Verified Solution • 2025–26 Session moderate

Q4.8: NCERT Physics - Part 1 LAWS OF MOTION 4.8 Solution: The driver of a three-wheeler moving with a speed of 36 km/h…

⚠️ Common Exam Traps & Conceptual Pitfalls

Students frequently insert the 36 km/h value directly as metres per second, producing a force about 3.6 times too large; always convert speed to SI units ($v_{\text{m/s}}=\frac{v_{\text{km/h}}}{3.6}$) before applying $a=(v-u)/t$ and $F=ma$.

The driver of a three-wheeler moving with a speed of 36 km/h sees a child standing in the middle of the road and brings his vehicle to rest in 4.0 s just in time to save the child. What is the average retarding force on the vehicle? The mass of the three-wheeler is 400 kg and the mass of the driver is 65 kg.

Step-by-Step Verified Solution

**Answer:** F = 1162.5 N **Step-by-Step Explanation:** Total mass = 400 + 65 = 465 kg. Initial speed = 36 km/h = 10 m/s. Deceleration:
a=v−ut=0−104=−2.5 m/s2a = \frac{v - u}{t} = \frac{0 - 10}{4} = -2.5 \text{ m/s}^2
Retarding force:
F=ma=465×2.5=1162.5 NF = ma = 465 \times 2.5 = 1162.5 \text{ N}
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All Questions in this Chapter

Q4.1 Q4.2 Q4.3 Q4.4 Q4.5 Q4.6 Q4.7 Q4.8 Q4.9 Q4.10 Q4.11 Q4.12 Q4.13 Q4.14 Q4.15 Q4.16 Q4.17 Q4.18 Q4.19 Q4.20 Q4.21 Q4.22 Q4.23

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