Motion and Force

Overview

Motion and Force forms the backbone of classical mechanics and is a high-weightage topic in WB TET Paper II Science. This chapter bridges everyday observations (a rolling ball, a falling mango, a moving bus) with fundamental physics principles that students encounter from Class 6–8. Expect 2–4 questions covering types of motion, effects of force, friction concepts and Newton's three laws.

Mastery here requires understanding that force is the cause of change in motion, not motion itself. Students must visualise real-life examples, recall Newton's laws verbally, and apply friction concepts to practical situations. The pedagogy angle often tests how to demonstrate these abstract ideas through activities and experiments in a classroom setting.

Key Concepts

• **Motion** is change in position of an object with respect to time and a reference point. An object at rest relative to one observer may be in motion relative to another.

• **Types of motion**: Rectilinear (straight line), Circular (around a fixed point), Rotational (spinning on an axis), Periodic (repeating at regular intervals), Oscillatory (to-and-fro about a mean position).

• **Force** is a push or pull that can change an object's state of rest or uniform motion, its shape, or its direction. SI unit: Newton (N).

• **Balanced forces** produce no change in motion; **unbalanced forces** cause acceleration or deceleration.

• **Friction** is the force that opposes relative motion between two surfaces in contact. It depends on the nature of surfaces and the normal force pressing them together.

• **Newton's First Law (Inertia)**: A body continues in its state of rest or uniform motion unless acted upon by an external unbalanced force.

• **Newton's Second Law**: Force equals mass times acceleration (F = ma). Greater mass needs greater force for the same acceleration.

• **Newton's Third Law**: For every action, there is an equal and opposite reaction. Action and reaction act on different bodies.

Formulas / Key Facts

| Concept | Formula / Fact | |---------|----------------| | Speed | Speed = Distance ÷ Time | | Velocity | Velocity = Displacement ÷ Time (has direction) | | Acceleration | a = (v − u) ÷ t, where u = initial velocity, v = final velocity | | Newton's Second Law | F = m × a (Force in N, mass in kg, acceleration in m/s²) | | Weight | W = m × g (g ≈ 10 m/s² on Earth's surface) | | Friction types | Static friction > Sliding friction > Rolling friction | | Factors affecting friction | Nature of surfaces, normal force (weight pressing down) | | Inertia depends on | Mass of the object — greater mass means greater inertia |

Worked Examples

**Example 1 — Calculating Force**

*A cart of mass 5 kg accelerates at 2 m/s². Find the force applied.*

Step 1: Identify given values. Mass m = 5 kg, acceleration a = 2 m/s².

Step 2: Apply Newton's Second Law. F = m × a = 5 × 2 = 10 N.

**Answer**: 10 Newton.

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**Example 2 — Identifying Type of Motion**

*Classify the motion: (a) Earth around the Sun (b) Pendulum of a clock (c) A car on a straight road*

(a) Circular motion (also periodic, as it repeats every year).

(b) Oscillatory motion (to-and-fro about a fixed point) and periodic.

(c) Rectilinear motion (straight-line path).

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**Example 3 — Newton's Third Law Application**

*A swimmer pushes water backward with her hands. Explain using Newton's Third Law.*

Action: Swimmer's hands push water backward.

Reaction: Water pushes the swimmer forward with equal force.

Result: The swimmer moves forward. Action and reaction act on different bodies (water and swimmer).

Common Mistakes

• **Thinking force is needed to maintain motion** → Correction: Force is needed only to *change* motion. A body in uniform motion continues without force if friction is absent (First Law).

• **Confusing speed and velocity** → Speed is a scalar (magnitude only); velocity is a vector (magnitude + direction). A car going round a circle at constant speed has changing velocity because direction changes.

• **Believing friction is always harmful** → Friction enables walking, writing, braking vehicles. Without friction, we could not grip anything.

• **Saying action and reaction cancel out** → They act on *different* bodies, so they never cancel. A book on a table: weight acts on table, reaction acts on book — different objects.

• **Mixing up mass and weight** → Mass is amount of matter (kg), constant everywhere. Weight is gravitational force (N), varies with location.

Quick Reference

1. Motion requires a reference point; rest and motion are relative.

2. Unbalanced force → change in motion; balanced force → no change.

3. First Law: Inertia — objects resist change in their state.

4. Second Law: F = ma — force produces acceleration proportional to mass.

5. Third Law: Action = Reaction, on different bodies.

6. Friction opposes motion; static > sliding > rolling.

7. Reduce friction: lubricants, ball bearings, polishing surfaces.