The Earth Pulls Everything ഭൂഗുരുത്വാകർഷണബലം
Mangoes drop, a stone thrown up comes back, a feather drifts down. Something pulls all of them towards the ground. On this page you find out what pulls them, in which direction, whether we can measure that pull, and what it depends on.
- A force (ബലം) is a push or a pull; its unit is the newton (N).
- A spring balance (സ്പ്രിങ് ബാലൻസ്) measures force: the bigger the force, the more the spring stretches.
- 100 g = 0.1 kg, 200 g = 0.2 kg.
1. Chandrayaan-3 and a classroom full of questions
The chapter opens with a class watching India's Chandrayaan-3 lander on the Moon (textbook page 73). The children say:
By the end of this chapter you will be able to answer all three. We start with something you see every day.
The mango and the stone speed up as they fall; the leaf and the feather drift slowly because the air holds them back (more about that on page 7). But all of them come down.
Q. കല്ലിനും തൂവലിനും താഴേക്ക് പതിക്കാൻ ആവശ്യമായ ബലം എവിടെ നിന്നാണ് ലഭിച്ചത്? – Where does the force that makes the stone and the feather fall come from?
2. Wells all round the globe – which way is “down”?
Imagine wells dug at many places all over the Earth (Fig 4.1), and people standing on every side of it (Fig 4.2). Step through.
Q. കല്ലുകൾ ആകർഷിക്കപ്പെടുന്നത് കിണറുകളുടെ അടിത്തട്ടിലേക്കല്ലേ? – In wells all over the Earth, are the stones not pulled to the bottom of each well?
Q. ഭൂമിയുടെ മറുവശത്തുള്ളവർ ഭൂമിയിൽ നിന്നു വീണുപോകുന്നില്ലല്ലോ! ഭൂമിയുടെ ആകർഷണമല്ലേ ഇതിനു കാരണം? – People on the other side of the Earth don't fall off. Isn't the Earth's attraction the reason?
The thinking boy in the textbook asks: ഈ ആകർഷണബലം അളക്കുവാൻ കഴിയുമോ? – Can this attractive force be measured? First, the textbook's key idea:
എല്ലാ വസ്തുക്കളെയും ഭൂമി ആകർഷിക്കുന്നു. ഇതിന്റെ ദിശ ഭൂമിയുടെ കേന്ദ്രത്തിലേക്കാണ്. ഈ ആകർഷണബലമാണ് ഭൂഗുരുത്വാകർഷണ ബലം.
The Earth attracts every object. The direction of this pull is towards the centre of the Earth (ഭൂകേന്ദ്രം). This attractive force (ആകർഷണബലം) is the Earth's gravitational force – gravity.
3. Activity 4.1 – Measuring the Earth's pull with a spring balance
The textbook's activity (പ്രവർത്തനം, page 74, Fig 4.3): fix a spring balance to a window bar, pull its hook with your hand, then hang 100 g and 200 g masses on it.
Textbook questions (page 74) – try them, then check.
- എന്തുകൊണ്ടാണ് സ്പ്രിങ് വലിഞ്ഞത്? Why did the spring stretch?
- സ്പ്രിങ് ബാലൻസ് റീഡിങ് എത്രയാണ്? ഇതല്ലേ നാം പ്രയോഗിച്ച ബലം? What is the reading? Isn't this the force we applied?
- ബലത്തിന്റെ യൂണിറ്റ് ഏതാണ്? What is the unit of force?
- With 100 g hanging: why did the spring stretch? Which force pulled the 100 g down? What is the reading?
- With 200 g: why more stretch? മാസ് കൂടുമ്പോൾ ആകർഷണബലത്തിന് എന്ത് സംഭവിക്കുന്നു? (കൂടുന്നു / കുറയുന്നു) What happens to the attractive force when the mass increases? Write one factor that affects it.
2. Whatever the pointer shows (in our lab, 3 N). Yes – the reading is the force we applied, because the balance is made to show the pull on its hook.
3. The newton (N).
4. Nobody is pulling, yet it stretches: the Earth pulls the 100 g mass downwards (gravity). Reading ≈ 0.98 N ≈ 1 N (0.1 kg × 9.8 N per kg).
5. The Earth pulls the 200 g mass harder: reading ≈ 1.96 N ≈ 2 N. The attractive force increases (കൂടുന്നു) when the mass increases. One factor: the mass of the object.
| Mass (g) | Mass (kg) | Reading (N) | Reading ÷ mass (N per kg) |
|---|
The thinking girl in the textbook asks: മാസ് മാത്രമായിരിക്കുമോ ആകർഷണബലത്തെ സ്വാധീനിക്കുന്നത്? – Is mass the only thing that affects the attractive force?
4. Table 4.1 – the pull gets weaker as you go up
The textbook gives the Earth's pull on the same 100 kg object at three heights above the Earth's surface (ഭൗമോപരിതലം):
| Mass of the object (kg) | Height above the Earth's surface (m) | Attractive force (N) |
|---|---|---|
| 100 | at the surface (0) | 980 |
| 100 | 1,00,000 (= 100 km) | 950 |
| 100 | 10,00,000 (= 1000 km) | 730 |
The curve is the Earth's pull worked out from the law you meet on the next page (force falls as 1 ÷ distance² from the Earth's centre). The three amber dots are the textbook's Table 4.1 values; the book rounds them, so they sit almost exactly on the curve.
Q. 100 kg മാസുള്ള വസ്തുവിൽ ആകർഷണബലം കൂടുതൽ അനുഭവപ്പെട്ടത് എവിടെ വച്ചാണ്? – Where did the 100 kg object feel the greatest force? (at the surface / at 1,00,000 m / at 10,00,000 m)
Q. ഭൂമിയിൽ നിന്ന് വസ്തുവിലേക്കുള്ള അകലം കൂടുമ്പോൾ ഭൂമി പ്രയോഗിക്കുന്ന ആകർഷണബലം (കൂടുന്നു / കുറയുന്നു) – As the distance from the Earth increases, the attractive force …?
📒 Science diary (സയൻസ് ഡയറി) – what we found
- ഭൂമി എല്ലാ വസ്തുക്കളെയും അതിന്റെ കേന്ദ്രത്തിലേക്ക് ആകർഷിക്കുന്നു. – The Earth attracts all objects towards its centre.
- വസ്തുവിന്റെ മാസ് കൂടുന്നതിനനുസരിച്ച് ആകർഷണബലം കൂടുന്നു. – The attractive force increases as the mass of the object increases.
- വസ്തുവിലേക്കുള്ള അകലം കൂടുന്നതിനനുസരിച്ച് ആകർഷണബലം കുറയുന്നു. – The attractive force decreases as the distance to the object increases.
5. Is the Earth the only one that pulls? – Tides
The thinking boy asks: ഭൂമി മാത്രമാണോ ഇത്തരത്തിൽ ആകർഷണബലം പ്രയോഗിക്കുന്നത്? – Is the Earth the only body that pulls like this? No! The sea at Kochi rises and falls twice a day: high tide (വേലിയേറ്റം) and low tide (വേലിയിറക്കം). These happen because the Moon and the Sun pull on the Earth – mostly on its oceans, which can move.
The bulges (exaggerated hugely) point towards and away from the Moon. As the Earth spins, Kochi passes through both bulges, so there are two high tides and two low tides in about 24 h 50 min.
Q. സൂര്യനും ചന്ദ്രനും ഭൂമിയിൽ ബലം പ്രയോഗിക്കുമെങ്കിൽ പ്രപഞ്ചത്തിലെ മറ്റു ഗോളങ്ങൾ തമ്മിലും പരസ്പരാകർഷണബലം ഉണ്ടാകില്ലേ? – If the Sun and the Moon exert a force on the Earth, won't all other bodies in the universe also attract each other?
Fishermen at Kochi, Munambam and Vizhinjam plan their trips with the tide tables; big ships enter Kochi port and the Vizhinjam harbour more easily at high tide. In Kerala the tide range is small (well under a metre) compared with places like the Gulf of Khambhat in Gujarat, where it can be several metres – but the cause is the same: the Moon's and Sun's gravity.
- Born in Thiruvananthapuram. BSc and MSc from University College, Thiruvananthapuram, with gold medals.
- Made fundamental contributions to cosmology (the study of the universe as a whole) and gravitation. He worked for many years at IUCAA (Inter-University Centre for Astronomy and Astrophysics), Pune.
- He presented a new way of looking at gravity, different from those of Newton and Einstein – gravity as an “emergent” effect, the way temperature emerges from the motion of countless molecules.
- Honours: Padma Shri, the Kerala Sasthra Puraskaram and many more.
A boy from Thiruvananthapuram became one of the world's leading thinkers on gravity – the very topic of this chapter.
- 📄 Thanu Padmanabhan (Wikipedia) Wikipedia
Life and work of the Thiruvananthapuram-born cosmologist who studied the nature of gravity. - 📄 T. Padmanabhan Tribute Page (IUCAA) IUCAA
IUCAA's tribute to 'Paddy' (1957–2021), where he worked from 1992. - ▶️ Thanu Padmanabhan: Atoms of Spacetime and the Nature of Gravity (EmQM15) YouTube – Fetzer Franklin Fund · 46:05
A real research talk by Padmanabhan himself – advanced, but nice to see him speak about gravity. - 📄 Tides – NASA Science NASA
How the Moon's gravity pulls on Earth's oceans to make high and low tides. - ▶️ What Causes Tides? YouTube – NOAA SciJinks · 3:34
Animated explanation of why there are two tidal bulges. - ▶️ An Apple Falls, But Not the Moon. Why? YouTube – BYJU'S · 2:28
Newton's big idea: the same gravity that drops the apple keeps the Moon in orbit.
6. Check yourself
- The Earth attracts every object towards its centre: this is the Earth's gravitational force (gravity).
- A spring balance measures it in newtons: 100 g → 0.98 N ≈ 1 N; 200 g → 1.96 N ≈ 2 N.
- The pull increases with mass and decreases with distance (Table 4.1: 980 N → 950 N → 730 N).
- The Moon and the Sun pull on the Earth too (tides) – every body attracts every other body.