CLASS 6 SCIENCE · CHAPTER 5
These notes follow the chapter’s sequence and explain its central ideas, examples and activities. Read each section, connect it with the textbook observations, and use the quick check to review the main concepts.
People measure length for many purposes, such as tailoring clothes, building objects and describing distances. Body-based units such as a handspan or cubit vary from one person to another, so they may give different results for the same object. Measurement becomes more reliable when people use standard units and suitable instruments. Before measuring, estimate the length and choose a tool whose scale is appropriate for the object. Record the value together with its unit so that the result can be understood by others.
The metre is a standard unit of length. Smaller lengths may be expressed in centimetres or millimetres, while longer distances may be expressed in kilometres. Unit conversion depends on the relationships between these units: 1 metre equals 100 centimetres, 1 centimetre equals 10 millimetres, and 1 kilometre equals 1000 metres. A ruler, metre scale or measuring tape may be selected according to the size and shape of the object. Use consistent units when comparing measurements.
Place the scale alongside the object and align the zero mark with one end. Read the mark at the other end while looking directly above it to avoid parallax error. If the zero edge is damaged, begin from another clear mark and subtract the starting reading from the ending reading. Keep the scale straight and avoid leaving gaps between the object and scale. A reading should match the smallest divisions the instrument can reasonably show; do not report unjustified precision.
A curved line cannot be measured accurately by simply placing a rigid ruler across it. A thread can be laid carefully along the curve, then straightened and measured with a scale. The thread should follow the curve without stretching. Position can be described by stating a reference point and the distance or direction from it. Clear descriptions of position help another person locate the same place or object. Always specify the reference point when describing where something is.
An object is in motion when its position changes with time relative to a reference point. Different objects move in different ways. Linear motion follows a straight path; circular motion follows a circular path; oscillatory motion repeats a to-and-fro movement about a position. A moving object may combine more than one kind of motion. For example, observing a rollercoaster or a rotating object can help identify the path. Describe the movement based on what is observed, and distinguish the path from the speed or distance travelled.
Key idea: People measure length for many purposes, such as tailoring clothes, building objects and describing distances.
Observe the examples in the chapter, identify the main idea in each section, and connect the concept with the textbook activities. Use accurate terms, explain your reasoning, and review the key ideas before answering questions.