Question:
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.
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, mprime = 65 kg
Total mass of the system, M = 400 + 65 = 465 kg
Using the first law of motion, the acceleration (a) of the three-wheeler can be calculated as:
v = u + at
∴ a = (v - u) / t = (0 - 10) / 4 = -2.5 ms-2
The negative sign indicates that the velocity of the three-wheeler is decreasing with time.
Using Newtons second law of motion, the net force acting on the three-wheeler can be calculated as:
F = Ma
= 465 × (–2.5) = –1162.5 N
The negative sign indicates that the force is acting against the direction of motion of the three-wheeler.
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