Exploring Forces

CHAPTER 5: EXPLORING FORCES

5.1 What Is a Force?

**Definition of Force:**

A **force** is a push or pull applied on an object. It is the result of an interaction between two objects. In simpler terms, whenever you push or pull something, you are applying a force on it.

**SI Unit of Force:** The SI (International System) unit of force is the **newton** (written with lowercase 'n'), with symbol **N**.

**Examples of Force in Daily Life:**

Pushing a door to open it

Pulling a rope to draw water from a well (common in Indian villages)

Pedaling a bicycle against the wind

Lifting a bucket of water

Kicking a football

**Key Understanding:**

Every force results from the interaction of at least two objects. When you push a table, your hand is one object and the table is another object. The force exists only as long as the two objects are interacting. Once the interaction stops, the force disappears.

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5.2 What Can a Force Do to Objects On Which It Is Applied?

When a force is applied to an object, it can produce various effects. Understanding these effects helps us recognize when forces are at work.

**Effects of Force:**

A force applied on an object may:

• **Make an object move from rest:** When you push a stationary bicycle, it starts moving. This is muscular force making a static object move.

• **Change the speed of an object if it is moving:** When your friend holds the moving bicycle from behind, the bicycle slows down. The force reduces the speed.

• **Change the direction of motion of an object:** When a cricket player hits a ball with a bat, the ball's direction changes from the bowler's path to a new direction. This is a push changing the direction of a moving object.

• **Bring about a change in the shape of an object:** When you press an inflated balloon, it changes shape. The force deforms the object temporarily.

• **Cause some or all of these effects simultaneously:** When you apply brakes on a moving bicycle, the force changes both the speed and direction of motion.

**Real-Life Examples:**

1. **Rolling a chapati:** A cook applies force with the rolling pin, changing the shape of the dough from a ball to a flat, circular disc.

2. **Turning the steering handle of an autorickshaw:** The driver applies force to change the direction of motion of the vehicle.

3. **Opening a drawer:** Force applied to the handle makes the drawer move from its rest position.

4. **Stretching a rubber band:** The force changes the shape and size of the rubber band.

5. **A fielder stopping a ball:** The fielder's hand applies force that brings the moving ball to rest.

**Important Conclusion:**

All changes in motion (starting, stopping, speeding up, slowing down, changing direction) and all changes in shape require the application of force. If you observe any of these changes, you can be certain that a force is acting on the object.

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5.3 Are Forces an Interaction Between Two or More Objects?

**Nature of Forces:**

A force always arises from the interaction between two or more objects. It is impossible for a force to act in isolation or on a single object. Let's understand this clearly:

**Examples of Force as Interaction:**

When you:

Push a table → Your hand (Object 1) interacts with the table (Object 2)

Hit a ball with a bat → The bat (Object 1) interacts with the ball (Object 2)

Press a balloon → Your fingers (Object 1) interact with the balloon (Object 2)

In every case, at least two objects are involved.

**Action and Reaction:**

An important principle: **When two objects interact, each object experiences a force from the other object.**

Example: When you push a wall, you are applying force on the wall. Simultaneously, the wall is also applying force on your hand. You feel this force as a resistance. As soon as you stop pushing (interaction ceases), the wall no longer exerts force on your hand.

**Formal Definition of Force:**

**A force is a push or pull on an object resulting from the object's interaction with another object.**

**At Rest Objects:**

You might think that an object at rest has no forces acting on it. This is incorrect. An object at rest means the forces acting on it are **balanced** or **neutralizing each other**. For example, a book lying on a table has the downward gravitational force (weight) balanced by the upward force from the table surface (normal force). You will study balanced forces in detail in higher grades.

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5.4 What Are the Different Types of Forces?

Forces are classified into two main categories based on whether physical contact is required:

5.4.1 CONTACT FORCES

**Definition:** Contact forces are forces that act only when there is direct physical contact between two objects. The contact can be:

**Direct:** Using your hands or body parts (pushing with your hand, kicking a ball)

**Indirect:** Using tools or implements (hitting with a bat, pulling with a rope)

#### **Muscular Force**

**Definition:** The force produced by the contraction and elongation (stretching) of muscles in living organisms is called **muscular force**.

**How It Works:**

Muscles are made of muscle fibers that can contract (shorten) and elongate (lengthen)

When muscles contract, they pull on bones, creating movement

When muscles elongate, they relax and prepare for the next contraction

This cyclical process produces force

**Uses of Muscular Force:**

In humans:

**Walking and running:** Leg muscles contract to push the body forward

**Lifting and pushing:** Arm and shoulder muscles contract to move objects

**Jumping:** Leg muscles contract suddenly to propel the body upward

**Stretching:** Muscles elongate as we reach or bend

**Chewing:** Jaw muscles contract to break down food

**Digestion:** The muscles of the alimentary canal contract to push food through (peristalsis)

**Circulation:** Heart muscles contract to pump blood throughout the body

In animals and other organisms:

**Birds:** Wing muscles contract and relax to create flight

**Fish:** Tail muscles create swimming movements

**Insects:** Small muscles create rapid wing beats for flight or crawling

**Snakes:** Body muscles contract in waves for slithering motion

**Horses and oxen:** Muscles used for pulling carts or plowing fields (common agricultural use in India)

**Indian Examples:**

A farmer using muscular force to push a plow through the field

A woman using muscular force to manually grind grain using a hand mill (chakki)

A cowherd using muscular force to control cattle

#### **Friction**

**Definition and Discovery:**

When objects slide or try to slide over another surface, a force opposes their motion. This force is called **friction** or **frictional force**.

**Example of Friction:**

A ball rolling on the ground slows down and stops without anyone pushing it backward. Something must be opposing its motion—that something is friction.

A bicycle slows down and stops when you stop pedaling on a flat road.

A stone sliding on ice travels much farther before stopping than the same stone sliding on sand. This tells us that friction varies with different surfaces.

**Direction of Friction:**

**Friction always acts in a direction opposite to the direction of motion** (or the direction in which an object is trying to move).

If a box slides rightward, friction acts leftward on the box.

If a bicycle moves forward, friction acts backward on the bicycle.

**Cause of Friction:**

Friction arises due to **irregularities (roughness) in the two surfaces in contact**. Even surfaces that appear smooth to our eyes have tiny bumps, dips, and imperfections when viewed under magnification. When two surfaces are placed together:

The irregularities of one surface lock into the irregularities of the other surface

This locking opposes any effort to slide one surface over the other

The result is the frictional force

**Diagram Description - Friction Between Two Surfaces:**

Imagine two magnified surfaces:

Surface A shows tiny peaks and valleys

Surface B shows tiny peaks and valleys

When placed together, the peaks of A fit into the valleys of B and vice versa

This interlocking creates resistance to motion

**Friction Depends on Surface Nature:**

The amount of friction is **not the same for all surfaces**. It depends on the nature and texture of the surfaces in contact:

**Very smooth surfaces** (glass, polished ceramic): Less friction - objects slide farther

**Moderately rough surfaces** (wooden table): Medium friction

**Very rough surfaces** (cloth, sand, concrete): More friction - objects stop sooner

**Experimental Observation (Activity 5.4):**

When the same object is pushed on different surfaces:

On glass: The object slides far before stopping (least friction)

On wood: The object slides a moderate distance

On cloth: The object stops sooner (more friction)

On sand: The object stops very quickly (maximum friction)

This proves: **Friction is greater on rough surfaces and less on smooth surfaces.**

**Friction in Different Media:**

Friction is not limited to solid surfaces. Objects moving through liquids and gases also experience friction:

**Air friction (air resistance):** Airplanes, high-speed trains, and racing cars are designed with streamlined (pointed) shapes to reduce air friction

**Water friction (water resistance):** Ships and submarines are designed with hydrodynamic shapes to reduce water friction

**Oil friction:** Objects moving through oil experience friction from the oil

**Real-Life Applications and Examples:**

1. **Stopping power:** Brakes on vehicles work by increasing friction between the brake pads and wheels

2. **Walking:** Without friction between our shoes and the ground, we would slip and fall

3. **Writing:** Friction between pen/pencil and paper allows us to write

4. **Climbing:** Friction between hands/feet and a surface allows climbing

5. **Matches:** Friction between the match stick and the matchbox creates the heat that ignites the match head

6. **Cooking:** Friction is used when rolling chapatis - the rolling pin creates friction with the dough

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5.4.2 NON-CONTACT FORCES

**Definition:** Non-contact forces are forces that can act on objects **even when they are not physically touching**. The force can be exerted from a distance.

#### **Magnetic Force**

**What is Magnetism:**

A **magnet** is an object that can attract certain materials (magnetic materials) without touching them. Iron, steel, nickel, and cobalt are magnetic materials.

**Magnetic Poles:**

Every magnet has two poles:

**North pole (N)**

**South pole (S)**

**Behavior of Magnetic Poles:**

When two magnets are brought close to each other:

• **Like poles repel each other:**

North pole repels North pole (N-N repulsion)

South pole repels South pole (S-S repulsion)

The magnets push away from each other

• **Unlike poles attract each other:**

North pole attracts South pole (N-S attraction)

The magnets pull toward each other

**Definition of Magnetic Force:**

**The force exerted by a magnet on another magnet or a magnetic material is called magnetic force.** It is a non-contact force because the magnet can attract or repel objects without touching them.

**Electromagnets:**

Electromagnets are created when electrical current flows through a coil of wire. They behave exactly like permanent magnets and produce magnetic force.

**Experimental Observation (Activity 5.5):**

Using two ring magnets on a vertical stick:

When like poles face each other (N-N or S-S), the upper magnet floats above the lower magnet - they repel

When unlike poles face each other (N-S), the upper magnet is pulled downward - they attract

If you push the repelling magnet down, you feel a strong force pushing it back up

This proves that magnetic force acts without contact.

**Real-Life Applications:**

1. **Compass:** Uses Earth's magnetic field to find direction

2. **Magnetic locks:** Used in refrigerators and cabinets

3. **Credit and debit cards:** Contain magnetic strips with encoded information

4. **Magnetic resonance imaging (MRI):** Used in medical diagnosis

5. **Magnetically levitated (maglev) trains:** Use magnetic force to float above tracks, reducing friction

6. **Loudspeakers:** Use magnets to convert electrical signals into sound

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#### **Electrostatic Force**

**What is Static Electricity:**

When certain materials are rubbed together vigorously, they accumulate **electrical charges** on their surfaces. These charges are called **static charges** because they remain stationary (do not move by themselves) on the object's surface. An object carrying static charges is called a **charged object**.

**Activity 5.6 Observation - Charging by Friction:**

When a plastic scale/straw is rubbed vigorously with polythene:

Both objects become charged

When the charged scale is brought near small pieces of paper (without touching), the paper pieces are attracted and stick to the scale

This happens without any contact between the scale and paper

This demonstrates that a charged object can exert force on other objects from a distance.

**Two Types of Charges:**

**Activity 5.7 Observation - Interaction Between Charges:**

When two balloons are rubbed with a woolen cloth:

Both balloons become charged

When released, they move away from each other (repulsion)

When the woolen cloth is brought near one balloon, they attract each other

**Conclusions from this activity:**

• The two balloons were charged in the same way, yet they repelled each other

This proves: **Like charges repel each other**

• The balloon and woolen cloth (which was used to charge the balloon) attract each other

This means they have opposite kinds of charges

This proves: **Unlike charges attract each other**

**The two kinds of charges are called:**

**Positive charge (+)** - present on one type of material

**Negative charge (-)** - present on another type of material

**Definition of Electrostatic Force:**

**The force exerted by a charged body on another charged body or an uncharged body is called electrostatic force.** It is a non-contact force because charged objects can attract or repel without touching.

**Important Note on Electric Current:**

When static charges move, they constitute an **electric current** in an electrical circuit. This is the same current that makes a lamp glow, produces heating effects (like in electric heaters), or creates magnetic effects (like in electromagnets). We will study electric current in more detail in later chapters.

**Real-Life Applications:**

1. **Photocopiers:** Use electrostatic forces to transfer toner to paper

2. **Electrostatic precipitators:** Use electric charges to remove dust from air

3. **Lightning:** Results from buildup of static charges in clouds

4. **Touchscreen phones:** Use electrostatic forces to detect finger positions

5. **Dust attraction to TV screens:** Dust is attracted by static charge buildup

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#### **Gravitational Force**

**Definition and Observation:**

When you throw a ball vertically upward, it goes up, slows down, stops momentarily at the highest point, and then falls back to the ground. No matter how hard you throw it, it always returns to the ground. This is true for all objects in our experience - they all eventually fall toward the Earth.

**Definition of Gravitational Force:**

**The force with which the Earth attracts objects toward itself is called the gravitational force, force of gravity, or simply gravity.**

**Why is Gravity a Non-Contact Force:**

Gravitational force acts without any contact between the Earth and the object. You don't need to touch an object to make it fall - the Earth's gravity pulls it down from a distance. This makes gravity a **non-contact force**.

**Characteristics of Gravitational Force:**

• **Always attractive:** Unlike magnetic force (which can attract or repel) and electrostatic force (which can attract or repel), gravitational force is **always attractive**. Objects are always pulled toward the Earth, never repelled.

• **Always acts downward:** Gravity pulls objects toward the center of the Earth, which is vertically downward from our perspective.

• **Acts on all objects:** Every object on Earth experiences gravitational force, regardless of its material or size.

• **Acts from a distance:** No contact is needed for gravity to act.

**Motion Under Gravity:**

**Vertical Motion - Upward Throw (Activity 5.8):**

When an object is thrown vertically upward:

1. **Going up phase:**

The object moves upward against gravity

Gravitational force acts downward (opposite to motion)

The object's speed **decreases** continuously

The object slows down progressively

2. **At the highest point:**

The object momentarily stops (velocity = 0)

Gravitational force still acts downward

The direction of motion is about to change

3. **Coming down phase:**

The object moves downward in the direction of gravitational force

The object's speed **increases** continuously

The object accelerates as it falls

**Vertical Motion - Downward Drop:**

When an object is dropped from a height:

1. The object takes a straight vertical path downward

2. Gravitational force acts in the same direction as motion

3. The object's speed continuously increases as it falls

4. The object reaches the ground with maximum speed

**Diagram Description - Vertical Motion:**

*Upward throw:*

Shows object at various heights

Shows decreasing distances between successive positions (indicating decreasing speed)

Arrow pointing downward shows direction of gravitational force

*Downward drop:*

Shows object at various heights

Shows increasing distances between successive positions (indicating increasing speed)

Arrow pointing downward shows direction of gravitational force

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5.5 Weight and Its Measurement

**Understanding Weight**

**Definition of Weight:**

**Weight is the force with which the Earth pulls an object toward itself.** It measures how strongly an object is attracted by Earth's gravity.

**Important Distinction:**

**Mass:** The amount of matter in an object (measured in kg or g)

**Weight:** The gravitational force acting on an object (measured in newtons, N)

**Note:** Mass is constant everywhere (on Earth, Moon, or in space), but weight changes depending on the gravitational force at that location. An object weighs less on the Moon than on Earth because the Moon's gravity is weaker.

**SI Unit of Weight:**

Since weight is a force, its SI unit is the **newton (N)**, the same as other forces.

**Does the Earth Pull Every Object with Equal Force?**

**Activity 5.9 Observation - Weight of Different Objects:**

Using a spring and different objects:

1. A pencil box is hung from a spring

The spring stretches by a certain amount

2. A tiffin box (heavier) is hung from the spring

The spring stretches more than in case 1

3. A small stone is hung from the spring

The spring stretches a different amount

**Observation:** The stretch in the spring is different for different objects.

**Conclusion:** **The Earth pulls different objects with different forces. Objects have different weights.** Heavier objects (with more mass) are pulled more strongly by gravity and have greater weight.

**Relationship Between Weight and Mass:**

Weight ∝ Mass

In other words, if an object has twice the mass, it will have twice the weight.

**Mathematical Relationship:**

Weight (W) = Mass (m) × Gravitational acceleration (g)

Where:

W = Weight (in newtons, N)

m = Mass (in kilograms, kg)

g = Gravitational acceleration ≈ 10 m/s² (or 9.8 m/s² for precise calculations)

**Example Calculation:**

If an object has a mass of 5 kg, its weight = 5 × 10 = 50 N

**Spring Balance - Device for Measuring Weight**

**What is a Spring Balance:**

A **spring balance** is a simple device used to measure the weight (force) of objects.

**Components of a Spring Balance:**

1. **Spring:** A coiled metal spring that stretches when force is applied

2. **Hook:** Attached to the lower end of the spring for hanging objects

3. **Scale:** Marked on the body with divisions showing weight values

4. **Unit markings:** Usually marked in newtons (N) on one scale and grams or kilograms (g or kg) on another scale

**How a Spring Balance Works:**

1. When an object is hung from the hook, the gravitational force (weight) pulls the object down

2. This force stretches the spring

3. The more the object weighs, the more the spring stretches

4. The amount of stretch is proportional to the weight

5. The scale pointer moves to indicate the weight

**Important Note on Scale Markings:**

The kilogram (kg) or gram (g) markings on a spring balance are **not measures of mass directly**. Instead, these markings show the weight in grams or kilograms **force** (a unit of force). These markings assume the spring balance is being used on Earth where g = 10 m/s².

For example:

"100 g" marking means 100 grams-force, which is approximately 1 N

"1 kg" marking means 1 kilogram-force, which is approximately 10 N

**Using a Spring Balance (Activity 5.10):**

**Steps to read a spring balance:**

1. **Identify the range:** Look at the maximum marking on the scale to determine what weights the balance can measure

Example: If maximum marking is 10, it can measure 0 to 10 N

2. **Check divisions:** Count the number of divisions between marked values to understand the scale intervals

Example: If there are 10 small divisions between 0 and 1 N, each division = 0.1 N

3. **Note the zero position:** Identify where the pointer rests when no weight is hung (should be at 0)

4. **Hang the object:** Carefully hang the object from the hook

The pointer will move to indicate the weight

Ensure the object hangs freely without touching anything

5. **Read the value:** Check where the pointer aligns with the scale markings

The reading should be at the pointer's position

For precise readings, if the pointer is between two markings, record the value between them

6. **Record the weight:** Write down the weight reading in newtons or the corresponding mass reading in grams/kilograms

**Precautions while using a spring balance:**

Don't overload the spring balance beyond its maximum range

Ensure the object hangs freely

Take the reading when the pointer stops moving

Use the same scale (N or kg) consistently

Handle carefully to avoid damaging the spring

**Real-Life Examples:**

1. **Weighing vegetables in markets:** Shopkeepers use spring balances to measure weight of vegetables

2. **Medical clinics:** Hanging spring balances are used to measure infant weight

3. **Luggage check:** Airlines use spring balances to check luggage weight

4. **Kitchen use:** Kitchen scales with spring mechanisms for measuring ingredients

5. **Industrial use:** Spring balances used in small factories to measure raw materials

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Summary of Key Concepts from Chapter 5

**Forces and Interactions:**

Force is a push or pull resulting from interaction between two objects

Forces are measured in newtons (N)

Forces can change motion, direction, or shape of objects

**Types of Forces:**

**Contact Forces** (require physical contact):

**Muscular force:** From contraction and elongation of muscles

**Friction:** Opposes motion, arises from surface irregularities

**Non-Contact Forces** (act from a distance):

**Magnetic force:** Attraction/repulsion between magnets or magnetic materials

**Electrostatic force:** Attraction/repulsion between charged objects

Like charges repel, unlike charges attract

**Gravitational force:** Earth's attraction on all objects

Always attractive

Always acts downward

Creates weight in objects

**Weight and Measurement:**

Weight = Force of gravity on an object = mg

Measured in newtons using a spring balance

Different objects have different weights

Weight increases with mass

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Important Diagrams to Draw and Label

**Diagram 1: Forces Changing Objects**

Show 4 sub-figures:

1. Object at rest → Apply force → Object moving

2. Moving object → Apply force in opposite direction → Object slowing

3. Moving object → Apply sideways force → Object changing direction

4. Balloon unpressed → Apply pressure force → Balloon deformed

All with clear arrows showing direction of applied force.

**Diagram 2: Friction Between Surfaces**

Draw two magnified surface profiles:

Top surface: Multiple peaks and valleys

Bottom surface: Multiple peaks and valleys

Show interlocking between peaks and valleys

Arrow showing direction of motion

Curved arrows showing friction acting opposite to motion

**Diagram 3: Magnetic Force - Attraction and Repulsion**

Show two scenarios:

1. **Repulsion:** Two magnets with like poles (N-N) facing, arrow showing them pushing apart

2. **Attraction:** Two magnets with unlike poles (N-S) facing, arrow showing them pulling together

Label poles clearly as N and S.

**Diagram 4: Gravitational Motion - Upward Throw**

Show trajectory of object:

Starting position with upward velocity arrow

Multiple positions going up with decreasing distances between them (showing decreasing speed)

Highest point with zero velocity

Multiple positions coming down with increasing distances between them (showing increasing speed)

Ground position

Downward force arrow at each position showing gravitational force direction

Label: "Speed decreasing" on way up, "Speed increasing" on way down

**Diagram 5: Spring Balance - Parts and Reading**

Show:

Full spring balance structure

Label: Spring, Hook, Scale, Pointer

Close-up of scale showing:

Markings in newtons (0, 1, 2, 3, ... up to 10 N)

Small divisions between markings

Pointer position at a specific reading

Show how to read the value

**Diagram 6: Electrostatic Force - Charged Objects**

Show:

1. Rubbed plastic scale with negative charges (-) marked

2. Paper pieces without charges

3. Paper pieces attracted to the scale with arrows showing attraction

4. Two balloons both with same charges (negative) marked

5. Balloons repelling with arrows showing repulsion

6. Balloon with negative charges and woolen cloth with positive charges attracting

**Diagram 7: Forces Acting on a Bicycle**

Draw a bicycle with arrows showing:

Muscular force from pedaling (forward)

Friction from ground (backward)

Wind resistance/air friction (backward)

Gravitational force (downward)

Normal force from ground (upward)

Arrows at appropriate positions with clear labels

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Key Terms Glossary

**Balanced Forces:** Forces acting on an object in opposite directions with equal magnitude, resulting in no net change in motion

**Charged Object:** An object that has accumulated static electrical charges on its surface

**Electrostatic Force:** Non-contact force between charged objects or between a charged object and an uncharged object

**Friction:** Contact force opposing motion between surfaces in contact

**Gravitational Force:** Non-contact force exerted by Earth attracting all objects toward itself

**Magnetic Force:** Non-contact force exerted by magnets on other magnets or magnetic materials

**Muscular Force:** Contact force produced by contraction and elongation of muscles

**Newton (N):** SI unit of force

**Static Charges:** Electrical charges that accumulate on object surfaces and remain stationary

**Weight:** Gravitational force exerted by Earth on an object; measured in newtons

**Spring Balance:** Device using a spring to measure weight of objects

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This comprehensive coverage provides students with complete understanding of all concepts, real-world applications, and the ability to answer any examination question from the chapter.