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CLASS 7 SCIENCE • CHAPTER 8

Measurement of Time and Motion

Understand how time and motion are measured, explore different types of motion, learn about distance, speed and periodic motion, and discover how these ideas are used in everyday life.

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📚 Chapter Contents

⏰ What Is Time?

Time is a fundamental quantity used to describe the sequence and duration of events.

We use time to determine when an event occurs and how long an activity takes.

Key Point: Time helps us compare the duration of different events and understand the order in which events happen.

Examples

📏 Measurement of Time

Time is measured by comparing the duration of an event with a standard unit of time.

Accurate measurement of time is important in science, sports, transport, communication and everyday activities.

Why Do We Need Standard Units?

Standard units allow people everywhere to communicate measurements accurately and consistently.

Remember: The SI unit of time is the second (s).

🕐 Units of Time

Different units are used depending on the duration being measured.

Unit Equivalent
1 minute 60 seconds
1 hour 60 minutes = 3600 seconds
1 day 24 hours
1 week 7 days
1 year 12 months

Smaller Measurements of Time

For events that happen very quickly, smaller units such as milliseconds and microseconds may be used.

⏱️ Clocks and Timekeeping

Humans have developed different instruments to measure time. Earlier methods depended on natural events such as the movement of the Sun and stars.

Examples of Timekeeping Devices

Modern Timekeeping: Modern clocks can measure time with very high accuracy. Precise time measurement is essential for science, navigation and technology.

🏃 What Is Motion?

An object is said to be in motion when its position changes with time relative to a reference point.

If the position of an object does not change with time relative to a reference point, the object is considered to be at rest.

Object at Rest

An object whose position does not change with respect to a chosen reference point is at rest.

Example: A book lying on a table.

Object in Motion

An object whose position changes with respect to a chosen reference point is in motion.

Example: A moving car on a road.

Important: Motion and rest are relative to the reference point chosen.

🔄 Types of Motion

Objects can move in different ways. Their motion can be classified according to the path they follow.

1. Linear Motion

Motion along a straight path is called linear motion.

Example: A car moving along a straight road.

2. Circular Motion

Motion along a circular path is called circular motion.

Example: The movement of the hands of a clock around its centre.

3. Rotational Motion

When an object spins around an axis, it undergoes rotational motion.

Example: A spinning wheel.

4. Oscillatory Motion

To-and-fro motion about a mean position is called oscillatory motion.

Example: A swinging pendulum.

📐 Distance and Its Measurement

Distance is the length of the path travelled by an object during its motion.

The SI unit of distance is the metre (m). Depending on the size of the distance, kilometres, centimetres and millimetres may also be used.

Unit Relationship
1 kilometre 1000 metres
1 metre 100 centimetres
1 centimetre 10 millimetres

Measuring Distance

Rulers, metre scales, measuring tapes and other instruments can be used to measure length or distance.

Measurement Tip: While measuring with a scale, place the object correctly and read the scale carefully to avoid errors.

⚡ Speed

Speed tells us how quickly an object covers a certain distance.

Speed = Distance ÷ Time

The SI unit of speed is metre per second (m/s). Kilometres per hour (km/h) is another commonly used unit.

Example

Suppose a cyclist travels 100 metres in 20 seconds.

Speed = 100 m ÷ 20 s = 5 m/s
Remember: Greater speed means an object covers more distance in the same amount of time.

🔁 Periodic Motion

Motion that repeats itself after equal intervals of time is called periodic motion.

Examples

Key Point: A periodic motion repeats after a definite or regular interval of time.

Time Period

The time taken by a periodic motion to complete one full cycle is called its time period.

🌍 Motion in Everyday Life

Motion is present everywhere around us. Some objects move in a straight path while others rotate, oscillate or follow curved paths.

Object Example of Motion
Moving car on a straight road Linear motion
Ceiling fan Rotational motion
Clock hand Circular motion
Swing Oscillatory motion
Earth around the Sun Periodic motion

📊 Uniform and Non-Uniform Motion

Uniform Motion

When an object covers equal distances in equal intervals of time, its motion is called uniform motion.

Example: A vehicle travelling at a constant speed on a straight road.

Non-Uniform Motion

When an object covers unequal distances in equal intervals of time, or its speed changes, its motion is called non-uniform motion.

Example: A bus moving through a busy city.

Feature Uniform Motion Non-Uniform Motion
Distance covered Equal distances in equal time intervals. Unequal distances in equal time intervals.
Speed Constant. Changes with time.
Example Object moving steadily at constant speed. Vehicle moving through traffic.

⚡ Quick Revision

Time: A quantity used to describe the duration and sequence of events.
SI Unit of Time: Second (s).
Motion: Change in position with time relative to a reference point.
Distance: Length of the path travelled by an object.
SI Unit of Distance: Metre (m).
Speed: Distance travelled per unit time.
Speed Formula: Speed = Distance ÷ Time.
Periodic Motion: Motion that repeats after regular intervals of time.
Uniform Motion: Equal distances covered in equal intervals of time.
Oscillatory Motion: Repeated to-and-fro motion about a mean position.

📝 Important Questions

Very Short Answer Questions

  1. What is time?
  2. What is the SI unit of time?
  3. What is motion?
  4. What is the SI unit of distance?
  5. Define speed.
  6. Write the formula for speed.
  7. What is periodic motion?
  8. Give one example of oscillatory motion.

Short Answer Questions

  1. Why are standard units necessary for measuring time and distance?
  2. Explain the difference between rest and motion.
  3. What is the difference between uniform and non-uniform motion?
  4. Explain periodic motion with two examples.
  5. How is speed calculated?
  6. Explain the different types of motion with suitable examples.

Numerical Questions

  1. A car travels 120 km in 3 hours. Find its average speed.
  2. A cyclist travels 200 metres in 40 seconds. Calculate the speed.
  3. A runner moves at a speed of 5 m/s for 20 seconds. Calculate the distance covered.

Long Answer Questions

  1. Explain the different types of motion with suitable examples.
  2. Explain how time and motion are related in the measurement of speed.
  3. What is periodic motion? Explain with suitable examples from everyday life.
Exam Tip: Remember the formula: Speed = Distance ÷ Time. Always write the correct unit with your final numerical answer.

❓ Frequently Asked Questions

1. What is the SI unit of time?

The SI unit of time is the second (s).

2. What is motion?

Motion is the change in position of an object with time relative to a reference point.

3. What is speed?

Speed is the distance travelled by an object per unit time.

4. What is the formula for speed?

Speed = Distance ÷ Time.

5. What is periodic motion?

Motion that repeats itself after equal intervals of time is called periodic motion.

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

In uniform motion, an object covers equal distances in equal intervals of time. In non-uniform motion, the distances covered in equal intervals of time are unequal or the speed changes.

7. What is the SI unit of speed?

The SI unit of speed is metre per second, written as m/s.

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