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.
Solution
Initial speed of the three-wheeler, u = 36 km/h = 10 m/s
Final speed of the three-wheeler, v = 0 m/sTime, 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
According to the first law of motion,
Acceleration (a) of the three-wheeler is,
v = u + at
Therefore,
a =
The negative sign indicates that the velocity of the three-wheeler is decreasing with time.
Now, using Newton’s second law of motion, the net force acting on the three-wheeler is,
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.
