Large distances between cities are measured in kilometres. Very small thicknesses are measured in millimetres. Small objects like erasers are conveniently measured in centimetres. A school ground is best measured in metres.
Distance between Delhi and Lucknow - kilometre; Thickness of a coin - millimetre; Length of an eraser - centimetre; Length of school ground - metre.
A car moving along a straight road shows linear motion. Motion is change in position with time relative to a reference point. Since 1 km = 1000 m and 1 m = 100 cm, 1 km = 100000 cm, not 100 cm.
(i) True (ii) True (iii) False
- i. millimetre
- ii. centimetre
- iii. kilometre
- iv. handspan
Millimetre, centimetre and kilometre are standard units. Handspan varies from person to person, so it is not a standard unit.
(iv) handspan
The smallest value that can be measured is the least count of the scale. It is found by reading the smallest division marked on the scale or tape.
Sample observations: 15-cm ruler - 1 mm; metre scale - 1 mm or 0.1 cm; measuring tape - 1 mm or 0.1 cm; tailor's tape - 1 mm or 0.1 cm. Actual observations may vary according to the scale used.
1 km = 1000 m. Therefore, 1.5 km = 1.5 x 1000 m = 1500 m.
1.5 km = 1500 m.
A flexible thread can follow a curved path. Once straightened, its length can be measured with a standard scale.
Use a thread to cover the curved rim or base once, mark the length on the thread, straighten it, and measure the marked length with a ruler. The recorded value will depend on the tumbler or bottle used.
1 m = 100 cm and 1 cm = 10 mm. Thus 1.45 m = 1.45 x 100 cm = 145 cm, and 145 cm = 145 x 10 mm = 1450 mm. Use the same conversion steps for the actual measured height.
Sample answer: If the measured height is 1.45 m, then it is 145 cm and 1450 mm.
Measure the notebook side and the coin diameter in the same unit. Number of coins required = length of notebook side ÷ diameter of coin. Round to the nearest whole coin if the division is not exact.
Sample answer: If the chosen notebook side is 24 cm and the coin diameter is 2 cm, then 24 ÷ 2 = 12 coins are required.
Linear motion is along a straight line, circular motion is along a circular path, and oscillatory motion is repeated to-and-fro motion about a mean position.
Linear motion: a car moving on a straight road; a stone falling straight down. Circular motion: blades of a fan; a point on a rotating wheel. Oscillatory motion: a swing moving to and fro; a simple pendulum.
Use millimetres for very small lengths, centimetres for small everyday objects, and metres for larger objects or distances.
Sample table: mm - thickness of a coin, thickness of a notebook page, width of a rice grain; cm - length of an eraser, length of a pencil, width of a book; m - height of a door, length of a classroom, length of a school ground.
The type of motion depends on the shape of the path followed by the ball. Straight track gives linear motion, while the loop makes the ball move along a circular path.
The ball shows linear motion on the straight portions of the track, such as near A-B and E-F. It shows circular motion on the loop portion of the track, such as C-D-E. On the curved connecting portions, its motion is along a curved path.
A reliable scale must keep a fixed length and have stable markings. Flexible or stretchable materials can give incorrect measurements.
She should not use paper, cloth or stretchable rubber. Paper may tear or bend, cloth can fold or stretch, and stretchable rubber changes length when pulled. A measuring scale should be rigid and should not change its length. Plywood or steel would be more suitable.
The game should use correct conversion facts such as 1 km = 1000 m, 1 m = 100 cm and 1 cm = 10 mm.
One possible game is to make two sets of cards. One set has lengths such as 1 m, 250 cm, 3 km and 40 mm. The other set has equivalent values such as 100 cm, 2.5 m, 3000 m and 4 cm. Players take turns matching equivalent cards. A correct match earns a point, and the player with the most points wins.