Question

The driver of a car 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 car is 400 kg and the mass
of the driver is 65 kg​

Answer 1

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 m/s2

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.