Motion And Time — for Class 7

What Is Motion — and Why Does Time Matter?

Every moving thing around us — a cricket ball after a sixer, an auto-rickshaw in traffic, water flowing in a river — is in motion. But motion alone doesn’t tell us the full story. We need time to make sense of it.

Think about it this way: if I tell you a bus travelled 60 km, you’d ask, “In how long?” That question is exactly what this chapter is about. Motion and time together give us speed, and speed lets us compare, predict, and understand the physical world.

This is a foundational chapter for Class 7 CBSE and sets up everything you’ll study in motion for Classes 9, 10, and eventually JEE/NEET. Get this right now, and later chapters feel easy.

Key Terms and Definitions

Motion — An object is said to be in motion when its position changes with respect to a fixed reference point (also called the reference point or origin).

A book lying on a table is not in motion. A book sliding off that table is.

Rest — An object is at rest when its position does not change with respect to a reference point.

Rest and motion are relative. You’re sitting still in a train, so you’re at rest relative to the seat — but you’re in motion relative to the platform outside. This is called relative motion and confuses many students in later classes. Getting comfortable with “relative to what?” now pays off.

Types of Motion:

Type	Description	Example
Rectilinear (Linear)	Motion along a straight line	A car on a straight highway
Circular	Motion along a circular path	A stone tied to a string, swung in a circle
Periodic	Motion that repeats at regular intervals	A pendulum, Earth’s revolution
Random	Motion with no fixed pattern	A fly buzzing around

Speed — The distance covered by an object per unit time.

\text{Speed} = \frac{\text{Distance}}{\text{Time}}

The SI unit of speed is metres per second (m/s). In everyday life, we also use km/h.

Uniform Motion — When an object covers equal distances in equal intervals of time. Speed stays constant.

Non-Uniform Motion — When an object covers unequal distances in equal intervals of time. Speed changes. This is the more common situation in real life.

Core Concepts — Explained Properly

The Distance-Time Relationship

The formula $\text{Speed} = \frac{\text{Distance}}{\text{Time}}$ is simple but needs to be understood in three forms:

\text{Speed} = \frac{d}{t} \quad \text{Distance} = \text{Speed} \times t \quad \text{Time} = \frac{d}{\text{Speed}}

The triangle trick many teachers show (cover the quantity you want) works fine for Class 7. Just make sure units are consistent before calculating.

Distance-Time Graphs

This is where most marks are scored — and lost.

What the graph tells us:

X-axis (horizontal): Time
Y-axis (vertical): Distance from starting point
Slope of the line: Speed (steeper slope = higher speed)

A straight line on a distance-time graph means uniform motion. A curved line means non-uniform motion (speed is changing). A horizontal line means the object is at rest (no change in distance, time is passing).

Measuring Time — From Sundials to Atomic Clocks

Time measurement has evolved over thousands of years:

Sundials — Used shadow of the sun. Only worked in daylight.
Water clocks (Clepsydra) — Water dripped at a constant rate. Used in ancient India and Egypt.
Mechanical clocks — Used pendulum motion (Galileo first noticed the pendulum property).
Quartz clocks — Vibrations of a quartz crystal. Used in modern watches.
Atomic clocks — Most accurate. Based on vibrations of caesium atoms.

The SI unit of time is the second (s). The standard second is defined using atomic clocks.

The Simple Pendulum

A simple pendulum consists of a small bob (mass) suspended from a fixed point by a string.

Time period (T) — The time taken to complete one full oscillation (swing to one side and back).

Oscillation — One complete back-and-forth movement.

What determines the time period?

The length of the string — longer string, longer time period. ✓
The mass of the bob — does NOT affect time period. ✗ (common misconception!)
The amplitude (how far you pull it) — does NOT affect time period for small amplitudes. ✗

CBSE Class 7 questions often ask: “If the length of a pendulum is increased, what happens to its time period?” Answer: Time period increases. A related question: “Does mass affect time period?” Answer: No.

Unit Conversion for Speed

Students lose easy marks by not converting units properly.

km/h to m/s:

1 \text{ km/h} = \frac{1000 \text{ m}}{3600 \text{ s}} = \frac{5}{18} \text{ m/s}

m/s to km/h:

1 \text{ m/s} = \frac{18}{5} \text{ km/h} = 3.6 \text{ km/h}

A useful trick: to convert km/h to m/s, multiply by $\frac{5}{18}$ . To go the other way, multiply by $\frac{18}{5}$ .

Solved Examples

Example 1 — Easy (CBSE Level)

A cyclist covers a distance of 15 km in 1 hour. Find her speed in km/h and m/s.

Solution:

\text{Speed} = \frac{\text{Distance}}{\text{Time}} = \frac{15 \text{ km}}{1 \text{ h}} = 15 \text{ km/h}

Converting to m/s:

15 \times \frac{5}{18} = \frac{75}{18} = 4.17 \text{ m/s}

Example 2 — Medium (CBSE Level)

A train travels at 72 km/h. How much time will it take to cover 360 km?

Solution:

First, note the units are consistent (both in km and km/h), so no conversion needed.

\text{Time} = \frac{\text{Distance}}{\text{Speed}} = \frac{360 \text{ km}}{72 \text{ km/h}} = 5 \text{ hours}

Example 3 — Medium (Graph-Based, CBSE Level)

From a distance-time graph, two objects A and B move for the same duration. A travels 40 m in 4 seconds; B travels 20 m in 4 seconds. Which is faster? What are their speeds?

Solution:

\text{Speed of A} = \frac{40}{4} = 10 \text{ m/s}

\text{Speed of B} = \frac{20}{4} = 5 \text{ m/s}

Object A is faster. On a distance-time graph, A’s line would be steeper than B’s.

Example 4 — Harder (Reasoning-Based, CBSE Olympiad Level)

Riya takes 25 minutes to reach school covering 5 km. Her brother Arjun takes 20 minutes to cover the same distance. Who has a higher speed, and by how much (in m/s)?

Solution:

Convert time to seconds before calculating:

Riya: 25 min = 1500 s
Arjun: 20 min = 1200 s

\text{Speed of Riya} = \frac{5000}{1500} = 3.33 \text{ m/s}

\text{Speed of Arjun} = \frac{5000}{1200} = 4.17 \text{ m/s}

Arjun is faster by $4.17 - 3.33 = 0.84 \text{ m/s}$ .

Always convert distance to metres and time to seconds when the final answer needs to be in m/s. Mixing km with seconds (or metres with hours) is the most common calculation error in this chapter.

Exam-Specific Tips

CBSE Class 7 Board Pattern

This chapter contributes 8-10 marks in the annual exam.
Expect 1-2 MCQs, 1 short-answer question on the pendulum, and 1 numerical on speed.
Graph-based questions (read a distance-time graph, or explain what a horizontal line means) appear almost every year.
Definitions of oscillation, time period, and the difference between uniform and non-uniform motion are frequently asked in 2-mark questions.

NCERT question: “Under what conditions is the distance-time graph a straight line?” This is a standard 2-mark question. The answer: when the object moves with uniform speed (covers equal distances in equal time intervals).

For Science Olympiads (NSO, NSTSE)

Expect questions where two objects’ distance-time graphs are drawn together — you need to find where they meet (same position at the same time).
Pendulum problems with changing lengths: if length quadruples, time period doubles (since $T \propto \sqrt{L}$ — this is beyond Class 7 syllabus but appears in olympiads).

Common Mistakes to Avoid

Mistake 1: Using distance in km and time in seconds directly. Speed = km/s is a valid unit, but it’s almost never what the question wants. Always check what unit the answer should be in before calculating.

Mistake 2: Thinking mass affects the pendulum’s time period. Heavier bob, lighter bob — doesn’t matter. Only the length of the string changes the time period. This trips up students every single year in the theory section.

Mistake 3: Confusing oscillation count. One oscillation = one complete cycle (left → right → left). Some students count just one swing (left → right) as one oscillation. That’s only half an oscillation.

Mistake 4: Reading distance-time graphs wrong. A steep line = high speed. A gentle slope = low speed. A flat (horizontal) line = object is at rest, NOT moving slowly. Many students say “moving slowly” for a horizontal line — that’s wrong.

Mistake 5: Forgetting to convert minutes to seconds (or hours). If time is given in minutes and distance in metres, you must convert minutes to seconds before using the formula. Speed = 100 m / 2 min is not correct until you write it as 100 m / 120 s = 0.83 m/s.

Practice Questions

Q1. A car travels 120 km in 2 hours. Find its speed in km/h and m/s.

Speed = 120 ÷ 2 = 60 km/h

Converting: 60 × (5/18) = 16.67 m/s

Q2. A pendulum completes 20 oscillations in 10 seconds. What is its time period?

Time period = Total time ÷ Number of oscillations = 10 ÷ 20 = 0.5 seconds

Q3. On a distance-time graph, what does a horizontal straight line represent?

The object is at rest — its distance from the reference point is not changing even as time passes.

Q4. Aman runs 400 m in 80 seconds. Raju runs 300 m in 50 seconds. Who is faster?

Aman’s speed = 400 ÷ 80 = 5 m/s

Raju’s speed = 300 ÷ 50 = 6 m/s

Raju is faster.

Q5. A bus starts from rest and reaches a speed of 54 km/h. Express this speed in m/s.

54 × (5/18) = 15 m/s

Q6. Two objects A and B start from the same point. A moves at 4 m/s, B moves at 6 m/s, both in the same direction. After 10 seconds, how far apart are they?

Distance by A = 4 × 10 = 40 m

Distance by B = 6 × 10 = 60 m

Gap = 60 − 40 = 20 m

Q7. If a pendulum’s length is increased, what happens to its time period? Give one real-life example where this matters.

Time period increases when length increases. A longer pendulum swings more slowly.

Real-life example: Grandfather clocks have long pendulums (about 1 metre) which tick once per second. Shorter pendulums tick faster — that’s why small desk clocks use different timing mechanisms.

Q8. A train takes 3 hours to cover a distance between two stations at 80 km/h. A new express train covers the same distance in 2 hours. Find the speed of the express train.

First, find the distance:

Distance = Speed × Time = 80 × 3 = 240 km

Express train speed = 240 ÷ 2 = 120 km/h

Frequently Asked Questions (FAQs)

What is the difference between uniform and non-uniform motion?

In uniform motion, an object covers equal distances in equal time intervals — its speed stays constant. In non-uniform motion, the distances covered in equal time intervals are different. A car in city traffic is always in non-uniform motion; a satellite in a circular orbit is in uniform motion (uniform speed, changing direction).

What is a reference point in motion?

A reference point (or origin) is a fixed location relative to which we measure whether an object is moving or at rest. For example, a tree by the roadside is a reference point for a moving car. Without specifying a reference point, “motion” and “rest” have no meaning.

Why do we need to measure time accurately?

Accurate time measurement is essential in sports (timing athletes to hundredths of a second), navigation (ships and planes use time to calculate position), and science experiments (measuring speed of chemical reactions or physical motion).

What is the SI unit of speed?

The SI unit of speed is metres per second (m/s). Other common units are km/h (for vehicles) and cm/s (for slow-moving objects in lab experiments).

Does the weight of a pendulum bob change its time period?

No. The time period of a simple pendulum depends only on the length of the string (and very slightly on the acceleration due to gravity). The mass or weight of the bob has no effect. This was first established by Galileo Galilei.

How do you read a distance-time graph?

Plot time on the x-axis and distance on the y-axis. The steepness (slope) of the line tells you the speed — a steeper line means higher speed. A straight line means uniform motion. A curve means non-uniform motion. A horizontal line means the object is stationary.

What was used to measure time before clocks were invented?

Ancient civilisations used sundials (shadow of the sun), water clocks (dripping water), hourglasses (falling sand), and even burning candles (marked at intervals). The motion of celestial bodies — the sun, moon, and stars — was the original clock for early humans.

What is the relationship between distance, speed, and time?

\text{Distance} = \text{Speed} \times \text{Time}

This simple relationship is the core of the entire chapter. Rearrange it to find speed $\left(\frac{d}{t}\right)$ or time $\left(\frac{d}{s}\right)$ as needed. Every numerical in this chapter is a variation of this formula.