### Q.8:- 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, *m*‘ = 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 Newton’s 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.