Topic Overview
📏 Chapter Overview: Measurement of Distances – Motion
This chapter is the foundation of physics in the AP TET syllabus. It covers how we measure the physical world (units) and how we describe the movement of objects within it (motion, speed, time). Understanding these basics is critical for solving numerical problems and answering conceptual questions.
1. Measurement: Theory & Units
Theory & Concept
Measurement is the process of comparing an unknown quantity with a known, fixed quantity called a Unit. The result has two parts: a number (magnitude) and a unit (e.g., 5 meters).
Standard Units (SI Units): To ensure uniformity worldwide, we use the International System of Units (SI Units).
Standard vs. Non-Standard Units
| Type | Description | Examples | Reliability |
|---|---|---|---|
| Non-Standard | Based on body parts or objects; varies from person to person. | Handspan, Cubit (elbow to fingertip), Foot, Pace (step). | Low (Unreliable) |
| Standard (SI) | Fixed, internationally accepted values. | Metre (m), Second (s), Kilogram (kg). | High (Reliable) |
- 1 kilometre (km) = 1000 metres (m)
- 1 metre (m) = 100 centimetres (cm)
- 1 centimetre (cm) = 10 millimetres (mm)
2. Motion and Rest: The Relative Concept
Concept & Analogy
Motion and Rest are relative terms. This means an object’s state depends entirely on the observer.
- Rest: Position does not change with time relative to surroundings.
- Motion: Position changes with time relative to surroundings.
Imagine you are sitting in a moving bus.
- Relative to a person standing on the road: You are in MOTION (because the bus is moving away from them).
- Relative to the passenger sitting next to you: You are at REST (your position relative to them is not changing).
3. Types of Motion
Objects move in different ways. We classify motion based on the path taken.
| Type of Motion | Definition (Concept) | Examples (Application) |
|---|---|---|
| Rectilinear | Motion along a straight line. | Soldiers marching, a stone falling freely, a car on a straight highway. |
| Curvilinear | Motion along a curved path. | A car turning a corner, a ball thrown at an angle. |
| Circular | Motion along a circular path; distance from a fixed center is constant. | Hands of a clock, a satellite orbiting Earth, tip of a fan blade. |
| Rotational | An object spins around its own internal axis. | Earth spinning on its axis (causing day/night), a spinning top, a potter’s wheel. |
| Periodic | Motion that repeats itself after a fixed interval of time. | A swinging pendulum, heartbeats, Earth revolving around the Sun (1 year). |
The Earth has Rotational motion on its axis (Day/Night) AND Periodic/Circular motion around the Sun (Seasons).
4. Speed, Time & Calculations
Measurement of Time
- SI Unit: second (s). Other units: minute (min), hour (h).
- Time Period: The time taken by a pendulum to complete one full oscillation (to-and-fro).
Speed (Concept & Formula)
Speed is the rate of motion. It tells us how fast or slow something moves.
- SI Unit of Speed: metres per second (m/s).
- Common Unit: kilometres per hour (km/h).
Average Speed
For non-uniform motion (when speed changes), we use Average Speed.
- km/h → m/s: Multiply by 5/18.
(e.g., 72 km/h = 72 × 5/18 = 20 m/s) - m/s → km/h: Multiply by 18/5.
(e.g., 10 m/s = 10 × 18/5 = 36 km/h)
5. Uniform Motion & Distance-Time Graphs
Uniform vs. Non-Uniform Motion
| Feature | Uniform Motion | Non-Uniform Motion |
|---|---|---|
| Concept | Equal distances in equal intervals of time. | Unequal distances in equal intervals of time. |
| Speed | Constant (Unchanging). | Variable (Changing). |
| Graph (D-T) | Straight line (slanted). | Curved line. |
Distance-Time (D-T) Graphs
Graphs visually represent motion. Time is on the X-axis, Distance on the Y-axis.
- Straight Line (slanted up): Uniform Motion (Constant Speed).
- Horizontal Line (parallel to X-axis): Object at REST (Distance not changing as time passes).
- Curved Line: Non-Uniform Motion (Changing Speed).
📝 Exam Tips, Formulas & Practice
Short Memory Formula Sheet
- S = D / T (Speed = Distance/Time)
- D = S × T (Distance = Speed × Time)
- T = D / S (Time = Distance/Speed)
- Avg Speed = Total D / Total T
- km/h to m/s = × 5/18
Typical AP TET Questions
- A train travels 120 km in 2 hours. What is its speed in m/s?
Step 1 (Find Speed in km/h): 120 / 2 = 60 km/h.
Step 2 (Convert to m/s): 60 × (5/18) = 16.67 m/s. - Which of the following pairs has the same type of motion?
A) Earth rotating on its axis and a spinning top.
Answer: A (Both are Rotational Motion).