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Question -

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



Answer -

Initial speed of the three-wheeler, u =36 km/h = 10 m/s

Final speed of the three-wheeler, v =0 m/s

Time, t = 4 s

Mass of the three-wheeler, m =400 kg

Mass of the driver, m‘ = 65 kg

Total mass of the system, M =400 + 65 = 465 kg

Using the first law of motion, theacceleration (a) of the three-wheeler can be calculated as:

v = u + at

Thenegative sign indicates that the velocity of the three-wheeler is decreasingwith time.

UsingNewton’s second law of motion, the net force acting on the three-wheeler can becalculated as:

F = Ma

=465 × (–2.5) = –1162.5 N

Thenegative sign indicates that the force is acting against the direction ofmotion of the three-wheeler.

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