Chapter 5: Measurement of Length and Motion (Physics)

Delhi–Lucknow: km; coin thickness: mm; eraser: cm; school ground: m.

(i) T (ii) T (iii) F: 1 km = 1000 m = 1,00,000 cm.

(i) T (ii) T (iii) F (1 km = 1,00,000 cm)

(iv) Handspan. Its length is different for different people, so it is not a standard unit.

(iv) handspan

(Record the devices you actually find; look at the smallest gap between two marks on each.)

Most scales and tapes measure up to 1 mm; a wooden metre scale may measure only up to 1 cm. Record what you find in a table.

$$1.5\text{km}=1.5\times 1000\text{m}=1500\text{m}$$

1500 m

A curved line cannot be measured directly with a straight scale. Method: wrap a thread once around the base of the tumbler, mark the point where the thread meets its starting end, straighten the thread and measure it on a scale. For example, for a tumbler of base diameter 7 cm, the curved edge measures about 22 cm. (Record your own reading.)

Wrap a thread around the base, mark it, straighten it and measure on a scale (e.g. about 22 cm for a 7 cm-wide tumbler).

Make your friend stand straight against a wall, mark the top of the head, and measure from the floor to the mark. For example, if the height is 140 cm:

(i) $140÷100=$ 1.40 m (ii) 140 cm (iii) $140\times 10=$ 1400 mm

e.g. 1.40 m = 140 cm = 1400 mm

Example: a ₹2 coin is about 2.5 cm across. The longer side of a notebook is about 24 cm.

Estimate: about 10 coins. Check: $24÷2.5=9.6$, so about 9 to 10 coins are needed.

(Use your own coin and notebook; measure the coin carefully because the 15 cm scale is shorter than the notebook, so measure the notebook in two parts.)

e.g. a 24 cm notebook side and a 2.5 cm coin need about 24 ÷ 2.5 ≈ 10 coins.

• Linear: a car moving on a straight road; an apple falling from a tree; a soldier marching in a straight line.
• Circular: the blades of a rotating ceiling fan; a merry-go-round; the tip of the hands of a clock.
• Oscillatory: a child on a swing; the pendulum of a clock; a plucked guitar or veena string.

Linear: car on a straight road, falling apple. Circular: fan blades, merry-go-round. Oscillatory: swing, pendulum.

mm: coin thickness, pencil tip, rice grain; cm: eraser, pencil, notebook; m: classroom, tree height, cricket pitch.

• A to B: the ball rolls down the straight slope: linear motion.
• B to C to D to E: the ball goes around the loop: circular motion.
• E to F: the ball moves along the straight track and leaves: linear motion.

A–B: linear; the loop B–C–D–E: circular; E–F: linear.

She should not use stretchable rubber or cloth: they stretch and sag, so the length between the marks would change every time she used the scale, and measurements would be wrong. Paper is also not a good choice because it tears, folds and wears out easily. Plywood and steel are good choices: they are rigid, do not stretch or bend easily, and last long.

Not stretchable rubber or cloth (they stretch, so the length changes), and preferably not paper (tears, folds); plywood or steel is suitable.

"Length Snap" card game:

1. Make 40 cards. On pairs of cards write equal lengths in different units, for example: 1 km and 1000 m; 1 m and 100 cm; 1 cm and 10 mm; 2.5 m and 250 cm; 3 km and 3000 m; 50 mm and 5 cm; 1.5 km and 1500 m; and so on.
2. Shuffle the cards and deal them equally among 2–4 players, face down.
3. Players take turns placing their top card face up on a central pile.
4. When a card is equal to the card just below it (e.g. 100 cm after 1 m), the first player to say "Snap!" wins the pile.
5. A wrong "snap" costs the player one card. The player with the most cards at the end wins.

(Variation: a memory game where all cards are face down and players turn over two at a time to find matching lengths.)

For example, a "snap" or memory game with pairs of cards showing equal lengths in different units (1 m and 100 cm, 1.5 km and 1500 m, …).