Motion and Measurement of Distances: Class 6 Science NCERT Guide

The Story of Transport and the Need for Measurement

Long, long ago, people moved around on foot. To travel across water, they used simple logs or boats. A huge change came with the invention of the wheel, which led to animal-pulled carts. These inventions made it possible to travel greater distances and transport goods. As people started travelling more and building things, they faced a new problem: how to describe how far they travelled or how big something was?

This led to the first forms of measurement. People used parts of their body. For example, the distance from the elbow to the fingertips was called a 'cubit'. The width of a hand was a 'hand span', and the length of a foot was, well, a 'foot'! But can you see a problem here? My hand span might be different from yours! If we both measure the same table, we might get different answers. This confusion showed that we needed a fixed, standard way to measure things that would be the same for everyone, everywhere. This is why standard units were created.

Standard Units of Measurement (SI Units)

Standard Unit

A unit of measurement that has a fixed value and does not change from person to person or place to place. It is accepted by everyone.

Metre (m)

The standard (SI) unit of length. We use it to measure things like the length of a room or the height of a tree. (1 metre = 100 centimetres).

Centimetre (cm)

A smaller unit of length, used for measuring shorter objects like a pencil or a notebook. (100 centimetres = 1 metre).

Millimetre (mm)

An even smaller unit of length, used for measuring very small things like the thickness of a coin. (10 millimetres = 1 centimetre).

Kilometre (km)

A larger unit of length, used for measuring long distances, like the distance between two cities. (1 kilometre = 1000 metres).

How to Measure Length Correctly

1. Step 1: Place the Scale Correctly — Place your ruler or measuring tape exactly along the length of the object you want to measure. Make sure it is straight and not tilted.
2. Step 2: Start from the Zero Mark — Align the zero mark (0) of the scale with one end of the object. If the zero mark is broken, you can start from another whole number (like 1 cm), but you must subtract this value from the final reading. For example, if you start at 1 cm and the object ends at 15 cm, the length is 15 - 1 = 14 cm.
3. Step 3: Avoid Parallax Error — Keep your eye positioned directly above the mark you are reading on the scale. If you look at it from the side (left or right), the reading will appear different and incorrect. This viewing error is called parallax error.
4. Step 4: Measuring a Curved Line — To measure a curved line, you can't use a straight ruler. Instead, take a piece of thread. Place one end of the thread at the beginning of the curved line. Carefully lay the thread along the entire curve. Mark the point on the thread where the line ends. Now, straighten the thread and measure its length from the start to your mark using a ruler. This gives you the length of the curved line.

Understanding Motion and Its Types

What does it mean for something to be in motion? Motion is simply the change in an object's position with respect to its surroundings over time. If you are sitting on a chair, you are at rest because your position isn't changing. But if you walk from the chair to the door, you are in motion.

Objects can move in different ways. Let's look at the main types of motion:

• Rectilinear Motion: This is motion in a straight line. Think of a soldier marching in a parade, a car on a straight highway, or an apple falling from a tree.
• Circular Motion: This is when an object moves along a circular path. The object itself stays the same distance from a central point. For example, the tip of a clock's hand or a stone tied to a string and swung around.
• Rotational Motion: This is when an object spins or rotates on its own axis. A spinning top, a rotating fan, and the Earth spinning on its axis are all examples of rotational motion.
• Periodic Motion: This is any motion that repeats itself after a fixed period of time. A child on a swing, the pendulum of a clock, and the ringing of a school bell at the same time every day are all periodic.

Sometimes, an object can show more than one type of motion at the same time. For example, a rolling ball is moving in a straight line (rectilinear) and also spinning (rotational).

Exam Tips: Measurement and Motion

A common question in exams asks you to identify the type of motion. Remember the difference between circular and rotational motion. In circular motion, the whole object moves in a circle around a point (like the Earth around the Sun). In rotational motion, the object spins on its own axis (like the Earth spinning to cause day and night). For measurement questions, always mention the units (cm, m, km) in your final answer. Forgetting units can lose you marks!

Practice Questions with Solutions

• Q: A tailor needs to measure the height of a person, which is 165 cm. How would you express this height in metres (m)? A: Step 1: Recall the relationship between metres and centimetres. We know that 100 cm = 1 m. Step 2: To convert centimetres to metres, we need to divide the number of centimetres by 100. Step 3: Calculate the value: 165 cm ÷ 100 = 1.65 m. Final answer: The height of the person is 1.65 m.
• Q: What type (or types) of motion are shown by the wheels of a moving bicycle on a straight road? A: Step 1: Analyze the motion of the wheel's centre. As the bicycle moves along a straight road, the centre of the wheel also moves in a straight line. This is rectilinear motion. Step 2: Analyze the motion of any point on the rim of the wheel. The wheel is constantly spinning around its central axle. This is rotational motion. Step 3: Combine the observations. The wheel as a whole is moving forward, and it is also spinning. Final answer: The wheels of a moving bicycle show both rectilinear motion and rotational motion.
• Q: While measuring the length of a box, you find that the zero mark on your ruler is broken. You place the 2 cm mark at one end of the box, and the other end reads 27.5 cm. What is the actual length of the box? A: Step 1: Identify the starting and ending readings on the scale. The starting reading is 2 cm. The final reading is 27.5 cm. Step 2: Understand that the measured length is the difference between the final reading and the starting reading, not just the final reading. Step 3: Subtract the starting reading from the final reading: 27.5 cm - 2 cm = 25.5 cm. Final answer: The actual length of the box is 25.5 cm.
• Q: Give two examples of periodic motion that you observe in your daily life. A: Step 1: Recall the definition of periodic motion. It is any motion that repeats itself after regular intervals of time. Step 2: Think of objects or events around you that follow this pattern. A clock's pendulum swings back and forth regularly. A child on a swing moves up and down in a predictable time. Step 3: List two distinct examples. Final answer: Two examples of periodic motion are: (i) The motion of the hands of a clock. (ii) The motion of a child on a swing.

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