laws of Motion it gives impulse, defined as force multiplied by time and equal to the change in momentum — which is precisely the fielder's catch, written as a formula. In the same chapter it gives conservation of momentum, which is the subject of the next part of this chapter. And in Class 11 System of Particles it extends to the centre of mass.
Inertia returns in Class 11 as inertial and non-inertial frames, where the jerk felt in a braking bus is re-examined and the pseudo-force is introduced precisely because inertia is not a force.
What the questions look like. Numericals dominate, and the most common shape gives a mass, a velocity change and a time, and asks for the force — the momentum form is usually faster than there. Assertion-reason items favour two statements from this page: that a body in uniform motion needs no force, and that inertia is not a force. Impulse questions asking why a longer contact time reduces force are standard, and they are this page's opening example with the word impulse attached.
How board and competitive emphasis differ. A board paper asks you to state the first law and explain the jerk in a bus, then sets a one-step sum. A competitive paper sets a problem where the mass, the friction and the applied force are all given and the net force has to be assembled first, or where a velocity reverses and the sign has to be handled. Board papers reward the verbal explanation; competitive papers reward the sign convention.
The single trap that costs the most marks. Getting the sign wrong on a reversal. A ball arriving at m/s and leaving at m/s has a velocity change of m/s, not and not . Students who treat the return speed as simply the same halve the answer, and the error survives into impulse and collision problems in Class 11.
A second trap worth naming. Confusing inertia with momentum. A stationary loaded truck has enormous inertia and zero momentum, so a question asking which of two bodies is harder to move is about mass, while one asking which is harder to stop is about . The two questions have different answers, and reading which is being asked is half the mark.
The second law becomes the central tool of that chapter. It is combined with free-body diagrams to handle blocks on tables, blocks on inclines, connected bodies over pulleys, and a person in a lift. In every one of those, the first step is the net force — which is why the Class 9 warning about substituting the applied force instead of the net force matters far beyond this page.
Momentum is developed in two directions. In Laws of Motion it gives impulse, defined as force multiplied by time and equal to the change in momentum — which is precisely the fielder's catch, written as a formula. In the same chapter it gives conservation of momentum, which is the subject of the next part of this chapter. And in Class 11 System of Particles it extends to the centre of mass.
Inertia returns in Class 11 as inertial and non-inertial frames, where the jerk felt in a braking bus is re-examined and the pseudo-force is introduced precisely because inertia is not a force.
What the questions look like. Numericals dominate, and the most common shape gives a mass, a velocity change and a time, and asks for the force — the momentum form is usually faster than there. Assertion-reason items favour two statements from this page: that a body in uniform motion needs no force, and that inertia is not a force. Impulse questions asking why a longer contact time reduces force are standard, and they are this page's opening example with the word impulse attached.
How board and competitive emphasis differ. A board paper asks you to state the first law and explain the jerk in a bus, then sets a one-step sum. A competitive paper sets a problem where the mass, the friction and the applied force are all given and the net force has to be assembled first, or where a velocity reverses and the sign has to be handled. Board papers reward the verbal explanation; competitive papers reward the sign convention.
The single trap that costs the most marks. Getting the sign wrong on a reversal. A ball arriving at m/s and leaving at m/s has a velocity change of m/s, not and not . Students who treat the return speed as simply the same halve the answer, and the error survives into impulse and collision problems in Class 11.
A second trap worth naming. Confusing inertia with momentum. A stationary loaded truck has enormous inertia and zero momentum, so a question asking which of two bodies is harder to move is about mass, while one asking which is harder to stop is about . The two questions have different answers, and reading which is being asked is half the mark.
Key takeaways
Inertia, F = ma and momentum: quick revision
- Newton's first law: a body at rest stays at rest and a body in uniform motion continues so, unless an unbalanced external force acts. Also the law of inertia.
- Inertia is the tendency to resist a change in the state of rest or motion, and it comes in rest, motion and direction.
- A bus starting jerks you backwards (inertia of rest); a bus stopping throws you forwards (inertia of motion), which is what seat belts are for.
- A coin falling into a glass when the card is flicked, and dust beaten from a carpet, are inertia of rest.
- The law does not say force is needed to keep something moving — real objects stop because of friction.
- Mass is the measure of inertia. A kg stone has five times the inertia of a kg stone; a loaded truck needs a longer stopping distance than an empty one.
- Inertia is a property, not a force — nothing pushes you forward when a bus stops, and inertia has no unit of its own.
- Newton's second law: , with .
- N on kg gives ; kg at needs N; a kg car reaching m/s in s needs N.
- A kg ball at m/s stopped in s has , so a retarding force of N.
- N on kg for s gives , m/s and m, confirmed by .
- For a fixed force, acceleration is inversely proportional to mass — which is mass-as-inertia written as an equation.
- ** is always the net force.** A N push against N friction accelerates with N.
- Momentum in , a vector. A kg body at m/s has .
- A kg truck at m/s and a kg car at m/s both have — equally hard to stop.
- ****, which gives the same N for the kg ball as did.
- A kg ball at m/s needs N if stopped in s, but only N in s — the fielder's soft hands.
- A reversal doubles the momentum change: kg going from to m/s changes by .
- Crumple zones, mats and bending knees all stretch the time, because the momentum change is fixed.
- A stationary truck has huge inertia and zero momentum — the two are different quantities.
Work out the force needed to stop a cricket ball in s and in s yourself, then try the same for a ball that is hit back rather than caught — the factor you get is the reason batsmen wear gloves.
- Inertia is the tendency to resist a change in the state of rest or motion, and it comes in rest, motion and direction.
- A bus starting jerks you backwards (inertia of rest); a bus stopping throws you forwards (inertia of motion), which is what seat belts are for.
- A coin falling into a glass when the card is flicked, and dust beaten from a carpet, are inertia of rest.
- The law does not say force is needed to keep something moving — real objects stop because of friction.
- Mass is the measure of inertia. A kg stone has five times the inertia of a kg stone; a loaded truck needs a longer stopping distance than an empty one.
- Inertia is a property, not a force — nothing pushes you forward when a bus stops, and inertia has no unit of its own.
- Newton's second law: , with .
- N on kg gives ; kg at needs N; a kg car reaching m/s in s needs N.
- A kg ball at m/s stopped in s has , so a retarding force of N.
- N on kg for s gives , m/s and m, confirmed by .
- For a fixed force, acceleration is inversely proportional to mass — which is mass-as-inertia written as an equation.
- ** is always the net force.** A N push against N friction accelerates with N.
- Momentum in , a vector. A kg body at m/s has .
- A kg truck at m/s and a kg car at m/s both have — equally hard to stop.
- ****, which gives the same N for the kg ball as did.
- A kg ball at m/s needs N if stopped in s, but only N in s — the fielder's soft hands.
- A reversal doubles the momentum change: kg going from to m/s changes by .
- Crumple zones, mats and bending knees all stretch the time, because the momentum change is fixed.
- A stationary truck has huge inertia and zero momentum — the two are different quantities.
Work out the force needed to stop a cricket ball in s and in s yourself, then try the same for a ball that is hit back rather than caught — the factor you get is the reason batsmen wear gloves.