Gravitation – Study Notes for AIIMS Physics
(Simple, beginner‑friendly, no heavy jargon)
1. What is Gravitation?
- Gravity is the invisible pull that keeps everything on the Earth.
- Think of it like a magnet that pulls you toward the ground.
- Every object with mass attracts every other object.
2. Key Ideas
| Concept | Everyday Analogy | Tiny Example |
|---|---|---|
| Mass | The amount of stuff in an object | A 1 kg brick has 1 kg of matter |
| Weight | How heavy you feel on a scale | On Earth, 1 kg brick weighs ~9.8 N |
| Gravity | The pull that makes weight | The Earth pulls you toward its center |
3. Newton’s Law of Universal Gravitation
-
Formula
[ F = G \frac{m_1 m_2}{r^2} ]- (F) = gravitational force
- (G) = 6.67 × 10⁻¹¹ Nm²/kg² (tiny constant)
- (m_1, m_2) = masses of two objects
- (r) = distance between their centers
-
What it means
- Bigger masses → stronger pull.
- Closer together → stronger pull.
-
Tiny Example
Two 1 kg masses 1 m apart:
(F = 6.67×10^{-11} \times \frac{1×1}{1^2} = 6.67×10^{-11}) N.
(Very weak – you can’t feel it.)
4. Gravity on Earth
| Quantity | Value (approx.) | Meaning |
|---|---|---|
| Acceleration due to gravity, (g) | 9.8 m/s² | How fast you fall if no air resistance |
| Weight of 1 kg on Earth | 9.8 N | Mass × g |
- Weight = Mass × g
[ W = m \times g ]
5. Gravitational Field Strength
-
Definition: Force per unit mass.
-
Formula:
[ g = G \frac{M}{r^2} ] where (M) = mass of the Earth (≈ 5.97 × 10²⁴ kg). -
Why it matters
- It tells you how strong the pull is at any point.
- Near the surface, (g ≈ 9.8) m/s².
- Far from Earth, (g) decreases with the square of distance.
6. Gravitational Potential Energy (GPE)
-
What it is: Energy stored because of height above ground.
-
Formula:
[ U = m g h ]- (h) = height above ground.
-
Tiny Example
2 kg box lifted 3 m:
(U = 2 × 9.8 × 3 = 58.8) J.
7. Gravitational Potential
-
Definition: Energy per unit mass.
-
Formula:
[ \Phi = -G \frac{M}{r} ] (Negative sign means energy is lower near the Earth.) -
Why it matters
- Helps compute GPE: (U = m \Phi).
8. Common AIIMS‑Style Questions
| Topic | Sample Question | Quick Hint |
|---|---|---|
| Force between two masses | Two 5 kg masses 2 m apart: find force. | Use (F = G m_1 m_2 / r^2). |
| Weight change with altitude | Weight of 70 kg person at 10 km altitude? | Use (g' = g (R/(R+h))^2). |
| Gravitational potential energy | 10 kg mass dropped from 5 m: find GPE lost. | (U = m g h). |
| Moon orbit speed | Rough speed of Earth’s moon? | Use (v = \sqrt{GM/r}). |
9. Study Tips
- Remember the core formula: (F = G m_1 m_2 / r^2).
- Practice quick calculations: Plug numbers into the formulas.
- Visualize: Think of Earth as a giant magnet pulling everything toward its center.
- Use the “big‑picture”: All gravitational problems boil down to mass, distance, and the constant (G).
10. Quick Self‑Check
- What is the weight of a 3 kg object on Earth?
- How does gravity change if the distance from Earth doubles?
- If you lift a 4 kg book 2 m, what is the gained gravitational potential energy?
(Answer them before looking at the solutions to test your understanding.)
Good luck with your AIIMS preparation!