# Force - Grade 10 (SEE) Physics
## Complete Chapter Notes with All Exercises - NEB Curriculum 2080

---

## 🎯 Chapter Overview
**Chapter 1: Force** is the foundation chapter of Grade 10 Physics for SEE examination. This chapter covers fundamental concepts of gravitational force, Newton's laws, weight, mass, and their applications in daily life.

**Weightage:** 12-15 marks in SEE examination
**Time Required:** 20-25 periods

---

## 📚 Learning Objectives
After studying this chapter, students will be able to:
- Define force and understand its types
- Explain Newton's Universal Law of Gravitation
- Distinguish between mass and weight
- Calculate gravitational force between objects
- Understand concepts of free fall and weightlessness

---

## ⚡ 1. FORCE

### 1.1 Definition
Force can be defined as a push or pull that tries to change the state of a body from motion to rest and rest to motion in a straight line.

**SI Unit:** Newton (N)
**CGS Unit:** dyne
**Conversion:** 1 N = 10⁵ dynes

### 1.2 Types of Force
1. **Pulling Force (Attraction Force)**
   - Example: Magnet attracting iron
2. **Pushing Force (Repulsion Force)**
   - Example: Two like poles of magnets

---

## 🌍 2. GRAVITATIONAL FORCE

### 2.1 Definition
The force of attraction between any two bodies of the universe due to their masses is called gravitational force.

### 2.2 Newton's Universal Law of Gravitation

**Statement:** "The force of gravitation between any two bodies in the universe is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centres."

**Mathematical Expression:**
```
F = G(m₁m₂)/d²

Where:
F = Gravitational force (N)
G = Universal gravitational constant = 6.67 × 10⁻¹¹ Nm²/kg²
m₁, m₂ = Masses of two bodies (kg)
d = Distance between their centers (m)
```

### 2.3 Universal Gravitational Constant (G)

**Definition:** G is defined as the gravitational force produced between two bodies of unit mass each when they are kept at a unit distance from their centre.

**Value:** G = 6.67 × 10⁻¹¹ Nm²/kg²
**SI Unit:** Nm²/kg²

**Significance:** If m₁ = 1 kg, m₂ = 1 kg, d = 1 m, then F = G

---

## 🔽 3. GRAVITY AND ACCELERATION DUE TO GRAVITY

### 3.1 Gravity
The force of attraction of the earth on a body is called gravity.

### 3.2 Acceleration due to Gravity (g)

**Definition:** The rate of change of velocity due to gravity is called acceleration due to gravity.

**Formula Derivation:**
```
For a body of mass 'm' on Earth's surface:
F = GMm/R² (from Newton's law)
F = mg (from Newton's second law)

Therefore: mg = GMm/R²
∴ g = GM/R²

Where:
g = acceleration due to gravity (m/s²)
G = gravitational constant
M = mass of Earth
R = radius of Earth
```

**Standard Value:** g = 9.8 m/s² (on Earth)

### 3.3 Variation of 'g'
- **g ∝ 1/R²** (inversely proportional to square of radius)
- g is greater at poles than at equator
- g decreases with altitude
- g varies from planet to planet

---

## ⚖️ 4. MASS AND WEIGHT

### 4.1 Mass
- Amount of matter contained in a body
- Remains constant everywhere in universe
- Scalar quantity
- SI Unit: kilogram (kg)

### 4.2 Weight
- Force with which a body is attracted towards center of Earth
- Changes from place to place
- Vector quantity
- SI Unit: Newton (N)

**Relationship:** Weight = mg

### 4.3 Comparison Table

| Mass | Weight |
|------|--------|
| Amount of matter in body | Force of attraction by Earth |
| Constant everywhere | Varies with location |
| Scalar quantity | Vector quantity |
| Unit: kg | Unit: N |
| Measured by beam balance | Measured by spring balance |

---

## 🍎 5. FREE FALL AND WEIGHTLESSNESS

### 5.1 Free Fall
**Definition:** When a body falls freely towards the center of Earth under the influence of gravitational pull, the motion is called free fall.

**Characteristics:**
- Initial velocity (u) = 0
- Acceleration = g = 9.8 m/s²
- Air resistance neglected

**Equations of Motion for Free Fall:**
```
v = u + gt = gt (since u = 0)
h = ut + ½gt² = ½gt²
v² = u² + 2gh = 2gh
```

### 5.2 Weightlessness
**Definition:** Weightlessness is the state of a body in which it feels that it is not influenced by any force.

**Examples:**
- Person in a falling lift
- Astronauts in satellites
- Objects in free fall

**Condition:** When acceleration of body equals acceleration due to gravity

---

## 📊 6. COMPLETE EXERCISE SOLUTIONS

### **2. Answer the following questions:**

**a. What is gravitation? What are the factors that affect gravitation? Write the SI unit of gravitation.**

**Answer:** The mutual force of attraction between two bodies is called gravitation.

**Factors affecting gravitation:**
- Mass of the bodies
- Distance between their centers

**SI unit:** Newton (N)

**b. State Newton's Universal Law of gravitation.**

**Answer:** Universal law of gravitation states that "everybody in this universe attracts another body with a force, which is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centres."

**c. What will happen to the gravitational force between two bodies if the distance between them is halved keeping their masses constant?**

**Answer:** 
Given: F = Gm₁m₂/d² ... (i)

If distance is halved: d₂ = d/2

New force: F₂ = Gm₁m₂/(d/2)² = Gm₁m₂/(d²/4) = 4Gm₁m₂/d² ... (ii)

From (i) and (ii): F₂ = 4F

**Therefore, the gravitational force becomes 4 times greater.**

**d. What will be the gravitational force between two heavenly bodies if the masses of both are tripled keeping the distance constant?**

**Answer:**
Given: F = Gm₁m₂/d² ... (i)

If masses are tripled: m₁' = 3m₁, m₂' = 3m₂

New force: F₂ = G(3m₁)(3m₂)/d² = 9Gm₁m₂/d² ... (ii)

From (i) and (ii): F₂ = 9F

**Therefore, the gravitational force becomes 9 times greater.**

**e. What will be the gravitational force between two bodies if the masses of each is doubled and the distance between them is halved?**

**Answer:**
Given: F = Gm₁m₂/d² ... (i)

If masses are doubled and distance halved:
- m₁' = 2m₁, m₂' = 2m₂
- d' = d/2

New force: F₂ = G(2m₁)(2m₂)/(d/2)² = 4Gm₁m₂/(d²/4) = 16Gm₁m₂/d² ... (ii)

From (i) and (ii): F₂ = 16F

**Therefore, the gravitational force becomes 16 times greater.**

**f. Define G. Also write its value and SI unit.**

**Answer:** G is defined as the gravitational force produced between two bodies of unit mass each when they are kept at a unit distance from their center.

**Value:** 6.67 × 10⁻¹¹ Nm²/kg²
**SI Unit:** Nm²/kg²

**g. In the formula g = GM/R², what do G, M and R stand for? Prove that g ∝ 1/R².**

**Answer:** 
- G = Gravitational constant
- M = Mass of the Earth
- R = Radius of the Earth

**Proof:**
Let 'm' be the mass of an object on Earth's surface.

From Newton's law: F = GMm/R² ... (i)
From Newton's second law: F = mg ... (ii)

Equating (i) and (ii): mg = GMm/R²

Therefore: g = GM/R²

Since G and M are constants: g ∝ 1/R²

**h. What will be the effect on the acceleration due to gravity of the earth if it is compressed to the size of the moon?**

**Answer:** If Earth is compressed to the size of moon, the acceleration due to gravity will increase because g ∝ 1/R². As radius decreases, g increases.

**i. The acceleration due to gravity of Jupiter is 25 m/s². What does it mean?**

**Answer:** It means that if any object is dropped near Jupiter's surface, it will accelerate downward at the rate of 25 m/s² due to Jupiter's gravity.

**j. A coin and a feather are dropped in vacuum. Which one will reach the ground first? Why?**

**Answer:** Both will reach the ground at the same time because in vacuum there is no air resistance, and acceleration due to gravity is same for all objects regardless of their mass.

**k. Where does a body have more weight - at the pole or at the equator of the earth? Explain with reason.**

**Answer:** A body has more weight at poles than at equator because polar radius is less than equatorial radius. Since g ∝ 1/R², gravity is stronger at poles, making weight greater.

**l. When an apple falls towards the earth, the earth moves up to meet the apple. Is this true? If yes, why is the earth's motion not noticeable?**

**Answer:** Yes, it is true according to Newton's third law. However, Earth's motion is not noticeable because Earth's mass is enormously larger than the apple's mass, so its acceleration is negligibly small.

**m. An astronaut is said to be weightless when he travels in a satellite. Does it mean that the earth does not attract him?**

**Answer:** No, Earth still attracts the astronaut. Weightlessness occurs because both the astronaut and satellite are in free fall together, creating the sensation of zero weight.

**n. What is the importance of gravitational force? Write any two points.**

**Answer:**
1. We are able to perform our daily activities due to gravitational force
2. The existence of atmosphere around Earth is possible due to gravitational force

**o. What is free fall? What happens to the weight of a body when it is falling freely under the action of gravity?**

**Answer:** When a body falls freely towards Earth's center under gravitational influence, the motion is called free fall. During free fall, the apparent weight becomes zero because the body accelerates at g.

**p. What is the effect of gravity on a falling object? Write the conclusion obtained from the coin and feather experiment.**

**Answer:** Gravity causes falling objects to accelerate toward Earth. The coin and feather experiment proves that in vacuum, all objects fall at the same rate independent of their masses.

**q. Write the differences between the fall of a parachute on Earth and its fall on the moon. On what condition does a body have free fall?**

**Answer:** On Earth, parachute falls slowly due to air resistance and stronger gravity. On moon, it falls faster (no air resistance) but with weaker gravity. 

**Condition for free fall:** When acceleration of body equals acceleration due to gravity.

**r. Write any two examples of weightlessness.**

**Answer:**
1. Person in a falling lift
2. Astronauts in satellites

**s. Define:**

**i. Acceleration due to gravity:** The acceleration produced on a body due to Earth's gravitational force.

**ii. Weightlessness:** State of a body when it feels no gravitational force acting on it.

**iii. Free fall:** Motion of a body falling freely under gravity with no external resistance.

**iv. Null point:** Point between two celestial bodies where net gravitational force is zero.

---

### **3. Distinguish between:**

**a. Gravity and Gravitation**

| Gravity | Gravitation |
|---------|-------------|
| Force of attraction of Earth on a body | Mutual force of attraction between any two bodies |
| Strong force | Weak force |
| Example: Earth pulls 1kg object with 9.8N | Example: Attraction between books, planets |

**b. Gravity and Acceleration due to gravity**

| Gravity | Acceleration due to gravity |
|---------|----------------------------|
| Force of attraction of Earth | Acceleration produced due to gravity |
| Cause of acceleration | Effect of gravity |
| SI unit: Newton | SI unit: m/s² |

**c. Mass and Weight**

| Mass | Weight |
|------|--------|
| Amount of matter in body | Force of attraction by Earth |
| Same everywhere | Varies with location |
| Scalar quantity | Vector quantity |

**d. g and G**

| g | G |
|---|---|
| Acceleration due to gravity | Universal gravitational constant |
| Unit: m/s² | Unit: Nm²/kg² |
| Vector quantity | Scalar quantity |

**e. Free fall and Weightlessness**

| Free fall | Weightlessness |
|-----------|----------------|
| Body falling under gravity only | State of feeling no gravitational force |
| Example: Fruit falling from tree | Example: Person in falling lift |

**f. Gravitational force and Acceleration due to gravity**

| Gravitational Force | Acceleration due to gravity |
|--------------------|----------------------------|
| Force of attraction between masses | Acceleration produced by gravity |
| SI unit: Newton | SI unit: m/s² |

**g. Weightlessness in space and weightlessness on Earth**

| Weightlessness in Space | Weightlessness on Earth |
|-------------------------|-------------------------|
| Apparent weight zero in space | Apparent weight zero on Earth |
| Occurs at null point | Occurs during free fall |

---

### **4. Give reason:**

**a. Newton's law of gravitation is called a universal law.**

**Answer:** Because this law applies to all bodies in the universe, from microscopic particles to massive celestial bodies.

**b. The weight of a body decreases with increase in distance from the earth's surface.**

**Answer:** Because g ∝ 1/R². As distance from Earth increases, value of g decreases, hence weight decreases.

**c. The weight of a stone is less at the top of a mountain than at its bottom.**

**Answer:** Because g is greater at bottom (closer to sea level) than at top due to less distance from Earth's center.

**d. The weight of a body is less on the moon than its weight on the earth.**

**Answer:** Because moon has weaker gravitational force than Earth due to smaller mass and radius.

**e. The moon has no atmosphere.**

**Answer:** Because moon's gravitational force is too weak to hold gas molecules in its atmosphere.

**f. The value of g is greater at the poles than at the equator.**

**Answer:** Because polar radius is smaller than equatorial radius, and g ∝ 1/R².

**g. There is more role of the moon than the sun to cause tides in oceans.**

**Answer:** Although sun is more massive, moon is much closer to Earth, making its gravitational effect stronger for causing tides.

**h. Tides occur in oceans but not in lakes.**

**Answer:** Because ocean water mass is much larger than lake water, making gravitational effects more noticeable.

**i. It is difficult to lift a large stone on the surface of the earth but it is easy to lift a small one.**

**Answer:** Because large stone has more mass, so Earth attracts it with greater gravitational force.

**j. Mass of Jupiter is 319 times more than Earth but its acceleration due to gravity is only 2.5 times more.**

**Answer:** Because Jupiter's radius is about 11 times larger than Earth's, and g ∝ 1/R².

**k. The weight of a body will be zero during free fall.**

**Answer:** Because during free fall, the body's acceleration equals g, making apparent weight zero.

**l. A paratrooper does not get injured while jumping from a flying aeroplane.**

**Answer:** Because parachute increases air resistance, reducing the falling acceleration and impact force.

**m. When a coin and feather are dropped from a building, the coin reaches ground earlier.**

**Answer:** Because coin has greater mass and experiences less air resistance compared to feather.

---

### **5. Diagrammatic Questions:**

**1. Earth's orbit around sun - gravitational force variation**

**Answer:** The gravitational force increases as Earth moves from point A to point B because distance to sun is less at B than A, and F ∝ 1/d².

**2. Feather and coin experiment**

**a. Name of experiment:** "Feather and coin" experiment

**b. Conclusion:** In vacuum, all objects fall with same acceleration regardless of mass.

**c. Effect with air:** Coin falls first because it has less air resistance than feather.

**3. Gravitational force calculation**

**Given:**
- m₁ = 800 kg
- m₂ = 400 kg  
- d = 8 m
- G = 6.67 × 10⁻¹¹ Nm²/kg²

**Solution:**
F = Gm₁m₂/d²
F = (6.67 × 10⁻¹¹ × 800 × 400)/8²
F = (6.67 × 10⁻¹¹ × 320000)/64
F = 3.335 × 10⁻⁷ N

---

## 🔢 6. NUMERICAL PROBLEMS

### **Problem Type 1: Gravitational Force Calculation**

**Q1:** Calculate the gravitational force between two objects of masses 50 kg and 80 kg separated by 10 m.

**Given:**
- m₁ = 50 kg
- m₂ = 80 kg
- d = 10 m
- G = 6.67 × 10⁻¹¹ Nm²/kg²

**Solution:**
F = Gm₁m₂/d²
F = (6.67 × 10⁻¹¹ × 50 × 80)/10²
F = (6.67 × 10⁻¹¹ × 4000)/100
F = 2.668 × 10⁻⁹ N

### **Problem Type 2: Weight Calculation**

**Q2:** Calculate the weight of a 75 kg person on Earth's surface.

**Given:**
- m = 75 kg
- g = 9.8 m/s²

**Solution:**
Weight = mg
Weight = 75 × 9.8 = 735 N

### **Problem Type 3: Free Fall Problems**

**Q3:** A ball is dropped from height of 45 m. Calculate time taken to reach ground.

**Given:**
- h = 45 m
- u = 0 (dropped)
- g = 9.8 m/s²

**Solution:**
h = ut + ½gt²
45 = 0 + ½ × 9.8 × t²
45 = 4.9t²
t² = 45/4.9 = 9.18
t = 3.03 s

### **Problem Type 4: Variation Problems**

**Q4:** If gravitational force between two objects is 100 N, what will be the force if distance is doubled?

**Given:**
- Initial force F₁ = 100 N
- New distance d₂ = 2d₁

**Solution:**
F₁ = Gm₁m₂/d₁²
F₂ = Gm₁m₂/(2d₁)² = Gm₁m₂/4d₁²

F₂ = F₁/4 = 100/4 = 25 N

### **Problem Type 5: Weight on Different Planets**

**Q5:** A person weighs 600 N on Earth. What will be his weight on Jupiter? (g_Jupiter = 25 m/s²)

**Given:**
- Weight on Earth = 600 N
- g_Earth = 9.8 m/s²
- g_Jupiter = 25 m/s²

**Solution:**
Mass = Weight_Earth/g_Earth = 600/9.8 = 61.22 kg

Weight on Jupiter = mass × g_Jupiter
Weight on Jupiter = 61.22 × 25 = 1530.5 N

---

## 📈 7. IMPORTANT FORMULAS SUMMARY

### **Force and Gravitation**
- F = Gm₁m₂/d²
- G = 6.67 × 10⁻¹¹ Nm²/kg²
- Weight = mg
- g = GM/R²

### **Free Fall Equations**
- v = gt (when u = 0)
- h = ½gt²
- v² = 2gh

### **Relationships**
- F ∝ m₁m₂ (masses)
- F ∝ 1/d² (distance)
- g ∝ 1/R² (radius)
- Weight ∝ g (gravity)

---

## 🎯 8. EXAM PREPARATION TIPS

### **Theory Questions (2-3 marks)**
- Learn exact definitions with units
- Practice distinguishing between concepts
- Memorize Newton's laws and statements
- Draw neat labeled diagrams

### **Give Reason Questions (2-3 marks)**
- Understand cause-effect relationships
- Use formulas to support reasoning
- Connect to daily life examples
- Practice explaining variations

### **Numerical Problems (3-4 marks)**
- Memorize all important formulas
- Show all calculation steps
- Check unit consistency
- Practice different problem types

### **Long Questions (5-6 marks)**
- Complete derivations with steps
- Detailed explanations with examples
- Multiple parts of same concept
- Diagrams where required

---

## 🏆 9. PREVIOUS YEAR SEE QUESTIONS PATTERN

### **Common Theory Questions**
- Define gravitation and its factors
- State Newton's universal law
- Distinguish between mass and weight
- Explain free fall and weightlessness
- Give reasons for gravitational variations

### **Common Numerical Types**
- Gravitational force between objects
- Weight calculations on different planets
- Free fall time and velocity problems
- Force variation with distance/mass changes
- Comparison problems

### **Important Diagrams**
- Earth's elliptical orbit showing gravitational variation
- Feather and coin experiment setup
- Free body diagrams showing forces
- Gravitational force between two spheres

---

## ✅ 10. CHAPTER CHECKLIST

**Before SEE Exam, ensure you can:**
- [ ] Define force, gravity, and gravitation clearly
- [ ] State and apply Newton's universal law
- [ ] Distinguish between all given concepts
- [ ] Solve all types of numerical problems
- [ ] Explain all "give reason" questions
- [ ] Draw required diagrams accurately
- [ ] Understand free fall and weightlessness
- [ ] Calculate weights on different planets

---

## 🎊 CONCLUSION

Force is a fundamental chapter that builds the foundation for understanding motion, energy, and other physics concepts. Master the definitions, formulas, and numerical problem-solving techniques. Practice distinguishing between similar concepts and explaining daily life phenomena using gravitational principles.

**Key Success Formula:** 
Concept Understanding + Formula Practice + Numerical Solving = SEE Success!

---

*This complete note covers all exercises from the Force chapter of NEB Science textbook for SEE examination preparation. Practice regularly and refer to your textbook for additional examples.*

**📞 Need Help?** Review the numerical problems and practice more variations for complete mastery.

---