Chapter 3 · ApplicationsLaws of motion in real lifeExplore online

Applications & Explore Online ചലനനിയമങ്ങൾ

Newton’s three laws are not only in the textbook: they decide why a seat belt saves lives, why an overloaded, speeding bus cannot stop in time, how a rocket rises from Sriharikota, why a fielder “gives” with the ball, how a snake boat moves and how a magician pulls a tablecloth from under the plates. Every lab below uses only the ideas of this chapter.

📖 The ideas behind every lab

1. Seat belts and airbags – impulse saves lives

In a crash the car stops almost at once. The passenger’s body, by inertia of motion (ചലനജഡത്വം), keeps moving forward. Something must now stop the body – and its change of momentum, mv − mu, is fixed by the mass and the speed. By the impulse–momentum principle (ആവേഗമൊമെന്റതത്വം) F = (mv − mu)/t: if the body is stopped by the hard dashboard in about a hundredth of a second, the force is enormous; a seat belt that stretches a little, and an airbag that squashes, make the stopping time ten or twenty times longer and the force ten or twenty times smaller.

🚗 Crash-test lab: a 60 kg passenger, F = (mv − mu)/t

The rectangles on the graph all have the same area F × t = impulse = change of momentum. A short, tall rectangle is a huge force; a long, flat one is a gentle force. The stopping times are typical rough values, used only to compare.

⚠ Always wear the seat belt, in the back seat too, and a helmet on a two-wheeler – a helmet’s soft lining also makes the stopping time of the head longer. An airbag protects only a passenger who is wearing the belt.

📝 In the exam this looks like: “A 60 kg passenger in a car moving at 72 km/h is stopped by the seat belt and airbag in 0.2 s. Find the force on the passenger. How many times bigger would it be if the body hit the dashboard and stopped in 0.02 s?” → u = 20 m/s, mv − mu = 60 × (0 − 20) = −1200 kg m/s; F = −1200/0.2 = −6000 N (opposite to the motion); with 0.02 s, F = −60,000 N – 10 times bigger.

2. The KSRTC bus – overloading, overspeeding and stopping distance

A bus full of passengers has a large mass, and on the highway a large velocity, so its momentum p = mv is huge. To stop it, the brakes must remove all of that momentum. The braking force is F = (mv − mu)/t = ma, where a is the retardation (മന്ദീകരണം) the brakes produce. While stopping uniformly, the bus covers s = u2/2a (from v2 = u2 + 2as with v = 0). Notice the u2: double the speed and the braking distance becomes four times.

🚌 Bus-braking lab: p = mv, F = ma, s = u²/2a

Distances along the road are to scale (one mark = 20 m); the bus itself is drawn about twice its real size so that you can see it. The brakes start at the left flag. The reaction-time button adds the distance travelled at full speed during the one second the driver needs to see the danger and press the brake.

🛣️ Road safety in numbers

The brakes and tyres of a vehicle can give only a limited force. An overloaded bus has more mass, so the same braking force F gives a smaller retardation a = F/m – it slides farther. An overspeeding bus needs four times the distance when its speed is doubled. Add a second of reaction time and the bus covers u metres more before the brakes even work. This is what the extended activity on overloading and overspeeding (തുടർപ്രവർത്തനങ്ങൾ) is about.

📝 In the exam this looks like: “A bus of mass 12,000 kg moving at 36 km/h is stopped in 4 s. Find its initial momentum and the braking force.” → u = 10 m/s, p = 12,000 × 10 = 120,000 kg m/s; F = 12,000 × (0 − 10)/4 = −30,000 N (the minus sign: opposite to the motion). Let us assess Q11: a 14,000 kg vehicle stopped with a retardation of 1.8 m/s2 needs F = 14,000 × 1.8 = 25,200 N.

3. Rocket launch from Sriharikota – the third law

At the Satish Dhawan Space Centre, Sriharikota, ISRO launches the PSLV and LVM3. A rocket does not push against the ground or the air. Its engines burn fuel and throw hot, high-pressure gases downwards at a very high speed: the rocket pushes the gases down (action), and the gases push the rocket up (reaction) – this upward push is the thrust. The rocket also has its weight acting downwards (the Earth’s pull, W = mg – you will meet it in Chapter 4). It rises only if the thrust is bigger than the weight: the resultant (സഫലബലം) F = thrust − weight is then upwards, and a = F/m.

🚀 Launch-pad lab: net force = thrust − weight, a = F/m

1 kN = 1000 N; 1 tonne = 1000 kg. Weight W = m × 9.8 N. Simple model: the mass and the thrust are kept constant for the first 10 s and air resistance is ignored. (In a real rocket the mass falls quickly as fuel burns, so the acceleration grows.) The numbers are round example numbers, not the data of a particular rocket.

📝 In the exam this looks like: “Explain how a rocket moves up using Newton’s third law.” → The burning fuel produces hot gases at high pressure which rush out downwards at high speed (action); the gases exert an equal and opposite force on the rocket upwards (reaction), so the rocket moves up. With numbers: “a 200,000 kg rocket has a weight of 1,960,000 N and an engine thrust of 2,960,000 N; find the acceleration” → F = 2,960,000 − 1,960,000 = 1,000,000 N upwards; a = 1,000,000/200,000 = 5 m/s2.

4. Catching in cricket and football – make the time longer

When a fielder catches a fast ball, the ball’s momentum must become zero. The fielder cannot change the change of momentum, mv − mu, but can change the time: by moving the hands back with the ball, the ball stops over a longer time, so the force on the hands is smaller. A goalkeeper does the same with a football. The textbook’s example: a 200 g ball at 30 m/s caught in 0.3 s, 0.2 s and 0.1 s needs forces of 20 N, 30 N and 60 N.

🏏 Catching lab: F = (mv − mu)/t

Hand movement: if the ball slows down uniformly, it travels ½ × u × t while stopping, so a longer catching time needs the hands to move back farther. The curve on the right shows F for every catching time with the same change of momentum.

📝 In the exam this looks like: “Give reasons: a cricketer pulls his hands back while catching a fast ball; a goalkeeper moves his hands back with the ball; a foam bed is kept in the pole-vault pit; glassware is packed with straw.” → In each case the time taken to stop the object is made longer, so for the same change of momentum the force (impact) is smaller (F = Δp/t).

5. Vallam kali – how a snake boat moves

In a vallam kali (വള്ളംകളി) race on Punnamada lake or the Pampa, dozens of rowers dip their oars together. Each oar pushes the water backwards (action); the water pushes the oar, and with it the boat, forwards (reaction) – Newton’s third law. As the boat speeds up, the water resistance on the hull grows. When the resistance becomes equal to the rowers’ push, the forces on the boat are balanced and it moves at a steady speed (first law). Stop rowing and the resistance alone acts – an unbalanced force against the motion – so the boat slows down.

🚣 Snake-boat lab: action on the water, reaction on the boat

Model: boat with crew ≈ 6000 kg; water resistance = 120 × v² newton (it grows quickly with speed). These are rough example numbers. The arrows on the boat are drawn to scale.

📝 In the exam this looks like: “How does a boat move forward when the oars push the water backwards? Which law of motion explains it?” → The oars exert a force on the water backwards (action); the water exerts an equal and opposite force on the oars and the boat forwards (reaction) – Newton’s third law. “When the boat moves with a constant velocity, are the forces on it balanced?” → Yes: the forward push equals the water resistance, so the resultant is zero.

6. Carrom coins and the tablecloth trick – inertia of rest

Pull a tablecloth slowly and the plate comes with it: friction has a long time to act and drags the plate along. Pull it very fast and friction acts only for a few hundredths of a second – too short to give the plate much velocity. The plate stays almost where it was: inertia of rest (നിശ്ചലജഡത്വം). The carrom-coin tower and the card-and-coin trick (Let us assess Q5) work the same way.

🍽️ Tablecloth lab: how fast must you pull?

Model: friction between the cloth and the plate gives the plate an acceleration of about 3 m/s² (friction coefficient 0.3); the cloth reaches 30 cm behind the plate; the table edge is 50 cm in front of it. The animation is in slow motion.

⚠ Practise with a plastic plate or a steel tumbler – never with glass or china – and only with an adult’s permission.
🎯 Carrom-coin tower (textbook Fig. 3.7 b)

📝 In the exam this looks like: “A card is placed on a cup and a coin on the card. The card is flicked away quickly. What happens to the coin? Why?” → The coin falls into the cup: it tends to remain at rest (inertia of rest), and the card moves away so quickly that friction has too little time to drag the coin along (Let us assess Q5).

✅ Check yourself

🌐 Explore online – resources hub

Everything in this guide works offline. The links below are extras: simulations, short videos and articles from trusted sources (PhET and others), grouped by topic. They need internet and open in a new tab, so you will not lose your place here. Ask a parent or teacher before you go online. The PhET simulations (for example “Forces and Motion: Basics”) run directly in your browser – nothing to install.

Resultant, balanced and unbalanced forces

🌐 Resultant, balanced and unbalanced forces needs internet · opens in a new tab

Galileo and the rolling ball

🌐 Galileo and the rolling ball needs internet · opens in a new tab

Isaac Newton

🌐 Isaac Newton needs internet · opens in a new tab

First law and inertia

🌐 First law and inertia needs internet · opens in a new tab

Momentum and Newton's cradle

🌐 Momentum and Newton's cradle needs internet · opens in a new tab

Second law: F = ma

🌐 Second law: F = ma needs internet · opens in a new tab

Impulse: catching, airbags and helmets

🌐 Impulse: catching, airbags and helmets needs internet · opens in a new tab

Third law: action and reaction, rockets

🌐 Third law: action and reaction, rockets needs internet · opens in a new tab

Road safety: speed, load and seat belts

🌐 Road safety: speed, load and seat belts needs internet · opens in a new tab

Malayalam explainers

🌐 Malayalam explainers needs internet · opens in a new tab

More to explore

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Now pull the whole chapter together: Summary & Quick Quiz ▶