What this chapter is about
This chapter helps you understand two important ideas: how we measure time and how we describe the motion of objects. Time is something we use every day — to wake up for school, to catch a bus, to know when a cricket match starts. But have you ever wondered how people measured time before clocks existed?
Motion is all around us. A car moving on the road, a ball thrown in the air, a fan spinning — all these are examples of motion. In this chapter, you will learn how to tell if something is moving, how fast it is moving, and how to measure these things properly.
After studying this chapter, you should be able to measure time using different devices, understand the difference between speed and other ways of describing motion, and solve simple problems about distance, speed and time.
Key ideas
- Motion means a change in the position of an object with time. If a thing stays in the same place, it is at rest.
- To know if something is moving, we compare its position to a fixed point called a reference point. A tree by the road is a good reference point.
- Speed tells us how fast an object is moving. It is the distance covered in a given time.
- Speed = Distance ÷ Time. The common unit is metres per second (m/s) or kilometres per hour (km/h).
- Uniform motion means an object covers equal distances in equal intervals of time. Non-uniform motion means distances covered are unequal.
- Time can be measured using sundials, water clocks, sand clocks, pendulums, and modern clocks or watches.
- A simple pendulum takes the same time to complete one swing if its length stays the same. This time is called the time period.
- The basic unit of time is the second (s). Larger units are minutes (60 s) and hours (60 min).
Formulas and facts to remember
- Speed = Distance ÷ Time — tells how much distance an object covers every unit of time.
- Distance = Speed × Time — use when you know speed and time but need distance.
- Time = Distance ÷ Speed — use when you know distance and speed but need time.
- 1 hour = 60 minutes = 3600 seconds.
- 1 kilometre = 1000 metres.
- To change km/h to m/s, multiply by 5/18. To change m/s to km/h, multiply by 18/5.
- A longer pendulum swings slower; a shorter pendulum swings faster.
- Ancient devices for measuring time: sundial (shadow), water clock (dripping water), sand clock (falling sand).
Worked examples
Example 1: Finding speed
Ravi cycles 6 km from his home to school in 30 minutes. What is his speed in km/h?
Step 1: Write what you know. Distance = 6 km, Time = 30 minutes.
Step 2: Change time to hours. 30 minutes = 30 ÷ 60 = 0.5 hour.
Step 3: Use Speed = Distance ÷ Time = 6 ÷ 0.5 = 12 km/h.
Ravi's speed is 12 km/h.
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Example 2: Finding distance
A train moves at a speed of 60 km/h. How far will it travel in 2 hours?
Step 1: Speed = 60 km/h, Time = 2 hours.
Step 2: Distance = Speed × Time = 60 × 2 = 120 km.
The train travels 120 km.
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Example 3: Finding time
Priya walks at 4 km/h. How long will she take to reach a market 2 km away?
Step 1: Distance = 2 km, Speed = 4 km/h.
Step 2: Time = Distance ÷ Speed = 2 ÷ 4 = 0.5 hour = 30 minutes.
Priya will take 30 minutes.
Common mistakes
- Forgetting to use the same units for distance and time → always convert minutes to hours (or seconds) before using the formula.
- Thinking that a faster pendulum has a longer string → actually, a shorter string makes the pendulum swing faster.
- Confusing rest and motion without a reference point → always choose a fixed reference point first.
- Mixing up the formula and writing Speed = Time ÷ Distance → remember, speed is distance divided by time, not the other way round.
- Believing an object moving in a circle is at rest because it comes back → it is still in motion because its position keeps changing.
Quick revision
- Motion is change of position with respect to a reference point.
- Speed = Distance ÷ Time; unit is m/s or km/h.
- Uniform motion: equal distances in equal times; non-uniform: unequal distances.
- A pendulum's time period depends on its length, not on its weight.
- Sundial, water clock, sand clock and pendulum clock are older ways to measure time.
- 1 hour = 3600 seconds; to convert km/h to m/s, multiply by 5/18.