Speed, Time and Direction വേഗവും സമയവും ദിശയും
The chapter's multiplication rules come from a point moving along a line at a steady speed. This page recalls speed, distance and time, and adds the two ideas the chapter needs: direction (right or left) and time before or after a chosen moment.
- Distance is measured in metres, time in seconds.
- The number line: positions right of O positive, left negative (page 1).
1. Speed = distance ÷ time
A runner covering 50 m in 5 s at a steady pace has speed 50 ÷ 5 = 10 metres per second (10 m/s). At that speed, in 10 s she covers 100 m. Distance = speed × time.
Bolt's world record: 100 m in 9.58 s (Berlin, 2009) – an average of about 10.4 m/s. His speed was not steady: he took 2.89 s for the first 20 m but only 1.61 s for 60–80 m. The textbook mentions this on page 133.
2. Direction matters
Starting at O and moving at 10 m/s for 3 s, the point is 30 m from O – but on which side? Moving right it reaches position 30; moving left, position −30.
3. Before and after a moment
Suppose we start our stopwatch when the point passes O, moving right at 10 m/s. Two seconds after, it is at 20. Where was it two seconds before? It still had 20 m to go to reach O, so it was at −20.
Direction 0 = moving right, 1 = moving left. Negative t means "seconds before" passing O. This is exactly the table on textbook page 134.
4. Speed that changes each second
A ball thrown straight up at 49 m/s slows by 9.8 m/s every second, stops after 5 s, and then falls faster and faster (9.8 m/s more each second). Changing speed like this appears in a side box on page 139.
In the textbook section സമയവും വേഗവും (page 9 of this guide), speeds to the left and times before the moment become negative numbers, and the single equation s = vt then works in every case – but only if (−10) × (−2) = 20.
5. Check yourself
- Speed = distance ÷ time; distance = speed × time.
- On a line, direction decides the side: 30 right of O is 30, 30 left is −30.
- "2 seconds before" passing O moving right means 20 m to the left of O: −20.