Q4. In which of these can the moving body be treated as a point object (NCERT Exercise 2.1, page 24)? (a) a railway carriage moving without jerks between two stations; (b) a monkey sitting on top of a man cycling smoothly on a circular track; (c) a spinning cricket ball that turns sharply on hitting the ground; (d) a tumbling beaker that has slipped off the edge of a table.
(a) and (b), for describing overall travel: the distances covered are much larger than the relevant object sizes. In (c) and (d), a point model cannot describe the spin, contact or tumbling being discussed. A point model may still track the centre of mass, but it discards the rotational information needed here.
Q5. A cyclist rides 600 m along a straight road in 2 min, rests for 1 min, then rides back to the start in 1.5 min. Find the velocity in each part, the average velocity and the average speed for the whole trip.
Out: +600 m/120 s = +5 m/s. Rest: 0. Back: −600 m/90 s, about −6.7 m/s. Whole trip: displacement 0, so average velocity 0; path 1200 m in 270 s, so average speed about 4.4 m/s.
Q6. Two cars start together from the same point and move along the same straight road, A at a steady 15 m/s and B at a steady 20 m/s. Sketch the two x-t graphs. Do they cross after t = 0?
Both are straight lines through the origin; B’s is steeper. They meet only at t = 0, and B stays ahead after that, so the graphs do not cross again. After 10 s, B is 200 m and A is 150 m from the start.