CBSE Notes Class 6 Science Chapter 7 Motion And Measurement Of Distances

Motion and Measurement of Distances is a key topic in physics that enables us to understand how objects move and how their movement can be quantified. Motion refers to the change in an object's position over time and can take various forms, including linear, circular, or oscillatory motion. The measurement of distances involves using different tools and units to quantify the length of the path traveled by an object.

1.0Measurements

• A measurement involves comparing an unknown quantity with a standard quantity of the same type.
• It is the precise value that tells us “how much”.
• The International System of Units, or SI, was established to provide a standard and reduce confusion.
• When a standard of measurement is established, all measurements are compared to the same exact quantity—the standard for example 1 km is equal to 1000 m
• A unit is the smallest quantity in terms of which other quantities can be measured.
• Without a unit, a measurement cannot be understood.
• In physics, we measure physical quantities, which serve as the fundamental elements through which the laws of physics are described.
• These quantities are measurable and represented by a number, followed by a unit.

2.0Need Of Measurement

• Measurement involves two components: a numerical value and a unit. Without a unit, a measurement has no meaning.
• We require a standard unit to make sense of the measurement. For example height is 4, it could mean 4 inches, 4 feet, etc. 

3.0Measurement Of Length

• Length is a fundamental quantity commonly used to measure the distance between two points.
• Any SI unit can be converted to any other related c.g.s. unit by multiplying by the appropriate conversion factor.
• There are two common systems of standardized units that are used for measuring length, the english system and the metric system.
• The English system uses inches (in.), feet (ft), yards (yd), and miles (mi) for length. 
• The metric system uses millimetres (mm), centimetres (cm), metres (m), and kilometres (km).
• The meter (m) is the SI unit of length.
• We use the centimetre (cm) to measure short distances,  such as the length of the book.
• The millimeter (mm) is used to measure tiny lengths, like the thickness of a coin.
• Long distances are measured in kilometres (km).

4.0Procedure To Measure Length

• For measuring the length of an object, you must choose a suitable device.
• A tailor uses a ‘measuring tape’, whereas a cloth merchant uses a metre rod. 
• Position the scale along the length of the object, ensuring it makes contact. In some scales, the ends may be damaged, making the zero mark difficult to see.
• Correct position of the eye is also important for taking measurements.
• For measuring the length of a curved line or surface using a measuring tape.
• A divider is a tool used to measure the distance between two points. The divider is placed such that its two points are at the two ends of the length to be measured
• In ancient days, long before measuring instruments were invented, people actually used different parts of their body to measure length. But since these measurements are dependent on the size of the person, they may vary from person to person.

Some hand units which were used in many places around the world are :

1. Span (or hand span-Stretch out your hand so that the tip of your thumb is as far away as possible from the tip of your little finger. This distance is referred to as a "span," which for most people is roughly half the length of a cubit.
2. Thumb-The width of a thumb, which was later used as the basis for the inch
3. Fathom-If you extend your arms fully to each side of your body, the distance between the tips of your middle fingers is known as a fathom.
4. Cubit-The cubit is the length from a person's elbow to the tip of the fully extended middle finger.
5. Pace-It is the measure of a full stride from the position of the heel when it is raised from the ground to the point the same heel is set down again at the end of the step.

Limitations of our senses and body parts:

• We use our senses and body parts for various measurements, we cannot trust them to measure exactly and accurately.
• The limitations of the use of senses and body parts have made us develop some devices and instruments for accurate measurements.

5.0Rest And Motion

• When an object doesn’t change its position with respect to the surroundings in a passage of time then it is said to be at rest. Example-A table lying in a room is at rest with respect to the wall of the room, etc.
• Motion can be defined as the practice of changing position in relation to a fixed reference point over a specific period of time. Example-Revolution of the Earth around the Sun for one complete revolution.

6.0Types Of Motion

1. Translational-Motion of a body in which all the points in the body follow parallel paths is called translational motion. Types of Translational Motion

• Rectilinear Motion-If an object moves in a straight line without changing its orientation, its motion is called rectilinear motion. 
• Curvilinear Motion-If an object moves along a curved path without changing its orientation, its motion is called curvilinear motion.
• Circular Motion-When an object moves along a circular path, this type of curvilinear motion is called circular motion.

2. Rotational Motion (Rotatory Motion)-Motion of a body turning about an axis is called rotational motion. The path of an object in a rotational motion is always circular (or curved).Example-The motion of a Potters wheel.

7.0Periodic And Oscillatory Motion

Periodic Motion-A motion that occurs when an object moves in a repeated pattern (a cycle) over equal periods of time is called a periodic motion. Example-Motion of a pendulum bob.

Oscillatory Motion-A motion that occurs when an object moves to and fro about its mean position over equal periods of time is called an oscillatory motion or vibratory motion. Example-Motion of a vibrating stretched spring