How Forces Affect Motion

The Concept of Force

**Force** is a push or pull that can change the state of motion of an object, change its direction, or alter its shape. Force is a **vector quantity**, meaning it has both magnitude and direction.

Effects of Force

Makes an object move from rest (e.g., kicking a ball)

Changes the speed of a moving object (e.g., braking a bicycle)

Changes the direction of motion (e.g., striking a cricket ball with a bat)

Changes the shape of an object (e.g., squeezing a lemon with fingers)

Measuring Force

**SI Unit:** Newton (N) — symbol written as capital N

**Magnitude** expresses the strength of force

**Spring Balance** is used to measure force magnitude

The smallest forces we can directly feel are of the order of **millinewtons (10⁻³ N)**, such as light touch

Scientists can measure forces as small as **yoctonewtons (10⁻²⁴ N)** using specialized equipment

Important Point

If either the magnitude or direction (or both) of a force changes, the effect of that force also changes.

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Balanced and Unbalanced Forces

When multiple forces act on an object, we need to determine their combined effect through **net force**.

Balanced Forces

**Balanced forces** are two or more forces that are equal in magnitude but opposite in direction. Their net force is zero, and the object remains in its original state.

**Example:** In a tug of war, when both teams pull the rope with equal force, the rope does not move. The rope experiences balanced forces.

Unbalanced Forces

**Unbalanced forces** are forces that are not equal in magnitude or not opposite in direction. They produce a non-zero net force.

Calculating Net Force

**Case 1: Forces Acting in the Same Direction**

Net force = Sum of individual forces

Direction = Same as the direction of both forces

**Formula:** F_net = F₁ + F₂

**Example:** Two people push a stalled car. Person A applies 30 N and Person B applies 20 N, both in the forward direction. Net force = 30 + 20 = 50 N forward.

**Case 2: Forces Acting in Opposite Directions**

Net force = Difference between magnitudes

Direction = Along the larger force

**Formula:** F_net = |F₁ - F₂|

**Example:** A block experiences 10 N force to the right and 6 N force to the left. Net force = 10 - 6 = 4 N to the right.

Worked Example (From Textbook)

Two forces of 10 N and 6 N act on a block:

**(a) Both towards right:** Net force = 10 + 6 = 16 N towards right

**(b) 10 N right, 6 N left:** Net force = 10 - 6 = 4 N towards right

**(c) 10 N left, 6 N right:** Net force = 10 - 6 = 4 N towards left

Important Concept

An object's motion depends only on the **net force**, not on individual forces

Forces at angles to each other will be studied in higher grades

Equal and opposite forces on extended objects cause rotation (e.g., turning a tap)

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The Force of Friction: Always Present

**Friction** is the force that opposes the relative motion between two surfaces in contact. It always acts opposite to the direction of motion or applied force.

Why Friction Occurs

When you push a box at rest, it doesn't move immediately because friction acts opposite to your applied force. The box moves only when your applied force exceeds the friction force, creating a net force in the forward direction.

Forces Acting on a Moving Object

When pushing an object on a surface, four forces act:

1. **Applied Force** — Force you apply (horizontal, forward)

2. **Gravitational Force (Weight)** — Earth's pull (vertical, downward) = mg

3. **Normal Force** — Perpendicular force from surface (vertical, upward)

4. **Friction Force** — Opposing motion (horizontal, backward)

**Important:** Gravitational force and normal force are **balanced** (equal and opposite in vertical direction), so they don't affect horizontal motion.

Friction and Continuous Motion

Once you stop pushing a moving object, friction continues to act opposite to motion, gradually decreasing velocity until the object stops. Examples:

A bicycle comes to rest after you stop pedaling

A ball stops rolling after you stop pushing it

To keep an object moving at constant velocity, you must continuously apply a force equal to the friction force to balance it.

Friction Depends on Surface Properties

Different surfaces have different friction forces. Smoother surfaces have less friction than rougher surfaces.

Activity 6.1 Observations (Rubber Band and Coins)

A stack of coins is pushed back by a stretched rubber band and released on different surfaces:

**On wooden table:** Coins travel shorter distance before stopping (larger friction)

**On laminated surface:** Coins travel greater distance (less friction than wood)

**On polished marble/tile:** Coins travel largest distance (least friction)

**Conclusion:** When friction is smaller, velocity decreases more slowly, and the object travels a larger distance.

Activity 6.2 Measurements (Spring Balance)

Using a spring balance to measure friction force on different surfaces:

1. Place a wooden block on the surface

2. Pull horizontally with gradually increasing force

3. Note the spring balance reading when the block just starts moving

4. This reading approximates the friction force on that surface

5. Smaller reading = smaller friction; larger reading = larger friction

**Result:** Polished surfaces show smallest spring balance reading (least friction), while rough surfaces show largest reading.

Thought Experiment: Zero Friction

If friction were zero (perfectly smooth surfaces), an object would continue moving forever at constant velocity and never come to rest, even without any applied force.

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Newton's First Law of Motion

**Statement:** An object at rest remains at rest, and an object in motion continues to move with a constant velocity, unless a net force acts upon the object.

**Alternative Statement:** If the net force acting on an object is zero, the body cannot begin to move or change its velocity. Its acceleration will be zero.

Key Concepts

**At Rest:** An object with zero velocity remains at rest when F_net = 0

**Constant Velocity Motion:** An object moves in a straight line at unchanging speed and direction when F_net = 0. No change in magnitude or direction of velocity means zero acceleration.

Inertia

**Inertia** is the tendency of objects to resist changes in their state of rest or uniform motion. This property depends on mass — larger mass means greater inertia.

Historical Context

Ancient belief: Force needed to maintain motion (incorrect)

**Galileo Galilei** (17th century): Through thought experiments, argued that if impediments (friction) are removed, an object continues moving indefinitely

**Isaac Newton** (1687): Formulated laws of motion using the concept of inertia

**Unit naming:** Newton (lowercase n) for full form, N for symbol

Worked Examples

**Example 6.2:**

A person pushes a moving box with force equal to friction force.

Applied force = Friction force (opposite directions)

Net force = 0

By Newton's First Law, the box continues moving at constant velocity

**Example 6.3:**

Graphs for an object with zero net force:

**When object is at rest:**

Position-time graph: Horizontal line (position unchanged)

Velocity-time graph: Horizontal line at v = 0

**When object moves with constant velocity:**

Position-time graph: Straight line with positive slope

Velocity-time graph: Horizontal line at constant non-zero velocity

Both show zero acceleration

Exam-Important Points for First Law

Zero net force → object cannot change motion state

An object moving at constant velocity has zero net force

An object at rest can only start moving if net force becomes non-zero

Friction must be overcome by applied force to initiate motion

If balanced forces act, motion state remains unchanged

This law applies to all objects in all reference frames

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Activity Summary and Conclusions

**Activity 6.1 (Friction Investigation):** Demonstrates that friction varies with surface material and affects how quickly objects decelerate.

**Activity 6.2 (Friction Measurement):** Shows that spring balance reading is proportional to friction force on different surfaces.

**Combined Learning:** Friction force depends on surface properties; less friction allows objects to travel farther before stopping; maintaining constant velocity requires force equal to friction.