A boy was hit by a car when he was crossing a zebra-crossing.
Let d be the distance between the car and the boy at the moment the driver first observed the boy. The driver applied the brakes and skid mark 36.0 m long was left on the road. After hitting the boy, the car travelled a distance of 19.7 m before coming to rest. You may neglect the change in speed of the car during the impact.
1) Estimate the thinking distance and the value of d. (with steps)
2) The speed limit of the road is 50 km h^-1(i.e. 13.9 m s^-1). If the car is travelling at this speed, explain whether it would hit the boy.
- Audrey HepburnLv 71 十年前最愛解答
This is a question in 2002 CE past paper 1
1. From the graph, since u = 21.5 m/s
So, think distance l = 15 m
From the figure,
19.7 + d = 36 + l
d = 36 + 15 - 19.7
d = 31.3 m
2. If the car is travelling at this speed (13.9 m/s),
the thinking distance will be approximately 15 m from the graph.
The braking distance would be approximately 10 m
And the stopping distance
= 15 + 10
= 25 m
The stopping distance is shorter than d and hence the car will not hit the boy.資料來源： Myself~~~