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Laws of Motion

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Question : 8 of 40
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The driver of a three-wheeler moving with a speed of 36 km h−1^{-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=36 km h−1u=36\,\text{km}\,\text{h}^{-1}
36×100060×60 m s−1=10 m s−1\frac{36 \times 1000}{60 \times 60}\,\text{m}\,\text{s}^{-1}=10\,\text{m}\,\text{s}^{-1}
v = 0; t = 4 s
Now, v = u + at
∴ 0 = 10 + a × 4 or a = – 2.5 m s−2^{-2}
m = 65 + 400 = 465 kg
Therefore, retarding force (in magnitude) on the vehicle,
F = 465 × 2.5
= 1162.5 N
= 1.2 × 103^{3}N
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