Chapter 3 · Section 1Textbook pages 53–57

Resultant, Balanced and Unbalanced Forces സഫലബലം / പരിണതബലം

Pushing a car, juggling balls, rowing a boat, throwing a javelin, swinging a bat – every one of these needs a force. But often several forces act on the same object at once. On this page you learn to add them into one resultant force, to tell balanced from unbalanced forces, and why pushing an autorickshaw from inside never moves it.

📖 What you need (pages 1 and 4 of this guide)

A force (ബലം) is a push or a pull; unit newton (N). A force has a size and a direction, so we draw it as an arrow.
Along a line, choose one direction as positive (+); a force the other way is then negative (−).
Friction (ഘർഷണം) is a force that opposes sliding or rolling.

1. The opening picture (page 53)

The chapter opens with a picture full of motion: a boy pushing a car, a man juggling, a boy rowing, an athlete throwing a javelin and a girl with a cricket bat. The textbook first asks about the child catching a cricket ball:

Q. പന്ത് പിടിച്ചപ്പോൾ കുട്ടി പന്തിനൊപ്പം കൈ കൂടി പുറകോട്ട് ചലിപ്പിച്ചതെന്തിനായിരിക്കും? – Why does the child move the hands backwards along with the ball while catching it?

To make the stopping time longer, so that the force on the hands is smaller and the catch does not hurt. Why: the ball has to lose all its motion either way; spreading that over a longer time needs a smaller force. You will prove this with numbers on page 11 (impulse).

2. Tug of war (Fig 3.1)

Two teams pull a rope (വടംവലി). Team 1 pulls to the left with force F₁, team 2 pulls to the right with force F₂. Set the two pulls and press Pull!

🪢 Tug of war lab

Team 1 (left) F₁ N Team 2 (right) F₂ N

Arrows are drawn to scale (0.3 px per newton). Taking right as positive, F₂ counts as + and F₁ as −. Try F₁ = F₂: both teams pull hard, yet the ribbon does not move.

Textbook questions (page 53)

രണ്ടു ടീമുകളും ബലം പ്രയോഗിച്ചുകൊണ്ടിരിക്കുന്നുവെങ്കിലും വടം ഒരു ദിശയിൽ മാത്രം ചലിക്കുന്നു. എന്തായിരിക്കും കാരണം? Both teams apply force, yet the rope moves in one direction only. Why?
ഇവിടെ ഇരു ടീമുകളും പ്രയോഗിച്ച ബലം തുല്യമാണോ? Are the forces of the two teams equal?
ഏതു ടീമാണ് കൂടുതൽ ബലം പ്രയോഗിച്ചത്? Which team applied more force?
കൂടുതലായി അനുഭവപ്പെട്ട ബലമല്ലേ ചലനം ഉണ്ടാക്കിയത്? Isn't it the extra force that caused the motion?

1. Because the two forces are not equal: the bigger pull wins and the rope moves towards that team.
2. No. If they were equal the rope would not move (try it in the lab).
3. The winning team – the team towards which the rope moves (team 1 in Fig 3.1, since the rope moves along F₁).
4. Yes. Only the difference (the “extra” force) decides the motion. That difference is the resultant force.

3. Resultant force (page 54)

In Fig 3.1, F₁ acts towards the winning team and F₂ in the opposite direction, so the resultant acts towards the winning team. Because force is a vector (സദിശ അളവ്) – it has a direction – we take forces in one chosen direction as positive and forces in the opposite direction as negative.

⭐ Resultant force (textbook page 54)

ഒരു വസ്തുവിൽ ഒന്നിലധികം ബലങ്ങൾ ഒരേ സമയത്ത് പ്രയോഗിക്കുമ്പോൾ ഈ ബലങ്ങൾ വസ്തുവിൽ ഉളവാക്കുന്ന ആകെ ബലമാണ് സഫലബലം അഥവാ പരിണതബലം.

When several forces act on an object at the same time, the total force they produce on it is the resultant force (net force).

Worked example · 100 N east and 150 N west

Textbook (page 54): ഒരു വസ്തുവിൽ കിഴക്ക് ദിശയിൽ 100 N ബലവും പടിഞ്ഞാറ് ദിശയിൽ 150 N ബലവും പ്രയോഗിച്ചാൽ പരിണതബലം എത്രയായിരിക്കും? – A force of 100 N towards the east and 150 N towards the west act on an object. What is the resultant?

Take east as positive. Then the westward 150 N is −150 N. Add with the signs. A negative answer means “towards the west”.

🎬 Exam layout, step by step

Answer: taking east as +, resultant = 100 N + (−150 N) = −50 N, that is 50 N towards the west. (You could equally take west as + and get +50 N towards the west – the same answer.)

4. Fig 3.2 and Table 3.1 – six situations

The textbook shows six pictures. Step through them; the table fills in as you go. For each picture we take the direction of F₁ as positive.

🎬 Fig 3.2 (a)–(f), one by one

Fig	Situation	Force F₁ (N)	Force F₂ (N)	Resultant (N)
(a)	two girls push a table	?	?	?
(b)	pushes from both sides	?	?	?
(c)	150 N push against 120 N push	?	?	?
(d)	man pulls a loaded wheelbarrow	?	?	?
(e)	two people pull a table	?	?	?
(f)	pulls of 200 N each way	?	?	?

Textbook questions (page 55)

ഏതൊക്കെ സന്ദർഭങ്ങളിലാണ് പരിണതബലം പൂജ്യമാകുന്നത്? In which cases is the resultant zero?
പരിണതബലം പൂജ്യമല്ലാത്ത സന്ദർഭങ്ങൾ ഏവ? In which cases is it not zero?
എതെല്ലാം സന്ദർഭങ്ങളിലാണ് ചലനമുണ്ടാകാത്തത്? In which cases is there no motion?

1. (b) and (f): 150 + (−150) = 0 and 200 + (−200) = 0.
2. (a), (c), (d), (e): 150 N, 30 N, 300 N and 200 N.
3. (b) and (f) – the forces cancel. Note: in real life a heavy table may also stay put in (c) if 30 N is not enough to beat the friction from the floor; then friction is one more force, and the resultant including friction is zero.

⭐ Balanced forces (textbook page 55)

ഒരു വസ്തുവിൽ അനുഭവപ്പെടുന്ന പരിണതബലം പൂജ്യമാണെങ്കിൽ പ്രയോഗിക്കപ്പെട്ട ബലങ്ങളെ സന്തുലിതബലങ്ങൾ എന്നു പറയുന്നു. ഇത്തരം ബലങ്ങൾക്ക് നിശ്ചലാവസ്ഥയിലുള്ള ഒരു വസ്തുവിനെ ചലിപ്പിക്കാനോ ചലനാവസ്ഥയിലുള്ള ഒരു വസ്തുവിന്റെ ചലനദിശയ്ക്കോ വേഗത്തിനോ മാറ്റം വരുത്താനോ കഴിയില്ല.

If the resultant force on an object is zero, the forces are balanced forces (സന്തുലിതബലങ്ങൾ). They cannot move an object at rest, and cannot change the speed or the direction of a moving object.

⭐ Unbalanced forces (textbook page 55)

ഒരു വസ്തുവിൽ അനുഭവപ്പെടുന്ന പരിണതബലം പൂജ്യമല്ലെങ്കിൽ പ്രയോഗിക്കപ്പെട്ട ബലങ്ങളെ അസന്തുലിതബലങ്ങൾ എന്നു പറയുന്നു. ഇത്തരം ബലങ്ങൾക്ക് നിശ്ചലാവസ്ഥയിലുള്ള ഒരു വസ്തുവിനെ ചലിപ്പിക്കാനും ചലനാവസ്ഥയിലുള്ള ഒരു വസ്തുവിന്റെ ചലനദിശയ്ക്കോ വേഗത്തിനോ മാറ്റം വരുത്താനും കഴിയും.

If the resultant force is not zero, the forces are unbalanced forces (അസന്തുലിതബലങ്ങൾ). They can move an object at rest and can change the speed or direction of a moving object.

Q. ചിത്രം 3.1 ലെ വടംവലി മത്സരത്തിൽ രണ്ട് ടീമുകളും വടത്തിൽ ബലം പ്രയോഗിച്ചപ്പോൾ വടത്തിൽ അനുഭവപ്പെട്ട പരിണതബലം സന്തുലിതമോ അസന്തുലിതമോ? – In the tug of war of Fig 3.1, was the resultant force on the rope balanced or unbalanced?

Unbalanced. The rope moved towards one team, so the resultant was not zero. (At a moment when the rope stands still with both teams straining, the forces are balanced.)

5. Activity – a toy car between two pans (Fig 3.3)

The thinking girl asks: എല്ലാ ബലങ്ങളും ചലനം ഉണ്ടാക്കുമോ? – Does every force cause motion? Let us test it. Fix a pulley at each end of a plank about 1.2 m long and 10 cm wide, put it on a table, place a toy car in the middle and tie it with strings to two pans of equal mass hanging over the pulleys. Put 200 g on each pan.

⚠ Real experiment: clamp the pulleys and the plank firmly, keep your feet away from under the pans, and add slotted masses gently – a pan that runs down to the floor can drop its masses. Stop the car with your hand before it crashes into a pulley.

🎬 Virtual lab: the textbook's steps (slow motion)

🧪 Your own trials – add masses, watch the car, record

Left pan: Right pan:

Left pan (g)	Right pan (g)	Pull of pans (N)	Balanced?	What the car did	a (m/s²)

Model: toy car 300 g, each pan 50 g, g = 9.8 m/s². Each pan pulls with (pan + load) × g; the pan weights cancel, so the pull = (difference of loads) × g. A little rolling friction acts on the car (up to 0.25 N while at rest, 0.20 N while rolling) – so 50 g extra (0.49 N) is enough to start it. Everything that moves – car, both pans, all loads – shares the acceleration: a = resultant ÷ total moving mass.

Textbook questions (page 56) – predict, try in the lab, then check.

ടോയ് കാർ ചലിക്കുന്നുണ്ടോ? Does the toy car move (200 g on each pan)?
ഇപ്പോൾ ബലങ്ങൾ സന്തുലിതമാണോ അതോ അസന്തുലിതമാണോ? Are the forces balanced or unbalanced now?
ഏതെങ്കിലും ഒരു പാനിൽ 50 g കൂടി വയ്ക്കുക. എന്തു നിരീക്ഷിക്കുന്നു? Add 50 g to one pan. What do you observe?
Balanced or unbalanced in this case?
കാർ ചലിച്ചുകൊണ്ടിരിക്കുന്ന അവസരത്തിൽ കാർ ഓടുന്ന ദിശയിലുള്ള പാനിൽ 50 g കൂടി ഇട്ടാൽ … While the car is moving, put 50 g more on the pan on the side it is moving towards. What change do you see?
… വിപരീത ദിശയിലുള്ള പാനിൽ 200 g കൂടി ഇട്ടാൽ എന്തു നിരീക്ഷിക്കും? While it is moving, put 200 g more on the opposite pan. What happens?
Balanced or unbalanced now?
ഈ പ്രവർത്തനങ്ങളിൽ നിന്നും എന്താണ് മനസ്സിലാക്കിയത്? What did you learn from these activities?
പരിണത ബലം അനുഭവപ്പെട്ട ദിശയിലേക്ക് വസ്തു ചലിക്കുന്നുണ്ടോ? Does the object move in the direction of the resultant force? (moves / does not move)
ചലനവേഗം വർധിക്കുന്നത് ഏത് സാഹചര്യത്തിലാണ്? When does the speed increase? (when the resultant increases / decreases)
കാറിനെ ചലിപ്പിച്ച ബലം കാറിന്റെ പുറത്ത് നിന്നാണോ അതോ അകത്ത് നിന്നാണോ പ്രയോഗിക്കപ്പെട്ടത്? Was the force that moved the car applied from outside or from inside the car?
ചലനദിശ മാറുന്നത് ഏത് സാഹചര്യത്തിലാണ്? When does the direction of motion change?

1. No, the car stays still.
2. Balanced: each pan pulls with the same force, so the resultant is zero.
3. The car starts moving towards the heavier pan.
4. Unbalanced: the heavier side pulls 0.05 kg × 9.8 m/s² = 0.49 N more.
5. The car speeds up faster – its acceleration increases, because the resultant force in its direction became bigger.
6. The car slows down, stops for an instant and then moves the other way: the resultant now points opposite to its motion.
7. Unbalanced (the resultant is towards the pan that got the 200 g).
8. An unbalanced force changes the speed and/or the direction of motion; balanced forces do not.
9. Yes – it moves (ചലിക്കുന്നു); more exactly, it accelerates in the direction of the resultant.
10. When the resultant force increases (in the direction of motion).
11. From outside – the strings, pulled by the weights of the pans, act on the car from outside.
12. When the resultant force acts opposite to the motion (long enough to stop the car and push it back).

In all these activities the force was given to the car from outside, so these are external forces (ബാഹ്യബലങ്ങൾ). An external force may be balanced or unbalanced.

⭐ Unbalanced external force (textbook page 56)

ഒരു വസ്തുവിൽ അസന്തുലിതബാഹ്യബലം പ്രയോഗിച്ചാൽ ആ വസ്തുവിന്റെ നിശ്ചലാവസ്ഥയ്ക്കോ ചലനാവസ്ഥയ്ക്കോ ചലനദിശയ്ക്കോ വേഗത്തിനോ മാറ്റം വരുത്താനോ അതിനുള്ള പ്രവണത ഉളവാക്കാനോ കഴിയും. ഈ മാറ്റം അസന്തുലിതബലത്തിന്റെ ദിശയിലായിരിക്കും.

An unbalanced external force (അസന്തുലിത ബാഹ്യബലം) acting on an object can change its state of rest or of motion, its direction or its speed – or create a tendency to do so. The change is in the direction of the unbalanced force.

6. Pushing an autorickshaw from inside (Fig 3.4)

Look at the man standing in the back of a goods autorickshaw, pushing hard on the cabin. Press Push and compare with a man pushing from the road.

🛺 Inside push vs outside push

Inside: the man's hands push the cabin forward, and his feet push the floor of the same autorickshaw backward just as hard. Both forces act on the autorickshaw, so they cancel. Outside: his feet push the road, not the autorickshaw – the only push on the autorickshaw is forward.

Textbook questions (page 56)

ഒരു വാഹനത്തെ അതിനകത്തു നിന്ന് തള്ളിയാൽ ആ വാഹനം ചലിക്കുമോ? If you push a vehicle from inside it, will it move?
ഈ ബലം ആന്തരികബലമല്ലേ? Isn't this an internal force?
ആന്തരിക ബലങ്ങൾ ……… ആയിരിക്കും. (സന്തുലിതം/അസന്തുലിതം) Internal forces are ……… (balanced / unbalanced).

1. No.
2. Yes – it is an internal force (ആന്തരികബലം): it is applied from inside the vehicle (the man is part of the moving system).
3. Balanced (സന്തുലിതം): every inside push on one part comes with an equal push the opposite way on another part, so the resultant on the vehicle is zero. (Page 12 explains this with Newton's third law.)

⭐ Internal forces (textbook page 57)

എല്ലാ ആന്തരികബലങ്ങളും സന്തുലിത ബലങ്ങളാണ്. അതിനാലാണ് ആന്തരികബലം പ്രയോഗിച്ചാൽ വസ്തുവിന്റെ നിശ്ചലാവസ്ഥയ്ക്കോ ചലനാവസ്ഥയ്ക്കോ മാറ്റമുണ്ടാകാത്തത്.

All internal forces are balanced forces. That is why internal forces cannot change the state of rest or of motion of an object.

7. The chart for your science diary (page 57)

The textbook asks you to complete a chart (ചാർട്ട് പൂർത്തിയാക്കി സയൻസ് ഡയറിയിൽ രേഖപ്പെടുത്തൂ). Fill it in step by step – the green boxes are the blanks of the textbook chart.

🗂️ Kinds of force – completing the chart

🌴 Kerala connection

At an Onam vadamvali (tug of war) the rope stays still for long seconds while both teams strain – balanced forces. The moment one team's pull grows, the forces become unbalanced and the rope races to that side. In a vallam kali snake-boat race, the oars of a hundred rowers add up (a resultant!) against the drag of the water; the boat speeds up only while their push is bigger than the drag.

🌐 Try it on a simulation

The textbook (page 56) suggests two PhET simulations: “Forces and Motion: Basics” (tug of war with little people, and pushing a box with and without friction) and “Friction”. If you have internet, the links are in the box below.

🌐 Explore online needs internet · opens in a new tab

🧪 Forces and Motion: Basics (PhET simulation) PhET
The sim named in your textbook: play tug of war, add forces and see when they balance or not.
🧪 Friction (PhET simulation) PhET
Rub two surfaces together and watch friction heat up the atoms; also named in the textbook.
🧪 Forces and Motion (PhET simulation, older Java version) PhET
Push a crate with and without friction and plot force, speed and acceleration. Older sim that runs in the browser via CheerpJ.
▶️ Balance and unbalanced forces | Laws of motion | Physics class 9 YouTube – Khan Academy India - English · 5:50
Short Class 9 explainer on balanced versus unbalanced forces.
📄 Balanced and Unbalanced Forces The Physics Classroom
Worked pictures showing when forces cancel and when they make an object speed up or slow down.
📄 Determining the Net Force The Physics Classroom
How to add forces to get the resultant (net) force.

8. Check yourself

🧭 Summary

Resultant force = the total of all forces on an object, adding with signs (one direction +, the other −).
Resultant zero → balanced forces → no change in rest or motion. Resultant not zero → unbalanced → the speed or direction changes, in the direction of the resultant.
Toy car: equal pans → stays; extra 50 g → moves towards it; more in its direction → speeds up more; more on the other side → slows, stops, reverses.
Internal forces are always balanced: you cannot move an autorickshaw by pushing it from inside.

Next: Galileo's channel and Newton's first law ▶