Laws of Motion

Question : 8

Total: 40

The driver of a three-wheeler moving with a speed of 36 km h‌–1 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.

Solution:

Here, u=36km h−1

36×1000

60×60

ms−1=10m s−1
v = 0; t = 4 s
Now, v = u + at
∴ 0 = 10 + a × 4 or a = – 2.5 m s‌–2
m = 65 + 400 = 465 kg
Therefore, retarding force (in magnitude) on the vehicle,
F = 465 × 2.5
= 1162.5 N
= 1.2 × 10‌3N

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