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NCERT Solutions for Class 9th Science Chapter 4 Activity 4.3: Let us plot a graph — Plotting graph

Book page 57 Updated on2026-09-08

Q1.
Refer to Table 4.3. We need to decide which quantity (time or position) to be shown along each axis.
Answer

Put time along the X-axis and position along the Y-axis.

Why it happens: time is the quantity we control and read off steadily — it marches on whatever the vehicle does. Position is the quantity that responds to it. The convention in every graph is that the controlling (independent) quantity goes on the X-axis and the responding (dependent) quantity on the Y-axis. That way the slope, rise ÷ run, automatically reads as "change in position ÷ change in time" — which is the velocity, a quantity we actually want. Swap the axes and the slope would come out as time per metre, which means nothing useful.
Tip: the same rule gives the velocity–time graph its meaning later: with time on X and velocity on Y, the slope is acceleration and the area is displacement.
Q2.
Determine a suitable scale for each quantity to represent it on the graph paper.
Answer

A good scale is the one that spreads the data over most of the sheet while keeping the numbers easy to plot. For Table 4.3 the book's choice is

X-axis: 5 divisions = 1 s  (time runs 0 s to 6 s)
Y-axis: 5 divisions = 20 m  (position runs 0 m to 120 m)

Check what that uses up: 6 s × 5 divisions = 30 divisions across, and 120 m ÷ 20 m × 5 = 30 divisions up — a comfortable square block on an ordinary sheet.

Why it happens: two conditions decide a scale. (a) The largest value must fit on the paper. (b) Each small division must stand for a round number (1, 2, 5, 10, 20 …) so that intermediate points can be read without arithmetic. A scale like "5 divisions = 17 m" would fit but nobody could plot 60 m on it. And a scale far too large, say 5 divisions = 100 m, would squeeze all six points into a corner and the slope would be impossible to measure accurately.
Check it yourself: the scale changes how the line looks, never what it means. Steepness on paper depends on the scales you chose; the velocity you calculate from it does not.
Q3.
Once all points are plotted, connect them to create the position-time graph for the vehicle's motion (Fig. 4.11c). It is a straight line for the data given in Table 4.3.
Answer

Plotting (0 s, 0 m), (1 s, 20 m), (2 s, 40 m) … (6 s, 120 m) and joining them gives a single straight line through the origin.

0123456 20406080100120 Time (s) Position (m)
Position–time graph for Table 4.3. Equal position gains of 20 m in every 1 s make the points fall on one straight line.
Why it happens: in each successive 1 s the vehicle's position increases by exactly 20 m. Equal rises for equal runs is precisely the geometric condition for a straight line. Its slope is the velocity:
v = (120 m − 0 m) / (6 s − 0 s) = 20 m s⁻¹, constant throughout
So a straight-line position–time graph means constant velocity, i.e. uniform motion.
Tip: this graph is not a picture of the road. It does not say the vehicle went "up" — it says its distance from the origin grew steadily with time.
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