Motion, Force & Newton's Laws

Why This Chapter Matters

Almost every SSC and RRB physics paper carries at least one question straight out of this chapter, usually on Newton's laws, friction, or the difference between speed and velocity. In papers like SSC MTS and RRB Group D, where physics is fact-recall and not calculation, this topic alone has appeared in some form in nearly every recent cycle. It is high-frequency, low-difficulty scoring territory if you get the concepts straight, and a guaranteed trap if you don't.

Here is the coming shape: we start with how motion is described (speed, velocity, acceleration), move into why motion changes at all (Newton's three laws), then cover momentum, friction, and circular motion, three ideas that examiners love to test through daily-life examples. The single biggest mistake aspirants make here is treating "speed" and "velocity" as interchangeable, and then getting a Newton's-law question wrong because they mixed up which law explains "why," which explains "how much," and which explains "the recoil." Keep that distinction sharp and this chapter becomes one of your easiest scoring zones.

1. Describing Motion — Distance, Speed, Velocity, Acceleration

Distance is the total path length a body covers, no matter how winding. Displacement is the straight-line shift from start point to end point, along with direction. A runner doing one full lap of a 400-metre track covers a distance of 400 metres but a displacement of zero, because they end up back where they started.

Exam trap: distance is always equal to or greater than displacement, never less. Questions often disguise this as "a person walks 3 km east, then 4 km north" and ask for displacement (5 km, using the 3-4-5 right triangle) versus distance (7 km). Read carefully which one is being asked.

Speed is distance covered per unit time. It is a scalar quantity, meaning it has only magnitude, no direction. The SI unit is metre per second (m/s), though km/h is common in daily life and in questions.

Velocity is displacement per unit time. It is a vector quantity, having both magnitude and direction. Two cars can have the same speed of 60 km/h but different velocities if one heads north and the other south.

Think of it like a train timetable versus a train's actual GPS track. Speed only tells you how fast the odometer is spinning. Velocity tells you exactly where the train would be if you drew a straight arrow from its starting station to right now.

Uniform velocity means equal displacement in equal time intervals, in the same direction, like a train cruising at a constant speed on a straight track. Non-uniform (variable) velocity means the rate or direction of motion keeps changing, like a car in city traffic.

Acceleration is the rate of change of velocity with time. Its SI unit is metre per second squared (m/s²). When velocity increases, acceleration is positive; when velocity decreases, it is called retardation or deceleration, effectively negative acceleration. A car speeding up as the signal turns green is accelerating; a car braking at the next signal is decelerating.

A useful daily-life way to fix this in memory: a speedometer needle shows speed at that instant, but how fast the needle itself is moving is acceleration. If you press the accelerator pedal hard, the needle jumps quickly, that's high acceleration. Ease off, and the needle creeps, that's low acceleration.

Memory hook: "SVA goes DDT" — Speed needs Distance, Velocity needs Displacement, Acceleration needs Time-rate-of-velocity-change. Say it fast three times before an exam and the definitions stop blurring together.

Uniform and Non-Uniform Acceleration

If velocity changes by equal amounts in equal time intervals, acceleration is uniform, as in a ball rolling down a smooth, constant slope. If the change is irregular, acceleration is non-uniform, as in a two-wheeler weaving through traffic. SSC questions occasionally ask you to identify which everyday scenario fits which category, so keep two or three examples of each ready in your head.

2. Newton's Laws of Motion

Sir Isaac Newton published his three laws of motion in 1687 in his work Principia Mathematica. These three laws together explain almost all everyday motion you will ever be asked about in an exam, from a cricket ball's flight to a rocket's launch.

Newton's First Law — The Law of Inertia

A body continues in its state of rest, or of uniform motion in a straight line, unless acted upon by an external unbalanced force.

This law is really about inertia, the natural tendency of any object to resist a change in its state of motion. A heavier object has more inertia and is harder to start moving or stop once moving. This is why a loaded truck takes longer to accelerate and longer to brake than an empty auto-rickshaw.

You have lived this law daily without naming it. When a bus suddenly brakes, your body keeps moving forward, that's your body's inertia resisting the change imposed by the bus stopping. When the bus suddenly starts, you get pushed backward into your seat, again your body resisting the change from rest to motion. Shaking a mango tree branch to make ripe mangoes fall works the same way: the branch moves, but the mango's inertia keeps it in place for a moment until the stem snaps under the strain.

Exam trap: students often say Newton's first law "creates" force. It does not. It defines force as whatever is needed to change a state of rest or uniform motion. The law is about what happens in the absence of an unbalanced force, not what force does.

Newton's Second Law — Force, Mass and Acceleration

The rate of change of momentum of a body is directly proportional to the applied force, and takes place in the direction of that force. In its simplest usable form: Force = mass × acceleration, written as F = ma.

This is the "how much" law. It tells you that for a given mass, more force produces more acceleration, and for a given force, more mass produces less acceleration. That is exactly why kicking a football sends it flying but kicking a parked car barely moves it, same force from your leg, wildly different mass, wildly different acceleration.

The SI unit of force is the newton (N), defined as the force needed to give a mass of 1 kilogram an acceleration of 1 m/s². Named, fittingly, after the man himself.

Think of pushing a shopping trolley at a supermarket. Push it empty and it darts forward with the lightest push. Load it with ten kilos of groceries and the same push barely nudges it. That is F = ma playing out in front of you every time you shop.

Newton's Third Law — Action and Reaction

For every action, there is an equal and opposite reaction. When body A exerts a force on body B, body B simultaneously exerts an equal and opposite force on body A.

This is the "recoil and push-back" law, and it is the one exam-setters love to test with device and sport examples. A gun recoils backward into the shoulder the instant a bullet is fired forward, both forces are equal in magnitude, opposite in direction. A swimmer pushes water backward with their hands and feet, and the water pushes the swimmer forward with equal force. A rocket expels burning gas downward at enormous speed, and the gas pushes the rocket upward with equal force, this is literally how every rocket and jet engine works.

Exam trap: action and reaction forces act on two different bodies, never on the same body, and they do not cancel each other out. If you push a wall, the wall pushes back on you with equal force, but you move (or feel strain) because that reaction acts on your body, while your action acted on the wall. Students often wrongly think action-reaction pairs cancel, forgetting they act on different objects entirely.

Memory hook for the three laws, in order: "Rest, Push, Recoil" — First law is about Rest (or staying uniform) until disturbed, Second law is about how hard you Push (force and acceleration), Third law is about the Recoil (equal, opposite reaction) that follows.

3. Momentum

Momentum is the quantity of motion contained in a moving body. It is defined as the product of mass and velocity: Momentum = mass × velocity, written p = mv. It is a vector quantity, its direction is the same as the velocity's direction. The SI unit is kilogram-metre per second (kg m/s).

Momentum explains why a slow-moving truck can do more damage in a collision than a fast-moving bicycle. The truck's far greater mass gives it far greater momentum even at modest speed. This is also why goalkeepers in football deliberately step back slightly while catching a hard-hit ball, they extend the time over which the ball's momentum changes to zero, which reduces the force felt by their hands. This same idea, giving impact more time to reduce the force involved, is why vehicles have crumple zones, why cricketers "give with the catch," and why gymnasium mats are padded, all of them stretch out the time of momentum change to soften the force.

Law of conservation of momentum: in a system free from external force, the total momentum before a collision equals the total momentum after it. This is why, in a game of carrom, when your striker hits a stationary coin, the coin moves off while the striker slows or stops, momentum simply transfers from one to the other, the total stays the same.

4. Friction

Friction is the force that opposes relative motion between two surfaces in contact. It acts along the surface, opposite to the direction of intended or actual motion. Without friction, you could not walk (your foot would simply slide), a vehicle could not brake or even move forward from rest, and a pencil could not write on paper.

Types of Friction

• Static friction — the friction that opposes the start of motion between two surfaces still at rest relative to each other. It is why a heavy almirah resists your first push before it finally budges.
• Sliding (kinetic) friction — the friction that acts once a body is already sliding over a surface. It is generally less than static friction, which is why an object is often hardest to get moving and slightly easier to keep moving once it has started.
• Rolling friction — the friction that acts when a body rolls over a surface, as a wheel or ball does. Rolling friction is much smaller than sliding friction, which is precisely why wheels, ball bearings, and roller skates were invented, to replace a sliding problem with a rolling one and save enormous effort.

Exam trap: the order of magnitude from most to least is almost always static friction > sliding friction > rolling friction. Questions frequently ask which type is "least," and the answer is rolling friction. Do not confuse this ordering with anything about speed; it is purely about how strongly each type resists motion, not how fast the body is travelling.

Friction — Friend and Foe

Friction is not purely a nuisance; the exam expects you to know both its uses and its drawbacks.

Where friction helps: walking and running, a matchstick igniting when struck, vehicle brakes and tyres gripping the road, writing with chalk or pen, nails and screws holding wood together.

Where friction hurts: it wears down machine parts, wastes energy as heat in engines, slows down moving vehicles and increases fuel consumption, and generates unwanted heat in bearings and axles.

Reducing friction: using lubricants like oil and grease between moving machine parts, using ball bearings to convert sliding into rolling friction, and streamlining the shape of vehicles, ships, and aircraft to cut down friction with air or water (this is specifically called fluid friction or drag, and reducing it is why cars and planes are shaped the way they are).

A kitchen-table analogy makes this permanent: think of friction as a strict traffic constable at a crowded chowk. Without the constable, nothing organised happens, vehicles would collide, walking would be impossible, nothing would ever "grip" anything else. But too much of the constable's interference and traffic slows to a crawl, wasting everyone's time and fuel. Engineers spend their careers deciding exactly how much of this "constable" to keep and where to remove it with oil, bearings, or sleek shapes.

5. Circular Motion and Centripetal Force

When a body moves along a circular path at constant speed, its direction of motion is continuously changing, even though its speed stays the same. Since velocity includes direction, this means the body is always accelerating, even at "constant speed." This acceleration is directed toward the centre of the circle and is called centripetal acceleration, and the force that causes it is called centripetal force.

Centripetal force is the force that acts on a body moving in a circular path, directed toward the centre of the circle, keeping the body on that curved track instead of flying off in a straight line. Without it, by Newton's first law, the body would simply travel in a straight line forever.

You experience this every time an auto-rickshaw takes a sharp turn and you feel yourself pushed toward the outer edge of the seat. What you are actually feeling is your own body's inertia trying to continue in a straight line while the vehicle curves under you; the seat and your grip supply the centripetal force that redirects you along the curve. This outward "push" you feel is often loosely called centrifugal force, but it is not a real force acting on you, it is simply the sensation of inertia resisting the centripetal force. Exam papers occasionally probe exactly this distinction.

Everyday examples of centripetal force at work: a stone tied to a string and whirled in a circle (the string's tension supplies the centripetal force, and if the string snaps, the stone flies off in a straight line, tangent to the circle, not outward from the centre); a satellite orbiting Earth (Earth's gravity supplies the centripetal force); a car taking a banked turn on a highway (the road's design and friction together supply it); and a washing machine's spin-dry drum, where the drum wall pushes water-soaked clothes inward while water, having no wall to hold it, escapes straight out through the holes, this is why the spin cycle "wrings" clothes dry.

Memory hook: think "Centre-seeking" for centripetal, the "petal" (pull) always points toward the centre. There is no separate outward force pulling you away in real circular motion, only inertia resisting the inward pull.

Quick Revision — One-Line Facts

• Distance is a scalar (path length); displacement is a vector (shortest straight-line shift).
• Speed is a scalar (distance/time); velocity is a vector (displacement/time).
• SI unit of speed and velocity is metre per second (m/s).
• Acceleration is the rate of change of velocity, SI unit m/s².
• Negative acceleration is called retardation or deceleration.
• Uniform velocity means equal displacement in equal time in a fixed direction.
• Newton's laws were published in 1687 in the Principia Mathematica.
• Newton's First Law is the law of inertia: a body resists a change in its state of rest or motion.
• Inertia depends directly on mass, heavier bodies have more inertia.
• Newton's Second Law: Force = mass × acceleration (F = ma).
• SI unit of force is the newton (N), equal to 1 kg m/s².
• Newton's Third Law: every action has an equal and opposite reaction, acting on two different bodies.
• Action-reaction forces never cancel each other because they act on different bodies.
• Momentum = mass × velocity, SI unit kg m/s, and it is a vector quantity.
• The law of conservation of momentum says total momentum stays constant when no external force acts.
• Rocket propulsion works entirely on Newton's third law, expelled gas pushes the rocket forward.
• Friction opposes relative motion between two surfaces in contact.
• Order of friction, generally: static > sliding (kinetic) > rolling.
• Rolling friction is the smallest type of friction, which is why wheels and ball bearings are used.
• Friction lets us walk, write, and brake vehicles; it also wastes energy as heat and wears out machines.
• Lubricants and ball bearings reduce friction between moving machine parts.
• Streamlined shapes reduce fluid friction (drag) in vehicles, ships, and aircraft.
• A body moving in a circle at constant speed is still accelerating, because direction keeps changing.
• Centripetal force acts toward the centre of a circular path and keeps a body on that path.
• "Centrifugal force" felt in a turning vehicle is the sensation of inertia, not a real inward-acting force.
• If a whirled string snaps, the object flies off in a straight line tangent to the circle, not straight outward.
• A satellite stays in orbit because Earth's gravity supplies the centripetal force.
• A washing machine's spin-dry cycle uses circular motion to fling water out of clothes through drum holes.
• Goalkeepers "give" with a catch to increase impact time and reduce the force on their hands, a momentum idea.
• Static friction is generally greater than kinetic friction, so starting motion needs more force than sustaining it.
• Isaac Newton is the scientist credited with all three laws of motion and the law of gravitation.

Memory Tables

Table 1: Motion Quantities at a Glance

Quantity	Type (Scalar/Vector)	Formula	SI Unit
Distance	Scalar	Total path length	metre (m)
Displacement	Vector	Shortest path with direction	metre (m)
Speed	Scalar	Distance / Time	m/s
Velocity	Vector	Displacement / Time	m/s
Acceleration	Vector	Change in velocity / Time	m/s²
Momentum	Vector	Mass × Velocity	kg m/s
Force	Vector	Mass × Acceleration	newton (N)

Table 2: Newton's Three Laws — One Line Each

Law	Core Idea	Everyday Example
First Law	A body resists change in its state of rest/motion (inertia)	Passenger jerks forward when a bus brakes suddenly
Second Law	Force equals mass times acceleration (F = ma)	Kicking a football moves it far; kicking a parked car barely moves it
Third Law	Every action has an equal, opposite reaction on another body	A gun recoils backward when a bullet fires forward

Table 3: Types of Friction Compared

Type of Friction	When It Acts	Relative Strength	Real-World Reduction Method
Static friction	Before motion begins	Highest	Reduced with lubricants
Sliding (kinetic) friction	While surfaces slide over each other	Medium	Reduced with lubricants or smoother surfaces
Rolling friction	While a body rolls over a surface	Lowest	Used deliberately via wheels and ball bearings
Fluid friction (drag)	Body moving through air or water	Depends on shape and speed	Reduced through streamlined design

Practice MCQs

Q1. A person walks 3 km east and then 4 km north. What is the magnitude of their displacement? (a) 3 km (b) 4 km (c) 5 km (d) 7 km

Q2. Which of the following is a vector quantity? (a) Speed (b) Distance (c) Velocity (d) Time

Q3. The SI unit of force is the: (a) joule (b) newton (c) watt (d) pascal

Q4. Newton's first law of motion is also known as the law of: (a) Gravitation (b) Momentum (c) Inertia (d) Friction

Q5. When a bus suddenly starts moving, standing passengers tend to fall backward. This is best explained by: (a) Newton's second law (b) Newton's third law (c) Newton's first law (d) The law of conservation of momentum

Q6. A gun recoils when a bullet is fired from it. This is an example of: (a) Newton's first law (b) Newton's second law (c) Newton's third law (d) Law of gravitation

Q7. Which type of friction acts when a ball rolls on the ground? (a) Static friction (b) Sliding friction (c) Rolling friction (d) Fluid friction

Q8. Momentum is defined as the product of: (a) mass and acceleration (b) mass and velocity (c) force and time (d) force and distance

Q9. According to Newton's second law of motion, force is directly proportional to: (a) mass alone (b) velocity alone (c) the product of mass and acceleration (d) distance travelled

Q10. Which of the following reduces friction between moving machine parts? (a) Increasing surface roughness (b) Using lubricants (c) Increasing the load (d) Increasing contact area

Q11. A stone tied to a string is whirled in a horizontal circle. If the string suddenly snaps, the stone will: (a) move toward the centre of the circle (b) stop immediately (c) fly off in a straight line tangent to the circle (d) continue moving in the same circle

Q12. In which of these examples does rolling friction play the biggest role in reducing effort? (a) A block sliding on ice (b) A suitcase dragged on the floor (c) A suitcase pulled on its wheels (d) A book pushed across a table

Q13. Two objects of different masses are acted upon by the same force. Which one will have greater acceleration? (a) The object with greater mass (b) The object with smaller mass (c) Both will have equal acceleration (d) Acceleration does not depend on mass

Q14. A washing machine's spin-dry cycle removes water from clothes mainly using the principle of: (a) Static friction (b) Centripetal force and circular motion (c) Newton's third law only (d) Gravitation alone

Q15. A goalkeeper pulls their hands backward slightly while catching a fast-moving ball. This technique works because it: (a) increases the force on the hands (b) reduces the ball's momentum to zero instantly (c) increases the time of impact, reducing the force felt (d) has no effect on the force experienced

Answer Key

Q	Answer	Reason
Q1	(c) 5 km	Displacement is the straight-line shift; 3-4-5 right triangle gives 5 km, distance would be 7 km.
Q2	(c) Velocity	Velocity has both magnitude and direction, making it a vector; speed and distance are scalars.
Q3	(b) newton	The newton is the force needed to give 1 kg an acceleration of 1 m/s², per Newton's second law.
Q4	(c) Inertia	The first law describes a body's natural resistance to any change in its state of rest or motion.
Q5	(c) Newton's first law	The passenger's body, at rest, resists the sudden forward motion of the bus and lags behind, falling backward.
Q6	(c) Newton's third law	The bullet's forward force (action) produces an equal, opposite backward force (reaction) on the gun.
Q7	(c) Rolling friction	Rolling friction acts specifically when one surface rolls, not slides, over another, and is the smallest of the friction types.
Q8	(b) mass and velocity	Momentum (p = mv) is the product of a body's mass and its velocity, and is a vector quantity.
Q9	(c) the product of mass and acceleration	Newton's second law states F = ma, force equals mass multiplied by acceleration, not either alone.
Q10	(b) Using lubricants	Lubricants like oil and grease reduce direct contact between surfaces, cutting down friction and wear.
Q11	(c) fly off in a straight line tangent to the circle	Once the centripetal force from the string vanishes, Newton's first law takes over and the stone travels straight, tangent to its last point on the circle.
Q12	(c) A suitcase pulled on its wheels	Wheels convert sliding friction into much smaller rolling friction, which is why wheeled suitcases need far less effort to move.
Q13	(b) The object with smaller mass	By F = ma, for the same force, acceleration is inversely proportional to mass, so the lighter object accelerates more.
Q14	(b) Centripetal force and circular motion	The spinning drum wall supplies centripetal force to the clothes while water, unrestrained, escapes outward through the holes in a straight path.
Q15	(c) increases the time of impact, reducing the force felt	Spreading the same change in momentum over a longer time lowers the average force, softening the impact on the hands.